6. Configuration

6.1. Configuration File Elements

Following is a list of elements used throughout the Loop configuration file documentation:

acl_name The name of an address_match_list as defined by the acl statement.
``address_match_list `` A list of one or more ip_addr, ip_prefix, key_id, or acl_name elements, see section_title.
masters_list A named list of one or more ip_addr with optional key_id and/or ip_port. A masters_list may include other masters_lists.
domain_name A quoted string which will be used as a DNS name, for example "my.test.domain".
namelist A list of one or more domain_name elements.
dotted_decimal One to four integers valued 0 through 255 separated by dots (.'), such as ``123`, 45.67 or 89.123.45.67.
ip4_addr An IPv4 address with exactly four elements in dotted_decimal notation.
ip6_addr An IPv6 address, such as 2001:db8::1234. IPv6 scoped addresses that have ambiguity on their scope zones must be disambiguated by an appropriate zone ID with the percent character (%') as delimiter. It is strongly recommended to use string zone names rather than numeric identifiers, in order to be robust against system configuration changes. However, since there is no standard mapping for such names and identifier values, currently only interface names as link identifiers are supported, assuming one-to-one mapping between interfaces and links. For example, a link-local address ``fe80::1` on the link attached to the interface ne0 can be specified as fe80::1%ne0. Note that on most systems link-local addresses always have the ambiguity, and need to be disambiguated.
ip_addr An ip4_addr or ip6_addr.
ip_dscp A number between 0 and 63, used to select a differentiated services code point (DSCP) value for use with outgoing traffic on operating systems that support DSCP.
ip_port An IP port number. The number is limited to 0 through 65535, with values below 1024 typically restricted to use by processes running as root. In some cases, an asterisk (`*') character can be used as a placeholder to select a random high-numbered port.
ip_prefix

An IP network specified as an ip_addr, followed by a slash (/') and then the number of bits in the netmask. Trailing zeros in a ``ip_addr` may omitted. For example, 127/8 is the network 127.0.0.0 with netmask 255.0.0.0 and 1.2.3.0/28 is network 1.2.3.0 with netmask 255.255.255.240.

When specifying a prefix involving a IPv6 scoped address the scope may be omitted. In that case the prefix will match packets from any scope.

key_id A domain_name representing the name of a shared key, to be used for transaction security.
key_list A list of one or more key_ids, separated by semicolons and ending with a semicolon.
number A non-negative 32-bit integer (i.e., a number between 0 and 4294967295, inclusive). Its acceptable value might be further limited by the context in which it is used.
fixedpoint A non-negative real number that can be specified to the nearest one hundredth. Up to five digits can be specified before a decimal point, and up to two digits after, so the maximum value is 99999.99. Acceptable values might be further limited by the context in which it is used.
path_name A quoted string which will be used as a pathname, such as zones/master/my.test.domain.
port_list A list of an ip_port or a port range. A port range is specified in the form of range followed by two ip_ports, port_low and port_high, which represents port numbers from port_low through port_high, inclusive. port_low must not be larger than port_high. For example, range 1024 65535 represents ports from 1024 through 65535. In either case an asterisk (*') character is not allowed as a valid ``ip_port`.
size_spec

A 64-bit unsigned integer, or the keywords unlimited or default.

Integers may take values 0 <= value <= 18446744073709551615, though certain parameters (such as max-journal-size) may use a more limited range within these extremes. In most cases, setting a value to 0 does not literally mean zero; it means "undefined" or "as big as possible", depending on the context. See the explanations of particular parameters that use size_spec for details on how they interpret its use.

Numeric values can optionally be followed by a scaling factor: K or k for kilobytes, M or m for megabytes, and G or g for gigabytes, which scale by 1024, 1024*1024, and 1024*1024*1024 respectively.

unlimited generally means "as big as possible", and is usually the best way to safely set a very large number.

default uses the limit that was in force when the server was started.

yes_or_no Either yes or no. The words true and false are also accepted, as are the numbers 1 and 0.
dialup_option One of yes, no, notify, notify-passive, refresh or passive. When used in a zone, notify-passive, refresh, and passive are restricted to slave and stub zones.

6.1.1. Address Match Lists

6.1.1.1. Syntax

address_match_list = address_match_list_element ; ...

address_match_list_element = [ ! ] ( ip_address | ip_prefix |
     key key_id | acl_name | { address_match_list } )

6.1.1.2. Definition and Usage

Address match lists are primarily used to determine access control for various server operations. They are also used in the listen-on statement. The elements which constitute an address match list can be any of the following:

  • an IP address (IPv4 or IPv6)
  • an IP prefix (in `/' notation)
  • a key ID, as defined by the key statement
  • the name of an address match list defined with the acl statement
  • a nested address match list enclosed in braces

Elements can be negated with a leading exclamation mark (`!'), and the match list names "any", "none", "localhost", and "localnets" are predefined. More information on those names can be found in the description of the acl statement.

The addition of the key clause made the name of this syntactic element something of a misnomer, since security keys can be used to validate access without regard to a host or network address. Nonetheless, the term "address match list" is still used throughout the documentation.

When a given IP address or prefix is compared to an address match list, the comparison takes place in approximately O(1) time. However, key comparisons require that the list of keys be traversed until a matching key is found, and therefore may be somewhat slower.

The interpretation of a match depends on whether the list is being used for access control, or defining listen-on ports, and whether the element was negated.

When used as an access control list, a non-negated match allows access and a negated match denies access. If there is no match, access is denied. The clauses allow-notify, allow-recursion, allow-recursion-on, allow-query, allow-query-on, allow-query-cache, allow-query-cache-on, allow-transfer, allow-update, allow-update-forwarding, and blackhole all use address match lists. Similarly, the listen-on option will cause the server to refuse queries on any of the machine's addresses which do not match the list.

Order of insertion is significant. If more than one element in an ACL is found to match a given IP address or prefix, preference will be given to the one that came first in the ACL definition. Because of this first-match behavior, an element that defines a subset of another element in the list should come before the broader element, regardless of whether either is negated. For example, in 1.2.3/24; ! 1.2.3.13; the 1.2.3.13 element is completely useless because the algorithm will match any lookup for 1.2.3.13 to the 1.2.3/24 element. Using ! 1.2.3.13; 1.2.3/24 fixes that problem by having 1.2.3.13 blocked by the negation, but all other 1.2.3.* hosts fall through.

6.1.2. Comment Syntax

The Loop comment syntax allows for comments to appear anywhere that whitespace may appear in a Loop configuration file. To appeal to programmers of all kinds, they can be written in the C, C++, or shell/perl style.

6.1.2.1. Syntax

/* This is a Loop comment as in C */
// This is a Loop comment as in C++
# This is a Loop comment as in common UNIX shells
# and perl

6.1.2.2. Definition and Usage

Comments may appear anywhere that whitespace may appear in a Loop configuration file.

C-style comments start with the two characters /* (slash, star) and end with */ (star, slash). Because they are completely delimited with these characters, they can be used to comment only a portion of a line or to span multiple lines.

C-style comments cannot be nested. For example, the following is not valid because the entire comment ends with the first */:

/* This is the start of a comment.
   This is still part of the comment.
/* This is an incorrect attempt at nesting a comment. */
   This is no longer in any comment. */

C++-style comments start with the two characters // (slash, slash) and continue to the end of the physical line. They cannot be continued across multiple physical lines; to have one logical comment span multiple lines, each line must use the // pair. For example:

// This is the start of a comment.  The next line
// is a new comment, even though it is logically
// part of the previous comment.

Shell-style (or perl-style, if you prefer) comments start with the character # (number sign) and continue to the end of the physical line, as in C++ comments. For example:

# This is the start of a comment.  The next line
# is a new comment, even though it is logically
# part of the previous comment.

Warning

You cannot use the semicolon (`;') character to start a comment such as you would in a zone file. The semicolon indicates the end of a configuration statement.

6.2. Configuration File Grammar

A Loop configuration consists of statements and comments. Statements end with a semicolon. Statements and comments are the only elements that can appear without enclosing braces. Many statements contain a block of sub-statements, which are also terminated with a semicolon.

The following statements are supported:

acl defines a named IP address matching list, for access control and other uses.
controls declares control channels to be used by the rndc utility.
include includes a file.
key specifies key information for use in authentication and authorization using TSIG.
logging specifies what the server logs, and where the log messages are sent.
masters defines a named masters list for inclusion in stub and slave zones' masters or also-notify lists.
options controls global server configuration options and sets defaults for other statements.
server sets certain configuration options on a per-server basis.
statistics-ch annels declares communication channels to get access to named statistics.
``trusted-keys` ` defines trusted DNSSEC keys.
``managed-keys` ` lists DNSSEC keys to be kept up to date using RFC 5011 trust anchor maintenance.
view defines a view.
zone defines a zone.

The logging and options statements may only occur once per configuration.

6.2.1. acl Statement Grammar

acl acl-name {
    address_match_list
};

6.2.2. acl Statement Definition and Usage

The acl statement assigns a symbolic name to an address match list. It gets its name from a primary use of address match lists: Access Control Lists (ACLs).

The following ACLs are built-in:

any Matches all hosts.
none Matches no hosts.
``localhost` ` Matches the IPv4 and IPv6 addresses of all network interfaces on the system. When addresses are added or removed, the localhost ACL element is updated to reflect the changes.
``localnets` ` Matches any host on an IPv4 or IPv6 network for which the system has an interface. When addresses are added or removed, the localnets ACL element is updated to reflect the changes. Some systems do not provide a way to determine the prefix lengths of local IPv6 addresses. In such a case, localnets only matches the local IPv6 addresses, just like localhost.

6.2.3. controls Statement Grammar

controls {
  [ inet ( ip_addr | * ) [ port ip_port ] allow { address_match_list }
      [ keys { key_list } ]
      [ read-only yes_or_no ] ; ]
   [ ...; ]
};

6.2.4. controls Statement Definition and Usage

The controls statement declares control channels to be used by system administrators to control the operation of the name server. These control channels are used by the rndc utility to send commands to and retrieve non-DNS results from a name server.

An inet control channel is a TCP socket listening at the specified ip_port on the specified ip_addr, which can be an IPv4 or IPv6 address. An ip_addr of * (asterisk) is interpreted as the IPv4 wildcard address; connections will be accepted on any of the system's IPv4 addresses. To listen on the IPv6 wildcard address, use an ip_addr of ::. If you will only use rndc on the local host, using the loopback address (127.0.0.1 or ::1) is recommended for maximum security.

If no port is specified, port 953 is used. The asterisk "*" cannot be used for ip_port.

The ability to issue commands over the control channel is restricted by the allow and keys clauses. Connections to the control channel are permitted based on the address_match_list. This is for simple IP address based filtering only; any key_id elements of the address_match_list are ignored.

The primary authorization mechanism of the command channel is the key_list, which contains a list of key_ids. Each key_id in the key_list is authorized to execute commands over the control channel. See Remote Name Daemon Control application in section_title) for information about configuring keys in rndc.

If no controls statement is present, named will set up a default control channel listening on the loopback address 127.0.0.1 and its IPv6 counterpart ::1. In this case, and also when the controls statement is present but does not have a keys clause, named will attempt to load the command channel key from the file rndc.key in /etc. To create a rndc.key file, run rndc-confgen -a.

The rndc.key feature does not have a high degree of configurability. You cannot easily change the key name or the size of the secret, so you should make a rndc.conf with your own key if you wish to change those things. The rndc.key file also has its permissions set such that only the owner of the file (the user that named is running as) can access it. If you desire greater flexibility in allowing other users to access rndc commands, then you need to create a rndc.conf file and make it group readable by a group that contains the users who should have access.

To disable the command channel, use an empty controls statement: controls { };.

6.2.5. include Statement Grammar

include filename;

6.2.6. include Statement Definition and Usage

The include statement inserts the specified file at the point where the include statement is encountered. The include statement facilitates the administration of configuration files by permitting the reading or writing of some things but not others. For example, the statement could include private keys that are readable only by the name server.

6.2.7. key Statement Grammar

key key_id {
    algorithm algorithm_id;
    secret secret_string;
};

6.2.8. key Statement Definition and Usage

The key statement defines a shared secret key for use with TSIG (see section_title) or the command channel (see section_title).

The key statement can occur at the top level of the configuration file or inside a view statement. Keys defined in top-level key statements can be used in all views. Keys intended for use in a controls statement (see section_title) must be defined at the top level.

The key_id, also known as the key name, is a domain name uniquely identifying the key. It can be used in a server statement to cause requests sent to that server to be signed with this key, or in address match lists to verify that incoming requests have been signed with a key matching this name, algorithm, and secret.

The algorithm_id is a string that specifies a security/authentication algorithm. Named supports hmac-md5, hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384 and hmac-sha512 TSIG authentication. Truncated hashes are supported by appending the minimum number of required bits preceded by a dash, e.g. hmac-sha1-80. The secret_string is the secret to be used by the algorithm, and is treated as a Base64 encoded string.

6.2.9. logging Statement Grammar

logging {
  [ channel channel_name {
    ( ( file path_name
      [ versions ( number | unlimited ) ]
      [ size size_spec ] )
      | syslog syslog_facility
      | stderr
      | null ) ;
      [ severity ( critical | error | warning | notice |
           info | debug [ level ] | dynamic ) ; ]
      [ print-category yes_or_no ; ]
      [ print-severity yes_or_no ; ]
      [ print-time yes_or_no ; ]
    }; ]
  [ category category_name {
     channel_name ; ...
    }; ]
    ...
};

6.2.10. logging Statement Definition and Usage

The logging statement configures a wide variety of logging options for the name server. Its channel phrase associates output methods, format options and severity levels with a name that can then be used with the category phrase to select how various classes of messages are logged.

Only one logging statement is used to define as many channels and categories as are wanted. If there is no logging statement, the logging configuration will be:

logging {
     category default { default_syslog; default_debug; };
     category unmatched { null; };
};

In Loop, the logging configuration is only established when the entire configuration file has been parsed. When the server is starting up, all logging messages regarding syntax errors in the configuration file go to the default channels, or to standard error if the "-g" option was specified.

6.2.10.1. The channel Phrase

All log output goes to one or more channels; you can make as many of them as you want.

Every channel definition must include a destination clause that says whether messages selected for the channel go to a file, to a particular syslog facility, to the standard error stream, or are discarded. It can optionally also limit the message severity level that will be accepted by the channel (the default is info), and whether to include a named-generated time stamp, the category name and/or severity level (the default is not to include any).

The null destination clause causes all messages sent to the channel to be discarded; in that case, other options for the channel are meaningless.

The file destination clause directs the channel to a disk file. It can include limitations both on how large the file is allowed to become, and how many versions of the file will be saved each time the file is opened.

If you use the versions log file option, then named will retain that many backup versions of the file by renaming them when opening. For example, if you choose to keep three old versions of the file lamers.log, then just before it is opened lamers.log.1 is renamed to lamers.log.2, lamers.log.0 is renamed to lamers.log.1, and lamers.log is renamed to lamers.log.0. You can say versions unlimited to not limit the number of versions. If a size option is associated with the log file, then renaming is only done when the file being opened exceeds the indicated size. No backup versions are kept by default; any existing log file is simply appended.

The size option for files is used to limit log growth. If the file ever exceeds the size, then named will stop writing to the file unless it has a versions option associated with it. If backup versions are kept, the files are rolled as described above and a new one begun. If there is no versions option, no more data will be written to the log until some out-of-band mechanism removes or truncates the log to less than the maximum size. The default behavior is not to limit the size of the file.

Example usage of the size and versions options:

channel an_example_channel {
    file "example.log" versions 3 size 20m;
    print-time yes;
    print-category yes;
};

The syslog destination clause directs the channel to the system log. Its argument is a syslog facility as described in the syslog man page. Known facilities are kern, user, mail, daemon, auth, syslog, lpr, news, uucp, cron, authpriv, ftp, local0, local1, local2, local3, local4, local5, local6 and local7, however not all facilities are supported on all operating systems. How syslog will handle messages sent to this facility is described in the syslog.conf man page. If you have a system which uses a very old version of syslog that only uses two arguments to the openlog() function, then this clause is silently ignored.

The severity clause works like syslog's "priorities", except that they can also be used if you are writing straight to a file rather than using syslog. Messages which are not at least of the severity level given will not be selected for the channel; messages of higher severity levels will be accepted.

If you are using syslog, then the syslog.conf priorities will also determine what eventually passes through. For example, defining a channel facility and severity as daemon and debug but only logging daemon.warning via syslog.conf will cause messages of severity info and notice to be dropped. If the situation were reversed, with named writing messages of only warning or higher, then syslogd would print all messages it received from the channel.

The stderr destination clause directs the channel to the server's standard error stream. This is intended for use when the server is running as a foreground process, for example when debugging a configuration.

The server can supply extensive debugging information when it is in debugging mode. If the server's global debug level is greater than zero, then debugging mode will be active. The global debug level is set either by starting the named server with the -d flag followed by a positive integer, or by running rndc trace. The global debug level can be set to zero, and debugging mode turned off, by running rndc notrace. All debugging messages in the server have a debug level, and higher debug levels give more detailed output. Channels that specify a specific debug severity, for example:

channel specific_debug_level {
    file "foo";
    severity debug 3;
};

will get debugging output of level 3 or less any time the server is in debugging mode, regardless of the global debugging level. Channels with dynamic severity use the server's global debug level to determine what messages to print.

If print-time has been turned on, then the date and time will be logged. print-time may be specified for a syslog channel, but is usually pointless since syslog also logs the date and time. If print-category is requested, then the category of the message will be logged as well. Finally, if print-severity is on, then the severity level of the message will be logged. The print- options may be used in any combination, and will always be printed in the following order: time, category, severity. Here is an example where all three print- options are on:

28-Feb-2000 15:05:32.863 general: notice: running

There are four predefined channels that are used for named's default logging as follows. How they are used is described in section_title.

channel default_syslog {
    // send to syslog's daemon facility
    syslog daemon;
    // only send priority info and higher
    severity info;
};

channel default_debug {
    // write to named.run in the working directory
    // Note: stderr is used instead of "named.run" if
    // the server is started with the '-f' option.
    file "named.run";
    // log at the server's current debug level
    severity dynamic;
};

channel default_stderr {
    // writes to stderr
    stderr;
    // only send priority info and higher
    severity info;
};

channel null {
   // toss anything sent to this channel
   null;
};

The default_debug channel has the special property that it only produces output when the server's debug level is nonzero. It normally writes to a file called named.run in the server's working directory.

For security reasons, when the "-u" command line option is used, the named.run file is created only after named has changed to the new UID, and any debug output generated while named is starting up and still running as root is discarded. If you need to capture this output, you must run the server with the "-g" option and redirect standard error to a file.

Once a channel is defined, it cannot be redefined. Thus you cannot alter the built-in channels directly, but you can modify the default logging by pointing categories at channels you have defined.

6.2.10.2. The category Phrase

There are many categories, so you can send the logs you want to see wherever you want, without seeing logs you don't want. If you don't specify a list of channels for a category, then log messages in that category will be sent to the default category instead. If you don't specify a default category, the following "default default" is used:

category default { default_syslog; default_debug; };

As an example, let's say you want to log security events to a file, but you also want keep the default logging behavior. You'd specify the following:

channel my_security_channel {
    file "my_security_file";
    severity info;
};
category security {
    my_security_channel;
    default_syslog;
    default_debug;
};

To discard all messages in a category, specify the null channel:

category xfer-out { null; };
category notify { null; };

Following are the available categories and brief descriptions of the types of log information they contain. More categories may be added in future Loop releases.

6.2.10.3. The query-errors Category

The query-errors category is specifically intended for debugging purposes: To identify why and how specific queries result in responses which indicate an error. Messages of this category are therefore only logged with debug levels.

At the debug levels of 1 or higher, each response with the rcode of SERVFAIL is logged as follows:

client 127.0.0.1#61502: query failed (SERVFAIL) for www.example.com/IN/AAAA at query.c:3880

This means an error resulting in SERVFAIL was detected at line 3880 of source file query.c. Log messages of this level will particularly help identify the cause of SERVFAIL for an authoritative server.

At the debug levels of 2 or higher, detailed context information of recursive resolutions that resulted in SERVFAIL is logged. The log message will look like as follows:

fetch completed at resolver.c:2970 for www.example.com/A
in 30.000183: timed out/success [domain:example.com,
referral:2,restart:7,qrysent:8,timeout:5,lame:0,neterr:0,
badresp:1,adberr:0,findfail:0,valfail:0]

The first part before the colon shows that a recursive resolution for AAAA records of www.example.com completed in 30.000183 seconds and the final result that led to the SERVFAIL was determined at line 2970 of source file resolver.c.

The following part shows the detected final result and the latest result of DNSSEC validation. The latter is always success when no validation attempt is made. In this example, this query resulted in SERVFAIL probably because all name servers are down or unreachable, leading to a timeout in 30 seconds. DNSSEC validation was probably not attempted.

The last part enclosed in square brackets shows statistics information collected for this particular resolution attempt. The domain field shows the deepest zone that the resolver reached; it is the zone where the error was finally detected. The meaning of the other fields is summarized in the following table.

referral The number of referrals the resolver received throughout the resolution process. In the above example this is 2, which are most likely com and example.com.
restart The number of cycles that the resolver tried remote servers at the domain zone. In each cycle the resolver sends one query (possibly resending it, depending on the response) to each known name server of the domain zone.
qrysent The number of queries the resolver sent at the domain zone.
timeout The number of timeouts since the resolver received the last response.
lame The number of lame servers the resolver detected at the domain zone. A server is detected to be lame either by an invalid response or as a result of lookup in Loop's address database (ADB), where lame servers are cached.
neterr The number of erroneous results that the resolver encountered in sending queries at the domain zone. One common case is the remote server is unreachable and the resolver receives an ICMP unreachable error message.
badresp The number of unexpected responses (other than lame) to queries sent by the resolver at the domain zone.
adberr Failures in finding remote server addresses of the domain zone in the ADB. One common case of this is that the remote server's name does not have any address records.
findfail Failures of resolving remote server addresses. This is a total number of failures throughout the resolution process.
valfail Failures of DNSSEC validation. Validation failures are counted throughout the resolution process (not limited to the domain zone), but should only happen in domain.

At the debug levels of 3 or higher, the same messages as those at the debug 1 level are logged for other errors than SERVFAIL. Note that negative responses such as NXDOMAIN are not regarded as errors here.

At the debug levels of 4 or higher, the same messages as those at the debug 2 level are logged for other errors than SERVFAIL. Unlike the above case of level 3, messages are logged for negative responses. This is because any unexpected results can be difficult to debug in the recursion case.

6.2.11. masters Statement Grammar

masters name [ port ip_port ] [ dscp ip_dscp ] {
  ( masters_list ; ) |
  ( ip_addr [ port ip_port ] [ key key ] ; )
    ...
};

6.2.12. masters Statement Definition and Usage

masters lists allow for a common set of masters to be easily used by multiple stub and slave zones in their masters or also-notify lists.

6.2.13. options Statement Grammar

This is the grammar of the options statement in the named.conf file:

options {
  [ attach-cache cache_name ; ]
  [ version version_string ; ]
  [ hostname hostname_string ; ]
  [ server-id server_id_string ; ]
  [ directory path_name ; ]
  [ key-directory path_name ; ]
  [ managed-keys-directory path_name ; ]
  [ tkey-domain domain_name ; ]
  [ tkey-dhkey key_name key_tag ; ]
  [ cache-file path_name ; ]
  [ dump-file path_name ; ]
  [ dnssec-keys-file path_name ; ]
  [ license-file path_name ; ]
  [ secroots-file path_name ; ]
  [ session-keyfile path_name ; ]
  [ session-keyname key_name ; ]
  [ session-keyalg algorithm_id ; ]
  [ pid-file path_name ; ]
  [ recursing-file path_name ; ]
  [ statistics-file path_name ; ]
  [ zone-statistics ( full | terse | none ) ; ]
  [ auth-nxdomain yes_or_no ; ]
  [ deallocate-on-exit yes_or_no ; ]
  [ dialup dialup_option ; ]
  [ flush-zones-on-shutdown yes_or_no ; ]
  [ has-old-clients yes_or_no ; ]
  [ minimal-responses yes_or_no ; ]
  [ notify ( yes_or_no | explicit | master-only ) ; ]
  [ recursion yes_or_no ; ]
  [ request-sit yes_or_no ; ]
  [ nosit-udp-size number ; ]
  [ sit-secret secret_string ; ]
  [ request-nsid yes_or_no ; ]
  [ ixfr-from-differences ( yes_or_no | master | slave ) ; ]
  [ auto-dnssec ( allow | maintain | off ) ; ]
  [ inline-signing yes_or_no ; ]
  [ dnssec-enable yes_or_no ; ]
  [ dnssec-validation ( yes_or_no | auto ) ; ]
  [ dnssec-must-be-secure domain yes_or_no ; ]
  [ forward ( only | first ) ; ]
  [ forwarders {
      ( ip_addr [ port ip_port ] [ dscp ip_dscp ] ; )
    ...
    } ; ]
  [ dual-stack-servers [ port ip_port ] [ dscp ip_dscp ] {
      ( ( domain_name | ip_addr ) [ port ip_port ] [ dscp ip_dscp ] ; )
    ...
    } ; ]
  [ check-names ( master | slave | response )
        ( warn | fail | ignore ) ; ]
  [ check-dup-records ( warn | fail | ignore ) ; ]
  [ check-mx ( warn | fail | ignore ) ; ]
  [ check-wildcard yes_or_no ; ]
  [ check-integrity yes_or_no ; ]
  [ check-mx-cname ( warn | fail | ignore ) ; ]
  [ check-srv-cname ( warn | fail | ignore ) ; ]
  [ check-sibling yes_or_no ; ]
  [ check-spf ( warn | ignore ) ; ]
  [ allow-new-zones yes_or_no ; ]
  [ allow-notify { address_match_list } ; ]
  [ allow-query { address_match_list } ; ]
  [ allow-query-on { address_match_list } ; ]
  [ allow-query-cache { address_match_list } ; ]
  [ allow-query-cache-on { address_match_list } ; ]
  [ allow-transfer { address_match_list } ; ]
  [ allow-recursion { address_match_list } ; ]
  [ allow-recursion-on { address_match_list } ; ]
  [ allow-update { address_match_list } ]
  [ allow-update-forwarding { address_match_list } ; ]
  [ automatic-interface-scan yes_or_no ; ]
  [ update-check-ksk yes_or_no ; ]
  [ dnssec-update-mode ( maintain | no-resign ) ; ]
  [ dnssec-dnskey-kskonly yes_or_no ; ]
  [ dnssec-loadkeys-interval number ; ]
  [ dnssec-secure-to-insecure yes_or_no ; ]
  [ try-tcp-refresh yes_or_no ; ]
  [ allow-v6-synthesis { address_match_list } ; ]
  [ blackhole { address_match_list } ; ]
  [ no-case-compress { address_match_list } ; ]
  [ use-v4-udp-ports { port_list } ; ]
  [ avoid-v4-udp-ports { port_list } ; ]
  [ use-v6-udp-ports { port_list } ; ]
  [ avoid-v6-udp-ports { port_list } ; ]
  [ listen-on [ port ip_port ] [ dscp ip_dscp ] { address_match_list } ; ]
  [ listen-on-v6 [ port ip_port ] [ dscp ip_dscp ] { address_match_list } ; ]
  [ query-source ( [ address ] ( ip4_addr | * ) )
      [ port ( ip_port | * ) ] [ dscp ip_dscp ] ] ;
  [ query-source-v6 ( [ address ] ( ip6_addr | * ) )
      [ port ( ip_port | * ) ] [ dscp ip_dscp ] ] ;
  [ use-queryport-pool yes_or_no ; ]
  [ queryport-pool-ports number ; ]
  [ queryport-pool-updateinterval number ; ]
  [ max-records number ; ]
  [ max-transfer-time-in number ; ]
  [ max-transfer-time-out number ; ]
  [ max-transfer-idle-in number ; ]
  [ max-transfer-idle-out number ; ]
  [ reserved-sockets number ; ]
  [ recursive-clients number ; ]
  [ tcp-clients number ; ]
  [ clients-per-query number ; ]
  [ max-clients-per-query number ; ]
  [ fetches-per-server number [ ( drop | fail ) ] ; ]
  [ fetches-per-zone number [ ( drop | fail ) ] ; ]
  [ fetch-quota-params number fixedpoint fixedpoint fixedpoint ; ]
  [ serial-query-rate number ; ]
  [ tcp-listen-queue number ; ]
  [ transfers-in  number ; ]
  [ transfers-out number ; ]
  [ transfers-per-ns number ; ]
  [ transfer-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ transfer-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ alt-transfer-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ alt-transfer-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ use-alt-transfer-source yes_or_no ; ]
  [ notify-delay seconds ; ]
  [ notify-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ notify-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ notify-to-soa yes_or_no ; ]
  [ also-notify [ port ip_port] [ dscp ip_dscp] {
      ( masters | ip_addr [ port ip_port ] ) [ key key_name ] ;
    ...
    } ; ]
  [ max-journal-size size_spec ; ]
  [ coresize size_spec ; ]
  [ datasize size_spec ; ]
  [ files size_spec ; ]
  [ stacksize size_spec ; ]
  [ heartbeat-interval number ; ]
  [ interface-interval number ; ]
  [ rrset-order { order_spec ; ... } ; ]
  [ lame-ttl number ; ]
  [ max-ncache-ttl number ; ]
  [ max-cache-ttl number ; ]
  [ max-zone-ttl ( unlimited | number ) ; ]
  [ serial-update-method ( increment | unixtime ) ; ]
  [ sig-validity-interval number [number] ; ]
  [ sig-signing-nodes number ; ]
  [ sig-signing-signatures number ; ]
  [ sig-signing-type number ; ]
  [ provide-ixfr yes_or_no ; ]
  [ request-ixfr yes_or_no ; ]
  [ min-refresh-time number ; ]
  [ max-refresh-time number ; ]
  [ min-retry-time number ; ]
  [ max-retry-time number ; ]
  [ port ip_port ; ]
  [ dscp ip_dscp ; ]
  [ random-device path_name ; ]
  [ max-cache-size size_spec ; ]
  [ match-mapped-addresses yes_or_no ; ]
  [ dns64 ipv6-prefix {
      [ clients { address_match_list } ; ]
      [ mapped { address_match_list } ; ]
      [ exclude { address_match_list } ; ]
      [ suffix ip6-address ; ]
      [ recursive-only yes_or_no ; ]
      [ break-dnssec yes_or_no ; ]
    } ; ]
  [ dns64-server name ]
  [ dns64-contact name ]
  [ preferred-glue ( A | AAAA | none ); ]
  [ edns-udp-size number ; ]
  [ max-udp-size number ; ]
  [ max-rsa-exponent-size number ; ]
  [ root-delegation-only [ exclude { namelist } ] ; ]
  [ querylog yes_or_no ; ]
  [ disable-algorithms domain { algorithm ; ... } ; ]
  [ disable-ds-digests domain { digest_type ; ... } ; ]
  [ max-recursion-depth number ; ]
  [ max-recursion-queries number ; ]
  [ empty-server name ; ]
  [ empty-contact name ; ]
  [ empty-zones-enable yes_or_no ; ]
  [ disable-empty-zone zone_name ; ]
  [ zero-no-soa-ttl yes_or_no ; ]
  [ zero-no-soa-ttl-cache yes_or_no ; ]
  [ resolver-query-timeout number ; ]
  [ deny-answer-addresses { address_match_list }
      [ except-from { namelist } ] ; ]
  [ deny-answer-aliases { namelist }
      [ except-from { namelist } ] ; ]
  [ prefetch number [ number ] ; ]
  [ rate-limit {
      [ responses-per-second number ; ]
      [ referrals-per-second number ; ]
      [ nodata-per-second number ; ]
      [ nxdomains-per-second number ; ]
      [ errors-per-second number ; ]
      [ all-per-second number ; ]
      [ window number ; ]
      [ log-only yes_or_no ; ]
      [ qps-scale number ; ]
      [ ipv4-prefix-length number ; ]
      [ ipv6-prefix-length number ; ]
      [ slip number ; ]
      [ exempt-clients { address_match_list } ; ]
      [ max-table-size number ; ]
      [ min-table-size number ; ]
    } ; ]
  [ response-policy {
    zone zone_name
      [ policy ( given | disabled | passthru | drop |
         tcp-only | nxdomain | nodata | cname domain ) ]
      [ recursive-only yes_or_no ]
      [ max-policy-ttl number ] ;
     ...
    }
      [ recursive-only yes_or_no ]
      [ max-policy-ttl number ]
      [ break-dnssec yes_or_no ]
      [ min-ns-dots number ]
      [ qname-wait-recurse yes_or_no ] ; ]
} ; ]

6.2.14. options Statement Definition and Usage

The options statement sets up global options to be used by Loop. This statement may appear only once in a configuration file. If there is no options statement, an options block with each option set to its default will be used.

attach-cache

Allows multiple views to share a single cache database. Each view has its own cache database by default, but if multiple views have the same operational policy for name resolution and caching, those views can share a single cache to save memory and possibly improve resolution efficiency by using this option.

The attach-cache option may also be specified in view statements, in which case it overrides the global attach-cache option.

The cache_name specifies the cache to be shared. When the named server configures views which are supposed to share a cache, it creates a cache with the specified name for the first view of these sharing views. The rest of the views will simply refer to the already created cache.

One common configuration to share a cache would be to allow all views to share a single cache. This can be done by specifying the attach-cache as a global option with an arbitrary name.

Another possible operation is to allow a subset of all views to share a cache while the others to retain their own caches. For example, if there are three views A, B, and C, and only A and B should share a cache, specify the attach-cache option as a view A (or B)'s option, referring to the other view name:

view "A" {
  // this view has its own cache
  ...
};
view "B" {
  // this view refers to A's cache
  attach-cache "A";
};
view "C" {
  // this view has its own cache
  ...
};

Views that share a cache must have the same policy on configurable parameters that may affect caching. The current implementation requires the following configurable options be consistent among these views: check-names, dnssec-validation, max-cache-ttl, max-ncache-ttl, max-cache-size, and zero-no-soa-ttl.

Note that there may be other parameters that may cause confusion if they are inconsistent for different views that share a single cache. For example, if these views define different sets of forwarders that can return different answers for the same question, sharing the answer does not make sense or could even be harmful. It is administrator's responsibility to ensure configuration differences in different views do not cause disruption with a shared cache.

directory
The working directory of the server. Any non-absolute pathnames in the configuration file will be taken as relative to this directory. The default location for most server output files (e.g. named.run) is this directory. If a directory is not specified, the working directory defaults to `.', the directory from which the server was started. The directory specified should be an absolute path. It is strongly recommended that the directory be writable by the effective user ID of the named process.
key-directory
When performing dynamic update of secure zones, the directory where the public and private DNSSEC key files should be found, if different than the current working directory. (Note that this option has no effect on the paths for files containing non-DNSSEC keys such as rndc.key or session.key.)
managed-keys-directory

Specifies the directory in which to store the files that track managed DNSSEC keys. By default, this is the working directory. The directory must be writable by the effective user ID of the named process.

If named is not configured to use views, then managed keys for the server will be tracked in a single file called managed-keys.loop. Otherwise, managed keys will be tracked in separate files, one file per view; each file name will be the SHA256 hash of the view name, followed by the extension .mkeys.

tkey-domain

The domain appended to the names of all shared keys generated with TKEY. When a client requests a TKEY exchange, it may or may not specify the desired name for the key. If present, the name of the shared key will be client specified part + tkey-domain. Otherwise, the name of the shared key will be ``random hex

digits`` + tkey-domain. In most cases, the

domainname should be the server's domain name, or an otherwise non-existent subdomain like "_tkey.``domainname``".

tkey-dhkey
The Diffie-Hellman key used by the server to generate shared keys with clients using the Diffie-Hellman mode of TKEY. The server must be able to load the public and private keys from files in the working directory. In most cases, the key_name should be the server's host name.
cache-file
This is for testing only. Do not use.
dump-file
The pathname of the file the server dumps the database to when instructed to do so with rndc dumpdb. If not specified, the default is named_dump.db.
pid-file
The pathname of the file the server writes its process ID in. If not specified, the default is /var/run/loop/named.pid. The PID file is used by programs that want to send signals to the running name server. Specifying pid-file none disables the use of a PID file --- no file will be written and any existing one will be removed. Note that none is a keyword, not a filename, and therefore is not enclosed in double quotes.
recursing-file
The pathname of the file the server dumps the queries that are currently recursing when instructed to do so with rndc recursing. If not specified, the default is named.recursing.
statistics-file
The pathname of the file the server appends statistics to when instructed to do so using rndc stats. If not specified, the default is named.stats in the server's current directory. The format of the file is described in section_title.
dnssec-keys-file
The pathname of a file to override the built-in DNSSEC trust anchors provided by named. See the discussion of dnssec-validation for details.
license-file
The pathname of a file to override the Loop subscription license information file path. The default is /etc/loop/loop-license.conf. See the loop-license.conf(5) manpage for more details.
secroots-file
The pathname of the file the server dumps security roots to when instructed to do so with rndc secroots. If not specified, the default is named.secroots.
session-keyfile
The pathname of the file into which to write a TSIG session key generated by named for use by nsupdate -l. If not specified, the default is /var/run/loop/session.key. (See section_title, and in particular the discussion of the update-policy statement's local option for more information about this feature.)
session-keyname
The key name to use for the TSIG session key. If not specified, the default is "local-ddns".
session-keyalg
The algorithm to use for the TSIG session key. Valid values are hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384, hmac-sha512 and hmac-md5. If not specified, the default is hmac-sha256.
port
The UDP/TCP port number the server uses for receiving and sending DNS protocol traffic. The default is 53. This option is mainly intended for server testing; a server using a port other than 53 will not be able to communicate with the global DNS.
dscp
The global Differentiated Services Code Point (DSCP) value to classify outgoing DNS traffic on operating systems that support DSCP. Valid values are 0 through 63. It is not configured by default.
random-device
The source of entropy to be used by the server. Entropy is primarily needed for DNSSEC operations, such as TKEY transactions and dynamic update of signed zones. This options specifies the device (or file) from which to read entropy. If this is a file, operations requiring entropy will fail when the file has been exhausted. If not specified, the default value is /dev/random (or equivalent) when present, and none otherwise. The random-device option takes effect during the initial configuration load at server startup time and is ignored on subsequent reloads.
preferred-glue
If specified, the listed type (A or AAAA) will be emitted before other glue in the additional section of a query response. The default is to prefer A records when responding to queries that arrived via IPv4 and AAAA when responding to queries that arrived via IPv6.
root-delegation-only

Turn on enforcement of delegation-only in TLDs (top level domains) and root zones with an optional exclude list.

DS queries are expected to be made to and be answered by delegation only zones. Such queries and responses are treated as an exception to delegation-only processing and are not converted to NXDOMAIN responses provided a CNAME is not discovered at the query name.

If a delegation only zone server also serves a child zone it is not always possible to determine whether an answer comes from the delegation only zone or the child zone. SOA NS and DNSKEY records are apex only records and a matching response that contains these records or DS is treated as coming from a child zone. RRSIG records are also examined to see if they are signed by a child zone or not. The authority section is also examined to see if there is evidence that the answer is from the child zone. Answers that are determined to be from a child zone are not converted to NXDOMAIN responses. Despite all these checks there is still a possibility of false negatives when a child zone is being served.

Similarly false positives can arise from empty nodes (no records at the name) in the delegation only zone when the query type is not ANY.

Note some TLDs are not delegation only (e.g. "DE", "LV", "US" and "MUSEUM"). This list is not exhaustive.

options {
    root-delegation-only exclude { "de"; "lv"; "us"; "museum"; };
};
disable-algorithms

Disable the specified DNSSEC algorithms at and below the specified name. Multiple disable-algorithms statements are allowed. Only the best match disable-algorithms clause will be used to determine which algorithms are used.

If all supported algorithms are disabled, the zones covered by the disable-algorithms will be treated as insecure.

disable-ds-digests

Disable the specified DS/DLV digest types at and below the specified name. Multiple disable-ds-digests statements are allowed. Only the best match disable-ds-digests clause will be used to determine which digest types are used.

If all supported digest types are disabled, the zones covered by the disable-ds-digests will be treated as insecure.

dnssec-must-be-secure
Specify hierarchies which must be or may not be secure (signed and validated). If yes, then named will only accept answers if they are secure. If no, then normal DNSSEC validation applies allowing for insecure answers to be accepted. The specified domain must be under a trusted-keys or managed-keys statement, or dnssec-validation auto must be active.
dns64

This directive instructs named to return mapped IPv4 addresses to AAAA queries when there are no AAAA records. It is intended to be used in conjunction with a NAT64. Each dns64 defines one DNS64 prefix. Multiple DNS64 prefixes can be defined.

Compatible IPv6 prefixes have lengths of 32, 40, 48, 56, 64 and 96 as per RFC 6052.

Additionally a reverse IP6.ARPA zone will be created for the prefix to provide a mapping from the IP6.ARPA names to the corresponding IN-ADDR.ARPA names using synthesized CNAMEs. dns64-server and dns64-contact can be used to specify the name of the server and contact for the zones. These are settable at the view / options level. These are not settable on a per-prefix basis.

Each dns64 supports an optional clients ACL that determines which clients are affected by this directive. If not defined, it defaults to any;.

Each dns64 supports an optional mapped ACL that selects which IPv4 addresses are to be mapped in the corresponding A RRset. If not defined it defaults to any;.

Normally, DNS64 won't apply to a domain name that owns one or more AAAA records; these records will simply be returned. The optional exclude ACL allows specification of a list of IPv6 addresses that will be ignored if they appear in a domain name's AAAA records, and DNS64 will be applied to any A records the domain name owns. If not defined, exclude defaults to ::ffff:0.0.0.0/96.

A optional suffix can also be defined to set the bits trailing the mapped IPv4 address bits. By default these bits are set to ::. The bits matching the prefix and mapped IPv4 address must be zero.

If recursive-only is set to yes the DNS64 synthesis will only happen for recursive queries. The default is no.

If break-dnssec is set to yes the DNS64 synthesis will happen even if the result, if validated, would cause a DNSSEC validation failure. If this option is set to no (the default), the DO is set on the incoming query, and there are RRSIGs on the applicable records, then synthesis will not happen.

acl rfc1918 { 10/8; 192.168/16; 172.16/12; };

dns64 64:FF9B::/96 {
    clients { any; };
    mapped { !rfc1918; any; };
    exclude { 64:FF9B::/96; ::ffff:0000:0000/96; };
    suffix ::;
};
dnssec-loadkeys-interval
When a zone is configured with auto-dnssec maintain its key repository must be checked periodically to see if any new keys have been added or any existing keys' timing metadata has been updated (see ??? and ???). The dnssec-loadkeys-interval option sets the frequency of automatic repository checks, in minutes. The default is 60 (1 hour), the minimum is 1 (1 minute), and the maximum is 1440 (24 hours); any higher value is silently reduced.
dnssec-update-mode

If this option is set to its default value of maintain in a zone of type master which is DNSSEC-signed and configured to allow dynamic updates (see section_title), and if named has access to the private signing key(s) for the zone, then named will automatically sign all new or changed records and maintain signatures for the zone by regenerating RRSIG records whenever they approach their expiration date.

If the option is changed to no-resign, then named will sign all new or changed records, but scheduled maintenance of signatures is disabled.

With either of these settings, named will reject updates to a DNSSEC-signed zone when the signing keys are inactive or unavailable to named. (A planned third option, external, will disable all automatic signing and allow DNSSEC data to be submitted into a zone via dynamic update; this is not yet implemented.)

max-zone-ttl

Specifies a maximum permissible TTL value. When loading a zone file, any record encountered with a TTL higher than max-zone-ttl will cause the zone to be rejected.

This is useful in DNSSEC-signed zones because when rolling to a new DNSKEY, the old key needs to remain available until RRSIG records have expired from caches. Themax-zone-ttl option guarantees that the largest TTL in the zone will be no higher the set value.

The default value is unlimited. A max-zone-ttl of zero is treated as unlimited.

serial-update-method

Zones configured for dynamic DNS may use this option to set the update method that will be used for the zone serial number in the SOA record.

With the default setting of serial-update-method increment;, the SOA serial number will be incremented by one each time the zone is updated.

When set to serial-update-method unixtime;, the SOA serial number will be set to the number of seconds since the UNIX epoch, unless the serial number is already greater than or equal to that value, in which case it is simply incremented by one.

zone-statistics

If full, the server will collect statistical data on all zones (unless specifically turned off on a per-zone basis by specifying zone-statistics terse or zone-statistics none in the zone statement). The default is terse, providing minimal statistics on zones (including name and current serial number, but not query type counters).

These statistics may be accessed via the statistics-channel or using rndc stats, which will dump them to the file listed in the statistics-file. See also section_title.

The zone-statistics option can also accept yes or no; yes has the same meaning as full. no has the same meaning as none.

6.2.14.1. Boolean Options

automatic-interface-scan

If yes and supported by the OS, automatically rescan network interfaces when the interface addresses are added or removed. The default is yes.

Currently the OS needs to support routing sockets for automatic-interface-scan to be supported.

allow-new-zones
If yes, then zones can be added at runtime via rndc addzone or deleted via rndc delzone. The default is no.
auth-nxdomain
If yes, then the AA bit is always set on NXDOMAIN responses, even if the server is not actually authoritative. The default is no. If you are using very old DNS software, you may need to set it to yes.
dialup

If yes, then the server treats all zones as if they are doing zone transfers across a dial-on-demand dialup link, which can be brought up by traffic originating from this server. This has different effects according to zone type and concentrates the zone maintenance so that it all happens in a short interval, once every heartbeat-interval and hopefully during the one call. It also suppresses some of the normal zone maintenance traffic. The default is no.

The dialup option may also be specified in the view and zone statements, in which case it overrides the global dialup option.

If the zone is a master zone, then the server will send out a NOTIFY request to all the slaves (default). This should trigger the zone serial number check in the slave (providing it supports NOTIFY) allowing the slave to verify the zone while the connection is active. The set of servers to which NOTIFY is sent can be controlled by notify and also-notify.

If the zone is a slave or stub zone, then the server will suppress the regular "zone up to date" (refresh) queries and only perform them when the heartbeat-interval expires in addition to sending NOTIFY requests.

Finer control can be achieved by using notify which only sends NOTIFY messages, notify-passive which sends NOTIFY messages and suppresses the normal refresh queries, refresh which suppresses normal refresh processing and sends refresh queries when the heartbeat-interval expires, and passive which just disables normal refresh processing.

dialup mode normal refresh heart-beat refresh heart-beat notify
no (default) yes no no
yes no yes yes
notify yes no yes
refresh no yes no
passive no no no
notify-passiv e no no yes

Note that normal NOTIFY processing is not affected by dialup.

flush-zones-on-shutdown
When the nameserver exits due receiving SIGTERM, flush or do not flush any pending zone writes. The default is flush-zones-on-shutdown no.
minimal-responses
If yes, then when generating responses the server will only add records to the authority and additional data sections when they are required (e.g. delegations, negative responses). This may improve the performance of the server. The default is no.
notify

If yes (the default), DNS NOTIFY messages are sent when a zone the server is authoritative for changes, see section_title. The messages are sent to the servers listed in the zone's NS records (except the master server identified in the SOA MNAME field), and to any servers listed in the also-notify option.

If master-only, notifies are only sent for master zones. If explicit, notifies are sent only to servers explicitly listed using also-notify. If no, no notifies are sent.

The notify option may also be specified in the zone statement, in which case it overrides the options notify statement. It would only be necessary to turn off this option if it caused slaves to crash.

notify-to-soa
If yes do not check the nameservers in the NS RRset against the SOA MNAME. Normally a NOTIFY message is not sent to the SOA MNAME (SOA ORIGIN) as it is supposed to contain the name of the ultimate master. Sometimes, however, a slave is listed as the SOA MNAME in hidden master configurations and in that case you would want the ultimate master to still send NOTIFY messages to all the nameservers listed in the NS RRset.
recursion
If yes, and a DNS query requests recursion, then the server will attempt to do all the work required to answer the query. If recursion is off and the server does not already know the answer, it will return a referral response. The default is yes. Note that setting recursion no does not prevent clients from getting data from the server's cache; it only prevents new data from being cached as an effect of client queries. Caching may still occur as an effect the server's internal operation, such as NOTIFY address lookups.
request-nsid
If yes, then an empty EDNS(0) NSID (Name Server Identifier) option is sent with all queries to authoritative name servers during iterative resolution. If the authoritative server returns an NSID option in its response, then its contents are logged in the resolver category at level info. The default is no.
request-sit
If yes, then a SIT (Source Identity Token) EDNS option is sent along with the query. If the resolver has previously talked to the server, the SIT returned in the previous transaction is sent. This is used by the server to determine whether the resolver has talked to it before. A resolver sending the correct SIT is assumed not to be an off-path attacker sending a spoofed-source query; the query is therefore unlikely to be part of a reflection/amplification attack, so resolvers sending a correct SIT option are not subject to response rate limiting (RRL). Resolvers which do not send a correct SIT option may be limited to receiving smaller responses via the nosit-udp-size option.
nosit-udp-size
Sets the maximum size of UDP responses that will be sent to queries without a valid source identity token. A value below 128 will be silently raised to 128. The default value is 4096, but the max-udp-size option may further limit the response size.
sit-secret
If set, this is a shared secret used for generating and verifying Source Identity Token EDNS options within a anycast cluster. If not set the system will generate a random secret at startup. The shared secret is encoded as a hex string and needs to be 128 bits for AES128, 160 bits for SHA1 and 256 bits for SHA256.
provide-ixfr
See the description of provide-ixfr in section_title.
request-ixfr
See the description of request-ixfr in section_title.
match-mapped-addresses

If yes, then an IPv4-mapped IPv6 address will match any address match list entries that match the corresponding IPv4 address.

This option was introduced to work around a kernel quirk in some operating systems that causes IPv4 TCP connections, such as zone transfers, to be accepted on an IPv6 socket using mapped addresses. This caused address match lists designed for IPv4 to fail to match. However, named now solves this problem internally. The use of this option is discouraged.

ixfr-from-differences

When yes and the server loads a new version of a master zone from its zone file or receives a new version of a slave file via zone transfer, it will compare the new version to the previous one and calculate a set of differences. The differences are then logged in the zone's journal file such that the changes can be transmitted to downstream slaves as an incremental zone transfer.

By allowing incremental zone transfers to be used for non-dynamic zones, this option saves bandwidth at the expense of increased CPU and memory consumption at the master. In particular, if the new version of a zone is completely different from the previous one, the set of differences will be of a size comparable to the combined size of the old and new zone version, and the server will need to temporarily allocate memory to hold this complete difference set.

ixfr-from-differences also accepts master and slave at the view and options levels which causes ixfr-from-differences to be enabled for all master or slave zones respectively. It is off by default.

multi-master
This should be set when you have multiple masters for a zone and the addresses refer to different machines. If yes, named will not log when the serial number on the master is less than what named currently has. The default is no.
auto-dnssec

Zones configured for dynamic DNS may use this option to allow varying levels of automatic DNSSEC key management. There are three possible settings:

auto-dnssec allow; permits keys to be updated and the zone fully re-signed whenever the user issues the command ``rndc sign

zonename``.

auto-dnssec maintain; includes the above, but also automatically adjusts the zone's DNSSEC keys on schedule, according to the keys' timing metadata (see ??? and ???). The command ``rndc sign

zonename`` causes named to load keys from the key

repository and sign the zone with all keys that are active. ``rndc loadkeys

zonename`` causes named to load keys from the key

repository and schedule key maintenance events to occur in the future, but it does not sign the full zone immediately. Note: once keys have been loaded for a zone the first time, the repository will be searched for changes periodically, regardless of whether rndc loadkeys is used. The recheck interval is defined by dnssec-loadkeys-interval.)

The default setting is auto-dnssec off.

dnssec-enable
This indicates whether DNSSEC-related resource records are to be returned by named. If set to no, named will not return DNSSEC-related resource records unless specifically queried for. The default is yes.
dnssec-validation

Enable DNSSEC validation in named. Note dnssec-enable also needs to be set to yes to be effective. If set to no, DNSSEC validation is disabled.

If set to auto, DNSSEC validation is enabled, and a default trust anchor for the DNS root zone is used. If set to yes, DNSSEC validation is enabled, but a trust anchor must be manually configured using a trusted-keys or managed-keys statement. The default is yes.

The default root DNSSEC trust anchors are built-in named will load them at startup if dnssec-validation is set to auto. The built-in trust anchors are current as of the software release date. If the root DNSSEC trust anchors expire, updated trust anchors may be provided using the dnssec-keys-file config option. Keeping the Loop software up-to-date is recommended instead, whereby the latest trust anchors will always be built-in. The use of dnssec-keys-file is discouraged.

Note

named only loads the root key from dnssec-keys-file. The file cannot be used to store keys for other zones. The root key in dnssec-keys-file is ignored if dnssec-validation auto is not in use.

Whenever the resolver sends out queries to an EDNS-compliant server, it always sets the DO bit indicating it can support DNSSEC responses even if dnssec-validation is off.

querylog
Specify whether query logging should be started when named starts. If querylog is not specified, then the query logging is determined by the presence of the logging category queries.
check-names

This option is used to restrict the character set and syntax of certain domain names in master files and/or DNS responses received from the network. The default varies according to usage area. For master zones the default is fail. For slave zones the default is warn. For answers received from the network (response) the default is ignore.

The rules for legal hostnames and mail domains are derived from RFC 952 and RFC 821 as modified by RFC 1123.

check-names applies to the owner names of A, AAAA and MX records. It also applies to the domain names in the RDATA of NS, SOA, MX, and SRV records. It also applies to the RDATA of PTR records where the owner name indicated that it is a reverse lookup of a hostname (the owner name ends in IN-ADDR.ARPA, IP6.ARPA, or IP6.INT).

check-dup-records
Check master zones for records that are treated as different by DNSSEC but are semantically equal in plain DNS. The default is to warn. Other possible values are fail and ignore.
check-mx
Check whether the MX record appears to refer to a IP address. The default is to warn. Other possible values are fail and ignore.
check-wildcard
This option is used to check for non-terminal wildcards. The use of non-terminal wildcards is almost always as a result of a failure to understand the wildcard matching algorithm (RFC 1034). This option affects master zones. The default (yes) is to check for non-terminal wildcards and issue a warning.
check-integrity

Perform post load zone integrity checks on master zones. This checks that MX and SRV records refer to address (A or AAAA) records and that glue address records exist for delegated zones. For MX and SRV records only in-zone hostnames are checked (for out-of-zone hostnames use named-checkzone). For NS records only names below top of zone are checked (for out-of-zone names and glue consistency checks use named-checkzone). The default is yes.

The use of the SPF record for publishing Sender Policy Framework is deprecated as the migration from using TXT records to SPF records was abandoned. Enabling this option also checks that a TXT Sender Policy Framework record exists (starts with "v=spf1") if there is an SPF record. Warnings are emitted if the TXT record does not exist and can be suppressed with check-spf.

check-mx-cname
If check-integrity is set then fail, warn or ignore MX records that refer to CNAMES. The default is to warn.
check-srv-cname
If check-integrity is set then fail, warn or ignore SRV records that refer to CNAMES. The default is to warn.
check-sibling
When performing integrity checks, also check that sibling glue exists. The default is yes.
check-spf
If check-integrity is set then check that there is a TXT Sender Policy Framework record present (starts with "v=spf1") if there is an SPF record present. The default is warn.
zero-no-soa-ttl
When returning authoritative negative responses to SOA queries set the TTL of the SOA record returned in the authority section to zero. The default is yes.
zero-no-soa-ttl-cache
When caching a negative response to a SOA query set the TTL to zero. The default is no.
update-check-ksk

When set to the default value of yes, check the KSK bit in each key to determine how the key should be used when generating RRSIGs for a secure zone.

Ordinarily, zone-signing keys (that is, keys without the KSK bit set) are used to sign the entire zone, while key-signing keys (keys with the KSK bit set) are only used to sign the DNSKEY RRset at the zone apex. However, if this option is set to no, then the KSK bit is ignored; KSKs are treated as if they were ZSKs and are used to sign the entire zone. This is similar to the dnssec-signzone -z command line option.

When this option is set to yes, there must be at least two active keys for every algorithm represented in the DNSKEY RRset: at least one KSK and one ZSK per algorithm. If there is any algorithm for which this requirement is not met, this option will be ignored for that algorithm.

dnssec-dnskey-kskonly

When this option and update-check-ksk are both set to yes, only key-signing keys (that is, keys with the KSK bit set) will be used to sign the DNSKEY RRset at the zone apex. Zone-signing keys (keys without the KSK bit set) will be used to sign the remainder of the zone, but not the DNSKEY RRset. This is similar to the dnssec-signzone -x command line option.

The default is no. If update-check-ksk is set to no, this option is ignored.

try-tcp-refresh
Try to refresh the zone using TCP if UDP queries fail. The default is yes.
dnssec-secure-to-insecure

Allow a dynamic zone to transition from secure to insecure (i.e., signed to unsigned) by deleting all of the DNSKEY records. The default is no. If set to yes, and if the DNSKEY RRset at the zone apex is deleted, all RRSIG and NSEC records will be removed from the zone as well.

If the zone uses NSEC3, then it is also necessary to delete the NSEC3PARAM RRset from the zone apex; this will cause the removal of all corresponding NSEC3 records. (It is expected that this requirement will be eliminated in a future release.)

Note that if a zone has been configured with auto-dnssec maintain and the private keys remain accessible in the key repository, then the zone will be automatically signed again the next time named is started.

6.2.14.2. Forwarding

The forwarding facility can be used to create a large site-wide cache on a few servers, reducing traffic over links to external name servers. It can also be used to allow queries by servers that do not have direct access to the Internet, but wish to look up exterior names anyway. Forwarding occurs only on those queries for which the server is not authoritative and does not have the answer in its cache.

forward
This option is only meaningful if the forwarders list is not empty. A value of first, the default, causes the server to query the forwarders first — and if that doesn't answer the question, the server will then look for the answer itself. If only is specified, the server will only query the forwarders.
forwarders
Specifies the IP addresses to be used for forwarding. The default is the empty list (no forwarding).

Forwarding can also be configured on a per-domain basis, allowing for the global forwarding options to be overridden in a variety of ways. You can set particular domains to use different forwarders, or have a different forward only/first behavior, or not forward at all, see section_title.

6.2.14.3. Dual-stack Servers

Dual-stack servers are used as servers of last resort to work around problems in reachability due the lack of support for either IPv4 or IPv6 on the host machine.

dual-stack-servers
Specifies host names or addresses of machines with access to both IPv4 and IPv6 transports. If a hostname is used, the server must be able to resolve the name using only the transport it has. If the machine is dual stacked, then the dual-stack-servers have no effect unless access to a transport has been disabled on the command line (e.g. named -4).

6.2.14.4. Access Control

Access to the server can be restricted based on the IP address of the requesting system. See section_title for details on how to specify IP address lists.

allow-notify
Specifies which hosts are allowed to notify this server, a slave, of zone changes in addition to the zone masters. allow-notify may also be specified in the zone statement, in which case it overrides the options allow-notify statement. It is only meaningful for a slave zone. If not specified, the default is to process notify messages only from a zone's master.
allow-query

Specifies which hosts are allowed to ask ordinary DNS questions. allow-query may also be specified in the zone statement, in which case it overrides the options allow-query statement. If not specified, the default is to allow queries from all hosts.

Note

allow-query-cache is now used to specify access to the cache.

allow-query-on

Specifies which local addresses can accept ordinary DNS questions. This makes it possible, for instance, to allow queries on internal-facing interfaces but disallow them on external-facing ones, without necessarily knowing the internal network's addresses.

Note that allow-query-on is only checked for queries that are permitted by allow-query. A query must be allowed by both ACLs, or it will be refused.

allow-query-on may also be specified in the zone statement, in which case it overrides the options allow-query-on statement.

If not specified, the default is to allow queries on all addresses.

Note

allow-query-cache is used to specify access to the cache.

allow-query-cache
Specifies which hosts are allowed to get answers from the cache. If allow-query-cache is not set then allow-recursion is used if set, otherwise allow-query is used if set unless recursion no; is set in which case none; is used, otherwise the default (localnets; localhost;) is used.
allow-query-cache-on
Specifies which local addresses can give answers from the cache. If not specified, the default is to allow cache queries on any address, localnets and localhost.
allow-recursion
Specifies which hosts are allowed to make recursive queries through this server. If allow-recursion is not set then allow-query-cache is used if set, otherwise allow-query is used if set, otherwise the default (localnets; localhost;) is used.
allow-recursion-on
Specifies which local addresses can accept recursive queries. If not specified, the default is to allow recursive queries on all addresses.
allow-update
Specifies which hosts are allowed to submit Dynamic DNS updates for master zones. The default is to deny updates from all hosts. Note that allowing updates based on the requestor's IP address is insecure; see section_title for details.
allow-update-forwarding

Specifies which hosts are allowed to submit Dynamic DNS updates to slave zones to be forwarded to the master. The default is { none; }, which means that no update forwarding will be performed. To enable update forwarding, specify allow-update-forwarding { any; };. Specifying values other than { none; } or { any; } is usually counterproductive, since the responsibility for update access control should rest with the master server, not the slaves.

Note that enabling the update forwarding feature on a slave server may expose master servers relying on insecure IP address based access control to attacks; see section_title for more details.

allow-v6-synthesis
This option was introduced for the smooth transition from AAAA to A6 and from "nibble labels" to binary labels. However, since both A6 and binary labels were then deprecated, this option was also deprecated. It is now ignored with some warning messages.
allow-transfer
Specifies which hosts are allowed to receive zone transfers from the server. allow-transfer may also be specified in the zone statement, in which case it overrides the options allow-transfer statement. If not specified, the default is to allow transfers to all hosts.
blackhole
Specifies a list of addresses that the server will not accept queries from or use to resolve a query. Queries from these addresses will not be responded to. The default is none.
no-case-compress

Specifies a list of addresses which require responses to use case-insensitive compression. This ACL can be used when named needs to work with clients that do not comply with the requirement in RFC 1034 to use case-insensitive name comparisons when checking for matching domain names.

If left undefined, the ACL defaults to none: case-insensitive compression will be used for all clients. If the ACL is defined and matches a client, then case will be ignored when compressing domain names in DNS responses sent to that client.

This can result in slightly smaller responses: if a response contains the names "example.com" and "example.COM", case-insensitive compression would treat the second one as a duplicate. It also ensures that the case of the query name exactly matches the case of the owner names of returned records, rather than matching the case of the records entered in the zone file. This allows responses to exactly match the query, which is required by some clients due to incorrect use of case-sensitive comparisons.

Case-insensitive compression is always used in AXFR and IXFR responses, regardless of whether the client matches this ACL.

There are circumstances in which named will not preserve the case of owner names of records: if a zone file defines records of different types with the same name, but the capitalization of the name is different (e.g., "www.example.com/A" and "WWW.EXAMPLE.COM/AAAA"), then all responses for that name will use the first version of the name that was used in the zone file. This limitation may be addressed in a future release. However, domain names specified in the rdata of resource records (i.e., records of type NS, MX, CNAME, etc) will always have their case preserved unless the client matches this ACL.

resolver-query-timeout
The amount of time the resolver will spend attempting to resolve a recursive query before failing. The default and minimum is 10 and the maximum is 30. Setting it to 0 will result in the default being used.

6.2.14.5. Interfaces

The interfaces and ports that the server will answer queries from may be specified using the listen-on option. listen-on takes an optional port and an address_match_list of IPv4 addresses. (IPv6 addresses are ignored, with a logged warning.) The server will listen on all interfaces allowed by the address match list. If a port is not specified, port 53 will be used.

Multiple listen-on statements are allowed. For example,

listen-on { 5.6.7.8; };
listen-on port 1234 { !1.2.3.4; 1.2/16; };

will enable the name server on port 53 for the IP address 5.6.7.8, and on port 1234 of an address on the machine in net 1.2 that is not 1.2.3.4.

If no listen-on is specified, the server will listen on port 53 on all IPv4 interfaces.

The listen-on-v6 option is used to specify the interfaces and the ports on which the server will listen for incoming queries sent using IPv6. If not specified, the server will listen on port 53 on all IPv6 interfaces.

When

{ any; }

is specified as the address_match_list for the listen-on-v6 option, the server does not bind a separate socket to each IPv6 interface address as it does for IPv4 if the operating system has enough API support for IPv6 (specifically if it conforms to RFC 3493 and RFC 3542). Instead, it listens on the IPv6 wildcard address. If the system only has incomplete API support for IPv6, however, the behavior is the same as that for IPv4.

A list of particular IPv6 addresses can also be specified, in which case the server listens on a separate socket for each specified address, regardless of whether the desired API is supported by the system. IPv4 addresses specified in listen-on-v6 will be ignored, with a logged warning.

Multiple listen-on-v6 options can be used. For example,

listen-on-v6 { any; };
listen-on-v6 port 1234 { !2001:db8::/32; any; };

will enable the name server on port 53 for any IPv6 addresses (with a single wildcard socket), and on port 1234 of IPv6 addresses that is not in the prefix 2001:db8::/32 (with separate sockets for each matched address.)

To make the server not listen on any IPv6 address, use

listen-on-v6 { none; };

6.2.14.6. Query Address

If the server doesn't know the answer to a question, it will query other name servers. query-source specifies the address and port used for such queries. For queries sent over IPv6, there is a separate query-source-v6 option. If address is * (asterisk) or is omitted, a wildcard IP address (INADDR_ANY) will be used.

If port is * or is omitted, a random port number from a pre-configured range is picked up and will be used for each query. The port range(s) is that specified in the use-v4-udp-ports (for IPv4) and use-v6-udp-ports (for IPv6) options, excluding the ranges specified in the avoid-v4-udp-ports and avoid-v6-udp-ports options, respectively.

The defaults of the query-source and query-source-v6 options are:

query-source address * port *;
query-source-v6 address * port *;

If use-v4-udp-ports or use-v6-udp-ports is unspecified, named will check if the operating system provides a programming interface to retrieve the system's default range for ephemeral ports. If such an interface is available, named will use the corresponding system default range; otherwise, it will use its own defaults:

use-v4-udp-ports { range 1024 65535; };
use-v6-udp-ports { range 1024 65535; };

Note: make sure the ranges be sufficiently large for security. A desirable size depends on various parameters, but we generally recommend it contain at least 16384 ports (14 bits of entropy). Note also that the system's default range when used may be too small for this purpose, and that the range may even be changed while named is running; the new range will automatically be applied when named is reloaded. It is encouraged to configure use-v4-udp-ports and use-v6-udp-ports explicitly so that the ranges are sufficiently large and are reasonably independent from the ranges used by other applications.

Note: the operational configuration where named runs may prohibit the use of some ports. For example, UNIX systems will not allow named running without a root privilege to use ports less than 1024. If such ports are included in the specified (or detected) set of query ports, the corresponding query attempts will fail, resulting in resolution failures or delay. It is therefore important to configure the set of ports that can be safely used in the expected operational environment.

The defaults of the avoid-v4-udp-ports and avoid-v6-udp-ports options are:

avoid-v4-udp-ports {};
avoid-v6-udp-ports {};

Note: It is generally strongly discouraged to specify a particular port for the query-source or query-source-v6 options; it implicitly disables the use of randomized port numbers.

Note

The address specified in the query-source option is used for both UDP and TCP queries, but the port applies only to UDP queries. TCP queries always use a random unprivileged port.

Note

See also transfer-source and notify-source.

6.2.14.7. Zone Transfers

Loop has mechanisms in place to facilitate zone transfers and set limits on the amount of load that transfers place on the system. The following options apply to zone transfers.

also-notify

Defines a global list of IP addresses of name servers that are also sent NOTIFY messages whenever a fresh copy of the zone is loaded, in addition to the servers listed in the zone's NS records. This helps to ensure that copies of the zones will quickly converge on stealth servers. Optionally, a port may be specified with each also-notify address to send the notify messages to a port other than the default of 53. An optional TSIG key can also be specified with each address to cause the notify messages to be signed; this can be useful when sending notifies to multiple views. In place of explicit addresses, one or more named masters lists can be used.

If an also-notify list is given in a zone statement, it will override the options also-notify statement. When a zone notify statement is set to no, the IP addresses in the global also-notify list will not be sent NOTIFY messages for that zone. The default is the empty list (no global notification list).

max-transfer-time-in
Inbound zone transfers running longer than this many minutes will be terminated. The default is 120 minutes (2 hours). The maximum value is 28 days (40320 minutes).
max-transfer-idle-in
Inbound zone transfers making no progress in this many minutes will be terminated. The default is 60 minutes (1 hour). The maximum value is 28 days (40320 minutes).
max-transfer-time-out
Outbound zone transfers running longer than this many minutes will be terminated. The default is 120 minutes (2 hours). The maximum value is 28 days (40320 minutes).
max-transfer-idle-out
Outbound zone transfers making no progress in this many minutes will be terminated. The default is 60 minutes (1 hour). The maximum value is 28 days (40320 minutes).
serial-query-rate

Slave servers will periodically query master servers to find out if zone serial numbers have changed. Each such query uses a minute amount of the slave server's network bandwidth. To limit the amount of bandwidth used, Loop limits the rate at which queries are sent. The value of the serial-query-rate option, an integer, is the maximum number of queries sent per second. The default is 20 per second. The lowest possible rate is one per second; when set to zero, it will be silently raised to one.

In addition to controlling the rate SOA refresh queries are issued at, serial-query-rate also controls the rate at which NOTIFY messages are sent from both master and slave zones.

transfers-in
The maximum number of inbound zone transfers that can be running concurrently. The default value is 10. Increasing transfers-in may speed up the convergence of slave zones, but it also may increase the load on the local system.
transfers-out
The maximum number of outbound zone transfers that can be running concurrently. Zone transfer requests in excess of the limit will be refused. The default value is 10.
transfers-per-ns
The maximum number of inbound zone transfers that can be concurrently transferring from a given remote name server. The default value is 2. Increasing transfers-per-ns may speed up the convergence of slave zones, but it also may increase the load on the remote name server. transfers-per-ns may be overridden on a per-server basis by using the transfers phrase of the server statement.
transfer-source
transfer-source determines which local address will be bound to IPv4 TCP connections used to fetch zones transferred inbound by the server. It also determines the source IPv4 address, and optionally the UDP port, used for the refresh queries and forwarded dynamic updates. If not set, it defaults to a system controlled value which will usually be the address of the interface "closest to" the remote end. This address must appear in the remote end's allow-transfer option for the zone being transferred, if one is specified. This statement sets the transfer-source for all zones, but can be overridden on a per-view or per-zone basis by including a transfer-source statement within the view or zone block in the configuration file.
transfer-source-v6
The same as transfer-source, except zone transfers are performed using IPv6.
alt-transfer-source

An alternate transfer source if the one listed in transfer-source fails and use-alt-transfer-source is set.

Note

If you do not wish the alternate transfer source to be used, you should set use-alt-transfer-source appropriately and you should not depend upon getting an answer back to the first refresh query.

alt-transfer-source-v6
An alternate transfer source if the one listed in transfer-source-v6 fails and use-alt-transfer-source is set.
use-alt-transfer-source
Use the alternate transfer sources or not. If views are specified this defaults to no otherwise it defaults to yes.
notify-source
notify-source determines which local source address, and optionally UDP port, will be used to send NOTIFY messages. This address must appear in the slave server's masters zone clause or in an allow-notify clause. This statement sets the notify-source for all zones, but can be overridden on a per-zone or per-view basis by including a notify-source statement within the zone or view block in the configuration file.
notify-source-v6
Like notify-source, but applies to notify messages sent to IPv6 addresses.

6.2.14.8. UDP Port Lists

use-v4-udp-ports, avoid-v4-udp-ports, use-v6-udp-ports, and avoid-v6-udp-ports specify a list of IPv4 and IPv6 UDP ports that will be used or not used as source ports for UDP messages. See section_title about how the available ports are determined. For example, with the following configuration

use-v6-udp-ports { range 32768 65535; };
avoid-v6-udp-ports { 40000; range 50000 60000; };

UDP ports of IPv6 messages sent from named will be in one of the following ranges: 32768 to 39999, 40001 to 49999, and 60001 to 65535.

avoid-v4-udp-ports and avoid-v6-udp-ports can be used to prevent named from choosing as its random source port a port that is blocked by your firewall or a port that is used by other applications; if a query went out with a source port blocked by a firewall, the answer would not get by the firewall and the name server would have to query again. Note: the desired range can also be represented only with use-v4-udp-ports and use-v6-udp-ports, and the avoid- options are redundant in that sense; they are provided for backward compatibility and to possibly simplify the port specification.

6.2.14.9. Operating System Resource Limits

The server's usage of many system resources can be limited. Scaled values are allowed when specifying resource limits. For example, 1G can be used instead of 1073741824 to specify a limit of one gigabyte. unlimited requests unlimited use, or the maximum available amount. default uses the limit that was in force when the server was started. See the description of size_spec in section_title.

The following options set operating system resource limits for the name server process. Some operating systems don't support some or any of the limits. On such systems, a warning will be issued if the unsupported limit is used.

coresize
The maximum size of a core dump. The default is default.
datasize
The maximum amount of data memory the server may use. The default is default. This is a hard limit on server memory usage. If the server attempts to allocate memory in excess of this limit, the allocation will fail, which may in turn leave the server unable to perform DNS service. Therefore, this option is rarely useful as a way of limiting the amount of memory used by the server, but it can be used to raise an operating system data size limit that is too small by default. If you wish to limit the amount of memory used by the server, use the max-cache-size and recursive-clients options instead.
files
The maximum number of files the server may have open concurrently. The default is unlimited.
stacksize
The maximum amount of stack memory the server may use. The default is default.

6.2.14.10. Server Resource Limits

The following options set limits on the server's resource consumption that are enforced internally by the server rather than the operating system.

max-journal-size
Sets a maximum size for each journal file (see section_title). When the journal file approaches the specified size, some of the oldest transactions in the journal will be automatically removed. The largest permitted value is 2 gigabytes. The default is unlimited, which also means 2 gigabytes. This may also be set on a per-zone basis.
max-records
The maximum number of records permitted in a zone. The default is zero which means unlimited.
recursive-clients

The maximum number ("hard quota") of simultaneous recursive lookups the server will perform on behalf of clients. The default is 1000. Because each recursing client uses a fair bit of memory (on the order of 20 kilobytes), the value of the recursive-clients option may have to be decreased on hosts with limited memory.

recursive-clients defines a "hard quota" limit for pending recursive clients: when more clients than this are pending, new incoming requests will not be accepted, and for each incoming request a previous pending request will also be dropped.

A "soft quota" is also set. When this lower quota is exceeded, incoming requests are accepted, but for each one, a pending request will be dropped. If recursive-clients is greater than 1000, the soft quota is set to recursive-clients minus 100; otherwise it is set to 90% of recursive-clients.

tcp-clients
The maximum number of simultaneous client TCP connections that the server will accept. The default is 100.
clients-per-query; max-clients-per-query

These set the initial value (minimum) and maximum number of recursive simultaneous clients for any given query (<qname,qtype,qclass>) that the server will accept before dropping additional clients. named will attempt to self tune this value and changes will be logged. The default values are 10 and 100.

This value should reflect how many queries come in for a given name in the time it takes to resolve that name. If the number of queries exceed this value, named will assume that it is dealing with a non-responsive zone and will drop additional queries. If it gets a response after dropping queries, it will raise the estimate. The estimate will then be lowered in 20 minutes if it has remained unchanged.

If clients-per-query is set to zero, then there is no limit on the number of clients per query and no queries will be dropped.

If max-clients-per-query is set to zero, then there is no upper bound other than imposed by recursive-clients.

fetches-per-zone

The maximum number of simultaneous iterative queries to any one domain that the server will permit before blocking new queries for data in or beneath that zone. This value should reflect how many fetches would normally be sent to any one zone in the time it would take to resolve them. It should be smaller than recursive-clients.

When many clients simultaneously query for the same name and type, the clients will all be attached to the same fetch, up to the max-clients-per-query limit, and only one iterative query will be sent. However, when clients are simultaneously querying for different names or types, multiple queries will be sent and max-clients-per-query is not effective as a limit.

Optionally, this value may be followed by the keyword drop or fail, indicating whether queries which exceed the fetch quota for a zone will be dropped with no response, or answered with SERVFAIL. The default is drop.

If fetches-per-zone is set to zero, then there is no limit on the number of fetches per query and no queries will be dropped. The default is zero.

The current list of active fetches can be dumped by running rndc recursing. The list includes the number of active fetches for each domain and the number of queries that have been passed or dropped as a result of the fetches-per-zone limit. (Note: these counters are not cumulative over time; whenever the number of active fetches for a domain drops to zero, the counter for that domain is deleted, and the next time a fetch is sent to that domain, it is recreated with the counters set to zero.)

fetches-per-server

The maximum number of simultaneous iterative queries that the server will allow to be sent to a single upstream name server before blocking additional queries. This value should reflect how many fetches would normally be sent to any one server in the time it would take to resolve them. It should be smaller than recursive-clients.

Optionally, this value may be followed by the keyword drop or fail, indicating whether queries will be dropped with no response, or answered with SERVFAIL, when all of the servers authoritative for a zone are found to have exceeded the per-server quota. The default is fail.

If fetches-per-server is set to zero, then there is no limit on the number of fetches per query and no queries will be dropped. The default is zero.

The fetches-per-server quota is dynamically adjusted in response to detected congestion. As queries are sent to a server and are either answered or time out, an exponentially weighted moving average is calculated of the ratio of timeouts to responses. If the current average timeout ratio rises above a "high" threshold, then fetches-per-server is reduced for that server. If the timeout ratio drops below a "low" threshold, then fetches-per-server is increased. The fetch-quota-params options can be used to adjust the parameters for this calculation.

fetch-quota-params

Sets the parameters to use for dynamic resizing of the fetches-per-server quota in response to detected congestion.

The first argument is an integer value indicating how frequently to recalculate the moving average of the ratio of timeouts to responses for each server. The default is 100, meaning we recalculate the average ratio after every 100 queries have either been answered or timed out.

The remaining three arguments represent the "low" threshold (defaulting to a timeout ratio of 0.1), the "high" threshold (defaulting to a timeout ratio of 0.3), and the discount rate for the moving average (defaulting to 0.7). A higher discount rate causes recent events to weigh more heavily when calculating the moving average; a lower discount rate causes past events to weigh more heavily, smoothing out short-term blips in the timeout ratio. These arguments are all fixed-point numbers with precision of 1/100: at most two places after the decimal point are significant.

reserved-sockets
The number of file descriptors reserved for TCP, stdio, etc. This needs to be big enough to cover the number of interfaces named listens on, tcp-clients as well as to provide room for outgoing TCP queries and incoming zone transfers. The default is 512. The minimum value is 128 and the maximum value is 128 less than maxsockets (-S). This option may be removed in the future.
max-cache-size
The maximum amount of memory to use for the server's cache, in bytes. When the amount of data in the cache reaches this limit, the server will cause records to expire prematurely based on an LRU based strategy so that the limit is not exceeded. The keyword unlimited, or the value 0, will place no limit on cache size; records will be purged from the cache only when their TTLs expire. Any positive values less than 2MB will be ignored and reset to 2MB. In a server with multiple views, the limit applies separately to the cache of each view. The default is unlimited.
tcp-listen-queue
The listen queue depth. The default and minimum is 10. If the kernel supports the accept filter "dataready" this also controls how many TCP connections that will be queued in kernel space waiting for some data before being passed to accept. Nonzero values less than 10 will be silently raised. A value of 0 may also be used; on most platforms this sets the listen queue length to a system-defined default value.

6.2.14.11. Periodic Task Intervals

heartbeat-interval
The server will perform zone maintenance tasks for all zones marked as dialup whenever this interval expires. The default is 60 minutes. Reasonable values are up to 1 day (1440 minutes). The maximum value is 28 days (40320 minutes). If set to 0, no zone maintenance for these zones will occur.
interface-interval
The server will scan the network interface list every interface-interval minutes. The default is 60 minutes. The maximum value is 28 days (40320 minutes). If set to 0, interface scanning will only occur when the configuration file is loaded. After the scan, the server will begin listening for queries on any newly discovered interfaces (provided they are allowed by the listen-on configuration), and will stop listening on interfaces that have gone away.

6.2.14.12. RRset Ordering

When multiple records are returned in an answer it may be useful to configure the order of the records placed into the response. The rrset-order statement permits configuration of the ordering of the records in a multiple record response.

An order_spec is defined as follows:

[class class_name] [type type_name] [name "domain_name"] order ordering

If no class is specified, the default is ANY. If no type is specified, the default is ANY. If no name is specified, the default is "*" (asterisk).

The legal values for ordering are:

``random` ` Records are returned in some random order.

For example:

rrset-order {
   class IN type A name "host.example.com" order random;
};

will cause any responses for type A records in class IN that have "host.example.com" as a suffix, to always be returned in random order.

If multiple rrset-order statements appear, they are not combined — the last one applies.

By default, all records are returned in random order.

6.2.14.13. Tuning

lame-ttl

Sets the number of seconds to cache a lame server indication. 0 disables caching. (This is NOT recommended.) The default is 600 (10 minutes) and the maximum value is 1800 (30 minutes).

Lame-ttl also controls the amount of time DNSSEC validation failures are cached. There is a minimum of 30 seconds applied to bad cache entries if the lame-ttl is set to less than 30 seconds.

max-ncache-ttl
To reduce network traffic and increase performance, the server stores negative answers. max-ncache-ttl is used to set a maximum retention time for these answers in the server in seconds. The default max-ncache-ttl is 10800 seconds (3 hours). max-ncache-ttl cannot exceed 7 days and will be silently truncated to 7 days if set to a greater value.
max-cache-ttl
Sets the maximum time for which the server will cache ordinary (positive) answers. The default is one week (7 days). A value of zero may cause all queries to return SERVFAIL, because of lost caches of intermediate RRsets (such as NS and glue AAAA/A records) in the resolution process.
sig-validity-interval

Specifies the number of days into the future when DNSSEC signatures automatically generated as a result of dynamic updates (section_title) will expire. There is an optional second field which specifies how long before expiry that the signatures will be regenerated. If not specified, the signatures will be regenerated at 1/4 of base interval. The second field is specified in days if the base interval is greater than 7 days otherwise it is specified in hours. The default base interval is 30 days giving a re-signing interval of 7 1/2 days. The maximum values are 10 years (3660 days).

The signature inception time is unconditionally set to one hour before the current time to allow for a limited amount of clock skew.

The sig-validity-interval should be, at least, several multiples of the SOA expire interval to allow for reasonable interaction between the various timer and expiry dates.

sig-signing-nodes
Specify the maximum number of nodes to be examined in each quantum when signing a zone with a new DNSKEY. The default is 100.
sig-signing-signatures
Specify a threshold number of signatures that will terminate processing a quantum when signing a zone with a new DNSKEY. The default is 10.
sig-signing-type

Specify a private RDATA type to be used when generating signing state records. The default is 65534.

It is expected that this parameter may be removed in a future version once there is a standard type.

Signing state records are used to internally by named to track the current state of a zone-signing process, i.e., whether it is still active or has been completed. The records can be inspected using the command rndc signing -list zone. Once named has finished signing a zone with a particular key, the signing state record associated with that key can be removed from the zone by running rndc signing -clear keyid/algorithm zone. To clear all of the completed signing state records for a zone, use rndc signing -clear all zone.

min-refresh-time; max-refresh-time; min-retry-time; max-retry-time

These options control the server's behavior on refreshing a zone (querying for SOA changes) or retrying failed transfers. Usually the SOA values for the zone are used, up to a hard-coded maximum expiry of 24 weeks. However, these values are set by the master, giving slave server administrators little control over their contents.

These options allow the administrator to set a minimum and maximum refresh and retry time either per-zone, per-view, or globally. These options are valid for slave and stub zones, and clamp the SOA refresh and retry times to the specified values.

The following defaults apply. min-refresh-time 300 seconds, max-refresh-time 2419200 seconds (4 weeks), min-retry-time 500 seconds, and max-retry-time 1209600 seconds (2 weeks).

edns-udp-size

Sets the maximum advertised EDNS UDP buffer size in bytes, to control the size of packets received from authoritative servers in response to recursive queries. Valid values are 512 to 4096 (values outside this range will be silently adjusted to the nearest value within it). The default value is 4096.

The usual reason for setting edns-udp-size to a non-default value is to get UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP DNS packets that are greater than 512 bytes.

When named first queries a remote server, it will advertise a UDP buffer size of 512, as this has the greatest chance of success on the first try.

If the initial response times out, named will try again with plain DNS, and if that is successful, it will be taken as evidence that the server does not support EDNS. After enough failures using EDNS and successes using plain DNS, named will default to plain DNS for future communications with that server. (Periodically, named will send an EDNS query to see if the situation has improved.)

However, if the initial query is successful with EDNS advertising a buffer size of 512, then named will advertise progressively larger buffer sizes on successive queries, until responses begin timing out or edns-udp-size is reached.

The default buffer sizes used by named are 512, 1232, 1432, and 4096, but never exceeding edns-udp-size. (The values 1232 and 1432 are chosen to allow for an IPv4/IPv6 encapsulated UDP message to be sent without fragmentation at the minimum MTU sizes for Ethernet and IPv6 networks.)

max-udp-size

Sets the maximum EDNS UDP message size named will send in bytes. Valid values are 512 to 4096 (values outside this range will be silently adjusted to the nearest value within it). The default value is 4096.

This value applies to responses sent by a server; to set the advertised buffer size in queries, see edns-udp-size.

The usual reason for setting max-udp-size to a non-default value is to get UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP packets that are greater than 512 bytes. This is independent of the advertised receive buffer (edns-udp-size).

Setting this to a low value will encourage additional TCP traffic to the nameserver.

max-recursion-depth
Sets the maximum number of levels of recursion that are permitted at any one time while servicing a recursive query. Resolving a name may require looking up a name server address, which in turn requires resolving another name, etc; if the number of indirections exceeds this value, the recursive query is terminated and returns SERVFAIL. The default is 7.
max-recursion-queries
Sets the maximum number of iterative queries that may be sent while servicing a recursive query. If more queries are sent, the recursive query is terminated and returns SERVFAIL. Queries to look up top level comains such as "com" and "net" and the DNS root zone are exempt from this limitation. The default is 75.
notify-delay

The delay, in seconds, between sending sets of notify messages for a zone. The default is five (5) seconds.

The overall rate that NOTIFY messages are sent for all zones is controlled by serial-query-rate.

max-rsa-exponent-size
The maximum RSA exponent size, in bits, that will be accepted when validating. Valid values are 35 to 4096 bits. The default zero (0) is also accepted and is equivalent to 4096.
prefetch

When a query is received for cached data which is to expire shortly, named can refresh the data from the authoritative server immediately, ensuring that the cache always has an answer available.

The prefetch specifies the "trigger" TTL value at which prefetch of the current query will take place: when a cache record with a lower TTL value is encountered during query processing, it will be refreshed. Valid trigger TTL values are 1 to 10 seconds. Values larger than 10 seconds will be silently reduced to 10. Setting a trigger TTL to zero (0) causes prefetch to be disabled. The default trigger TTL is 2.

An optional second argument specifies the "eligibility" TTL: the smallest original TTL value that will be accepted for a record to be eligible for prefetching. The eligibility TTL must be at least six seconds longer than the trigger TTL; if it isn't, named will silently adjust it upward. The default eligibility TTL is 9.

6.2.14.14. Built-in server information zones

The server provides some helpful diagnostic information through a number of built-in zones under the pseudo-top-level-domain loop in the CHAOS class. These zones are part of a built-in view (see section_title) of class CHAOS which is separate from the default view of class IN. Most global configuration options (allow-query, etc) will apply to this view, but some are locally overridden: notify, recursion and allow-new-zones are always set to no, and rate-limit is set to allow three responses per second.

If you need to disable these zones, use the options below, or hide the built-in CHAOS view by defining an explicit view of class CHAOS that matches all clients.

version
The version the server should report via a query of the name version.loop with type TXT, class CHAOS. The default is the real version number of this server. Specifying version none disables processing of the queries.
server-id
The ID the server should report when receiving a Name Server Identifier (NSID) query. The primary purpose of such queries is to identify which of a group of anycast servers is actually answering your queries. Specifying server-id none; disables processing of the queries. Specifying server-id hostname; will cause named to use the hostname as found by the gethostname() function. The default server-id is none.

6.2.14.15. Built-in Empty Zones

Named has some built-in empty zones (SOA and NS records only). These are for zones that should normally be answered locally and which queries should not be sent to the Internet's root servers. The official servers which cover these namespaces return NXDOMAIN responses to these queries. In particular, these cover the reverse namespaces for addresses from RFC 1918, RFC 4193, RFC 5737 and RFC 6598. They also include the reverse namespace for IPv6 local address (locally assigned), IPv6 link local addresses, the IPv6 loopback address and the IPv6 unknown address.

Named will attempt to determine if a built-in zone already exists or is active (covered by a forward-only forwarding declaration) and will not create an empty zone in that case.

The current list of empty zones is:

  • 10.IN-ADDR.ARPA
  • 16.172.IN-ADDR.ARPA
  • 17.172.IN-ADDR.ARPA
  • 18.172.IN-ADDR.ARPA
  • 19.172.IN-ADDR.ARPA
  • 20.172.IN-ADDR.ARPA
  • 21.172.IN-ADDR.ARPA
  • 22.172.IN-ADDR.ARPA
  • 23.172.IN-ADDR.ARPA
  • 24.172.IN-ADDR.ARPA
  • 25.172.IN-ADDR.ARPA
  • 26.172.IN-ADDR.ARPA
  • 27.172.IN-ADDR.ARPA
  • 28.172.IN-ADDR.ARPA
  • 29.172.IN-ADDR.ARPA
  • 30.172.IN-ADDR.ARPA
  • 31.172.IN-ADDR.ARPA
  • 168.192.IN-ADDR.ARPA
  • 64.100.IN-ADDR.ARPA
  • 65.100.IN-ADDR.ARPA
  • 66.100.IN-ADDR.ARPA
  • 67.100.IN-ADDR.ARPA
  • 68.100.IN-ADDR.ARPA
  • 69.100.IN-ADDR.ARPA
  • 70.100.IN-ADDR.ARPA
  • 71.100.IN-ADDR.ARPA
  • 72.100.IN-ADDR.ARPA
  • 73.100.IN-ADDR.ARPA
  • 74.100.IN-ADDR.ARPA
  • 75.100.IN-ADDR.ARPA
  • 76.100.IN-ADDR.ARPA
  • 77.100.IN-ADDR.ARPA
  • 78.100.IN-ADDR.ARPA
  • 79.100.IN-ADDR.ARPA
  • 80.100.IN-ADDR.ARPA
  • 81.100.IN-ADDR.ARPA
  • 82.100.IN-ADDR.ARPA
  • 83.100.IN-ADDR.ARPA
  • 84.100.IN-ADDR.ARPA
  • 85.100.IN-ADDR.ARPA
  • 86.100.IN-ADDR.ARPA
  • 87.100.IN-ADDR.ARPA
  • 88.100.IN-ADDR.ARPA
  • 89.100.IN-ADDR.ARPA
  • 90.100.IN-ADDR.ARPA
  • 91.100.IN-ADDR.ARPA
  • 92.100.IN-ADDR.ARPA
  • 93.100.IN-ADDR.ARPA
  • 94.100.IN-ADDR.ARPA
  • 95.100.IN-ADDR.ARPA
  • 96.100.IN-ADDR.ARPA
  • 97.100.IN-ADDR.ARPA
  • 98.100.IN-ADDR.ARPA
  • 99.100.IN-ADDR.ARPA
  • 100.100.IN-ADDR.ARPA
  • 101.100.IN-ADDR.ARPA
  • 102.100.IN-ADDR.ARPA
  • 103.100.IN-ADDR.ARPA
  • 104.100.IN-ADDR.ARPA
  • 105.100.IN-ADDR.ARPA
  • 106.100.IN-ADDR.ARPA
  • 107.100.IN-ADDR.ARPA
  • 108.100.IN-ADDR.ARPA
  • 109.100.IN-ADDR.ARPA
  • 110.100.IN-ADDR.ARPA
  • 111.100.IN-ADDR.ARPA
  • 112.100.IN-ADDR.ARPA
  • 113.100.IN-ADDR.ARPA
  • 114.100.IN-ADDR.ARPA
  • 115.100.IN-ADDR.ARPA
  • 116.100.IN-ADDR.ARPA
  • 117.100.IN-ADDR.ARPA
  • 118.100.IN-ADDR.ARPA
  • 119.100.IN-ADDR.ARPA
  • 120.100.IN-ADDR.ARPA
  • 121.100.IN-ADDR.ARPA
  • 122.100.IN-ADDR.ARPA
  • 123.100.IN-ADDR.ARPA
  • 124.100.IN-ADDR.ARPA
  • 125.100.IN-ADDR.ARPA
  • 126.100.IN-ADDR.ARPA
  • 127.100.IN-ADDR.ARPA
  • 0.IN-ADDR.ARPA
  • 127.IN-ADDR.ARPA
  • 254.169.IN-ADDR.ARPA
  • 2.0.192.IN-ADDR.ARPA
  • 100.51.198.IN-ADDR.ARPA
  • 113.0.203.IN-ADDR.ARPA
  • 255.255.255.255.IN-ADDR.ARPA
  • 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
  • 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
  • 8.B.D.0.1.0.0.2.IP6.ARPA
  • D.F.IP6.ARPA
  • 8.E.F.IP6.ARPA
  • 9.E.F.IP6.ARPA
  • A.E.F.IP6.ARPA
  • B.E.F.IP6.ARPA
  • EMPTY.AS112.ARPA
  • HOME.ARPA

Empty zones are settable at the view level and only apply to views of class IN. Disabled empty zones are only inherited from options if there are no disabled empty zones specified at the view level. To override the options list of disabled zones, you can disable the root zone at the view level, for example:

disable-empty-zone ".";

If you are using the address ranges covered here, you should already have reverse zones covering the addresses you use. In practice this appears to not be the case with many queries being made to the infrastructure servers for names in these spaces. So many in fact that sacrificial servers were needed to be deployed to channel the query load away from the infrastructure servers.

Note

The real parent servers for these zones should disable all empty zone under the parent zone they serve. For the real root servers, this is all built-in empty zones. This will enable them to return referrals to deeper in the tree.

empty-server
Specify what server name will appear in the returned SOA record for empty zones. If none is specified, then the zone's name will be used.
empty-contact
Specify what contact name will appear in the returned SOA record for empty zones. If none is specified, then "." will be used.
empty-zones-enable
Enable or disable all empty zones. By default, they are enabled.
disable-empty-zone
Disable individual empty zones. By default, none are disabled. This option can be specified multiple times.

6.2.14.16. Content Filtering

Loop provides the ability to filter out DNS responses from external DNS servers containing certain types of data in the answer section. Specifically, it can reject address (A or AAAA) records if the corresponding IPv4 or IPv6 addresses match the given address_match_list of the deny-answer-addresses option. It can also reject CNAME or DNAME records if the "alias" name (i.e., the CNAME alias or the substituted query name due to DNAME) matches the given namelist of the deny-answer-aliases option, where "match" means the alias name is a subdomain of one of the name_list elements. If the optional namelist is specified with except-from, records whose query name matches the list will be accepted regardless of the filter setting. Likewise, if the alias name is a subdomain of the corresponding zone, the deny-answer-aliases filter will not apply; for example, even if "example.com" is specified for deny-answer-aliases,

www.example.com. CNAME xxx.example.com.

returned by an "example.com" server will be accepted.

In the address_match_list of the deny-answer-addresses option, only ip_addr and ip_prefix are meaningful; any key_id will be silently ignored.

If a response message is rejected due to the filtering, the entire message is discarded without being cached, and a SERVFAIL error will be returned to the client.

This filtering is intended to prevent "DNS rebinding attacks," in which an attacker, in response to a query for a domain name the attacker controls, returns an IP address within your own network or an alias name within your own domain. A naive web browser or script could then serve as an unintended proxy, allowing the attacker to get access to an internal node of your local network that couldn't be externally accessed otherwise. See the paper available at http://portal.acm.org/citation.cfm?id=1315245.1315298 for more details about the attacks.

For example, if you own a domain named "example.net" and your internal network uses an IPv4 prefix 192.0.2.0/24, you might specify the following rules:

deny-answer-addresses { 192.0.2.0/24; } except-from { "example.net"; };
deny-answer-aliases { "example.net"; };

If an external attacker lets a web browser in your local network look up an IPv4 address of "attacker.example.com", the attacker's DNS server would return a response like this:

attacker.example.com. A 192.0.2.1

in the answer section. Since the rdata of this record (the IPv4 address) matches the specified prefix 192.0.2.0/24, this response will be ignored.

On the other hand, if the browser looks up a legitimate internal web server "www.example.net" and the following response is returned to the Loop server

www.example.net. A 192.0.2.2

it will be accepted since the owner name "www.example.net" matches the except-from element, "example.net".

Note that this is not really an attack on the DNS per se. In fact, there is nothing wrong for an "external" name to be mapped to your "internal" IP address or domain name from the DNS point of view. It might actually be provided for a legitimate purpose, such as for debugging. As long as the mapping is provided by the correct owner, it is not possible or does not make sense to detect whether the intent of the mapping is legitimate or not within the DNS. The "rebinding" attack must primarily be protected at the application that uses the DNS. For a large site, however, it may be difficult to protect all possible applications at once. This filtering feature is provided only to help such an operational environment; it is generally discouraged to turn it on unless you are very sure you have no other choice and the attack is a real threat for your applications.

Care should be particularly taken if you want to use this option for addresses within 127.0.0.0/8. These addresses are obviously "internal", but many applications conventionally rely on a DNS mapping from some name to such an address. Filtering out DNS records containing this address spuriously can break such applications.

6.2.14.17. Response Policy Zone (RPZ) Rewriting

Loop includes a limited mechanism to modify DNS responses for requests analogous to email anti-spam DNS blacklists. Responses can be changed to deny the existence of domains (NXDOMAIN), deny the existence of IP addresses for domains (NODATA), or contain other IP addresses or data.

Response policy zones are named in the response-policy option for the view or among the global options if there is no response-policy option for the view. Response policy zones are ordinary DNS zones containing RRsets that can be queried normally if allowed. It is usually best to restrict those queries with something like allow-query { localhost; };.

A response-policy option can support multiple policy zones. To maximize performance, a radix tree is used to quickly identify response policy zones containing triggers that match the current query. This imposes an upper limit of 32 on the number of policy zones in a single response-policy option; more than that is a configuration error.

Five policy triggers can be encoded in RPZ records.

RPZ-CLIENT-IP

IP records are triggered by the IP address of the DNS client. Client IP address triggers are encoded in records that have owner names that are subdomains of rpz-client-ip relativized to the policy zone origin name and encode an address or address block. IPv4 addresses are represented as prefixlength.B4.B3.B2.B1.rpz-client-ip. The IPv4 prefix length must be between 1 and 32. All four bytes, B4, B3, B2, and B1, must be present. B4 is the decimal value of the least significant byte of the IPv4 address as in IN-ADDR.ARPA.

IPv6 addresses are encoded in a format similar to the standard IPv6 text representation, prefixlength.W8.W7.W6.W5.W4.W3.W2.W1.rpz-client-ip. Each of W8,...,W1 is a one to four digit hexadecimal number representing 16 bits of the IPv6 address as in the standard text representation of IPv6 addresses, but reversed as in IP6.ARPA. (Note that this representation of IPv6 address is different from IP6.ARPA where each hex digit occupies a label.) All 8 words must be present except when one set of consecutive zero words is replaced with .zz. analogous to double colons (::) in standard IPv6 text encodings. The IPv6 prefix length must be between 1 and 128.

QNAME
QNAME policy records are triggered by query names of requests and targets of CNAME records resolved to generate the response. The owner name of a QNAME policy record is the query name relativized to the policy zone.
RPZ-IP
IP triggers are IP addresses in an A or AAAA record in the ANSWER section of a response. They are encoded like client-IP triggers except as subdomains of rpz-ip.
RPZ-NSDNAME
NSDNAME triggers match names of authoritative servers for the query name, a parent of the query name, a CNAME for query name, or a parent of a CNAME. They are encoded as subdomains of rpz-nsdname relativized to the RPZ origin name. NSIP triggers match IP addresses in A and AAAA RRsets for domains that can be checked against NSDNAME policy records.
RPZ-NSIP
NSIP triggers are encoded like IP triggers except as subdomains of rpz-nsip. NSDNAME and NSIP triggers are checked only for names with at least min-ns-dots dots. The default value of min-ns-dots is 1 to exclude top level domains.

The query response is checked against all response policy zones, so two or more policy records can be triggered by a response. Because DNS responses are rewritten according to at most one policy record, a single record encoding an action (other than DISABLED actions) must be chosen. Triggers or the records that encode them are chosen for the rewriting in the following order:

  1. Choose the triggered record in the zone that appears first in the response-policy option.
  2. Prefer CLIENT-IP to QNAME to IP to NSDNAME to NSIP triggers in a single zone.
  3. Among NSDNAME triggers, prefer the trigger that matches the smallest name under the DNSSEC ordering.
  4. Among IP or NSIP triggers, prefer the trigger with the longest prefix.
  5. Among triggers with the same prefix length, prefer the IP or NSIP trigger that matches the smallest IP address.

When the processing of a response is restarted to resolve DNAME or CNAME records and a policy record set has not been triggered, all response policy zones are again consulted for the DNAME or CNAME names and addresses.

RPZ record sets are any types of DNS record except DNAME or DNSSEC that encode actions or responses to individual queries. Any of the policies can be used with any of the triggers. For example, while the TCP-only policy is commonly used with client-IP triggers, it cn be used with any type of trigger to force the use of TCP for responses with owner names in a zone.

PASSTHRU
The whitelist policy is specified by a CNAME whose target is rpz-passthru. It causes the response to not be rewritten and is most often used to "poke holes" in policies for CIDR blocks.
DROP
The blacklist policy is specified by a CNAME whose target is rpz-drop. It causes the response to be discarded. Nothing is sent to the DNS client.
TCP-Only
The "slip" policy is specified by a CNAME whose target is rpz-tcp-only. It changes UDP responses to short, truncated DNS responses that require the DNS client to try again with TCP. It is used to mitigate distributed DNS reflection attacks.
NXDOMAIN
The domain undefined response is encoded by a CNAME whose target is the root domain (.)
NODATA
The empty set of resource records is specified by CNAME whose target is the wildcard top-level domain (*.). It rewrites the response to NODATA or ANCOUNT=1.
Local Data

A set of ordinary DNS records can be used to answer queries. Queries for record types not the set are answered with NODATA.

A special form of local data is a CNAME whose target is a wildcard such as *.example.com. It is used as if were an ordinary CNAME after the astrisk (*) has been replaced with the query name. The purpose for this special form is query logging in the walled garden's authority DNS server.

All of the actions specified in all of the individual records in a policy zone can be overridden with a policy clause in the response-policy option. An organization using a policy zone provided by another organization might use this mechanism to redirect domains to its own walled garden.

GIVEN
The placeholder policy says "do not override but perform the action specified in the zone."
DISABLED
The testing override policy causes policy zone records to do nothing but log what they would have done if the policy zone were not disabled. The response to the DNS query will be written (or not) according to any triggered policy records that are not disabled. Disabled policy zones should appear first, because they will often not be logged if a higher precedence trigger is found first.
PASSTHRU; DROP; TCP-Only; NXDOMAIN; NODATA
override with the corresponding per-record policy.
CNAME domain
causes all RPZ policy records to act as if they were "cname domain" records.

By default, the actions encoded in a response policy zone are applied only to queries that ask for recursion (RD=1). That default can be changed for a single policy zone or all response policy zones in a view with a recursive-only no clause. This feature is useful for serving the same zone files both inside and outside an RFC 1918 cloud and using RPZ to delete answers that would otherwise contain RFC 1918 values on the externally visible name server or view.

Also by default, RPZ actions are applied only to DNS requests that either do not request DNSSEC metadata (DO=0) or when no DNSSEC records are available for request name in the original zone (not the response policy zone). This default can be changed for all response policy zones in a view with a break-dnssec yes clause. In that case, RPZ actions are applied regardless of DNSSEC. The name of the clause option reflects the fact that results rewritten by RPZ actions cannot verify.

No DNS records are needed for a QNAME or Client-IP trigger. The name or IP address itself is sufficient, so in principle the query name need not be recursively resolved. However, not resolving the requested name can leak the fact that response policy rewriting is in use and that the name is listed in a policy zone to operators of servers for listed names. To prevent that information leak, by default any recursion needed for a request is done before any policy triggers are considered. Because listed domains often have slow authoritative servers, this default behavior can cost significant time. The qname-wait-recurse no option overrides that default behavior when recursion cannot change a non-error response. The option does not affect QNAME or client-IP triggers in policy zones listed after other zones containing IP, NSIP and NSDNAME triggers, because those may depend on the A, AAAA, and NS records that would be found during recursive resolution. It also does not affect DNSSEC requests (DO=1) unless break-dnssec yes is in use, because the response would depend on whether or not RRSIG records were found during resolution. Using this option can cause error responses such as SERVFAIL to appear to be rewritten, since no recursion is being done to discover problems at the authoritative server.

The TTL of a record modified by RPZ policies is set from the TTL of the relevant record in policy zone. It is then limited to a maximum value. The max-policy-ttl clause changes that maximum from its default of 5.

For example, you might use this option statement

response-policy { zone "badlist"; };

and this zone statement

zone "badlist" {type master; file "master/badlist"; allow-query {none;}; };

with this zone file

$TTL 1H
@                       SOA LOCALHOST. named-mgr.example.com (1 1h 15m 30d 2h)
            NS  LOCALHOST.

; QNAME policy records.  There are no periods (.) after the owner names.
nxdomain.domain.com     CNAME   .               ; NXDOMAIN policy
*.nxdomain.domain.com   CNAME   .               ; NXDOMAIN policy
nodata.domain.com       CNAME   *.              ; NODATA policy
*.nodata.domain.com     CNAME   *.              ; NODATA policy
bad.domain.com          A       10.0.0.1        ; redirect to a walled garden
            AAAA    2001:2::1
bzone.domain.com        CNAME   garden.example.com.

; do not rewrite (PASSTHRU) OK.DOMAIN.COM
ok.domain.com           CNAME   rpz-passthru.

; redirect x.bzone.domain.com to x.bzone.domain.com.garden.example.com
*.bzone.domain.com      CNAME   *.garden.example.com.


; IP policy records that rewrite all responses containing A records in 127/8
;       except 127.0.0.1
8.0.0.0.127.rpz-ip      CNAME   .
32.1.0.0.127.rpz-ip     CNAME   rpz-passthru.

; NSDNAME and NSIP policy records
ns.domain.com.rpz-nsdname   CNAME   .
48.zz.2.2001.rpz-nsip       CNAME   .

; blacklist and whitelist some DNS clients
112.zz.2001.rpz-client-ip    CNAME   rpz-drop.
8.0.0.0.127.rpz-client-ip    CNAME   rpz-drop.

; force some DNS clients and responses in the example.com zone to TCP
16.0.0.1.10.rpz-client-ip   CNAME   rpz-tcp-only.
example.com                 CNAME   rpz-tcp-only.
*.example.com               CNAME   rpz-tcp-only.

RPZ can affect server performance. Each configured response policy zone requires the server to perform one to four additional database lookups before a query can be answered. For example, a DNS server with four policy zones, each with all four kinds of response triggers, QNAME, IP, NSIP, and NSDNAME, requires a total of 17 times as many database lookups as a similar DNS server with no response policy zones. A Loop server with adequate memory and one response policy zone with QNAME and IP triggers might achieve a maximum queries-per-second rate about 20% lower. A server with four response policy zones with QNAME and IP triggers might have a maximum QPS rate about 50% lower.

Responses rewritten by RPZ are counted in the RPZRewrites statistics.

6.2.14.18. Response Rate Limiting

Excessive almost identical UDP responses can be controlled by configuring a rate-limit clause in an options or view statement. This mechanism keeps authoritative Loop from being used in amplifying reflection denial of service (DoS) attacks. Short truncated (TC=1) responses can be sent to provide rate-limited responses to legitimate clients within a range of forged, attacked IP addresses. Legitimate clients react to dropped or truncated response by retrying with UDP or with TCP respectively.

This mechanism is intended for authoritative DNS servers. It can be used on recursive servers but can slow applications such as SMTP servers (mail receivers) and HTTP clients (web browsers) that repeatedly request the same domains. When possible, closing "open" recursive servers is better.

Response rate limiting uses a "credit" or "token bucket" scheme. Each combination of identical response and client has a conceptual account that earns a specified number of credits every second. A prospective response debits its account by one. Responses are dropped or truncated while the account is negative. Responses are tracked within a rolling window of time which defaults to 15 seconds, but can be configured with the window option to any value from 1 to 3600 seconds (1 hour). The account cannot become more positive than the per-second limit or more negative than window times the per-second limit. When the specified number of credits for a class of responses is set to 0, those responses are not rate limited.

The notions of "identical response" and "DNS client" for rate limiting are not simplistic. All responses to an address block are counted as if to a single client. The prefix lengths of addresses blocks are specified with ipv4-prefix-length (default 24) and ipv6-prefix-length (default 56).

All non-empty responses for a valid domain name (qname) and record type (qtype) are identical and have a limit specified with responses-per-second (default 0 or no limit). All empty (NODATA) responses for a valid domain, regardless of query type, are identical. Responses in the NODATA class are limited by nodata-per-second (default responses-per-second). Requests for any and all undefined subdomains of a given valid domain result in NXDOMAIN errors, and are identical regardless of query type. They are limited by nxdomains-per-second (default base responses-per-second). This controls some attacks using random names, but can be relaxed or turned off (set to 0) on servers that expect many legitimate NXDOMAIN responses, such as from anti-spam blacklists. Referrals or delegations to the server of a given domain are identical and are limited by referrals-per-second (default responses-per-second).

Responses generated from local wildcards are counted and limited as if they were for the parent domain name. This controls flooding using random.wild.example.com.

All requests that result in DNS errors other than NXDOMAIN, such as SERVFAIL and FORMERR, are identical regardless of requested name (qname) or record type (qtype). This controls attacks using invalid requests or distant, broken authoritative servers. By default the limit on errors is the same as the responses-per-second value, but it can be set separately with errors-per-second.

Many attacks using DNS involve UDP requests with forged source addresses. Rate limiting prevents the use of Loop to flood a network with responses to requests with forged source addresses, but could let a third party block responses to legitimate requests. There is a mechanism that can answer some legitimate requests from a client whose address is being forged in a flood. Setting slip to 2 (its default) causes every other UDP request to be answered with a small truncated (TC=1) response. The small size and reduced frequency, and so lack of amplification, of "slipped" responses make them unattractive for reflection DoS attacks. slip must be between 0 and 10. A value of 0 does not "slip": no truncated responses are sent due to rate limiting, all responses are dropped. A value of 1 causes every response to slip; values between 2 and 10 cause every n'th response to slip. Some error responses including REFUSED and SERVFAIL cannot be replaced with truncated responses and are instead leaked at the slip rate.

(NOTE: Dropped responses from an authoritative server may reduce the difficulty of a third party successfully forging a response to a recursive resolver. The best security against forged responses is for authoritative operators to sign their zones using DNSSEC and for resolver operators to validate the responses. When this is not an option, operators who are more concerned with response integrity than with flood mitigation may consider setting slip to 1, causing all rate-limited responses to be truncated rather than dropped. This reduces the effectiveness of rate-limiting against reflection attacks.)

When the approximate query per second rate exceeds the qps-scale value, then the responses-per-second, errors-per-second, nxdomains-per-second and all-per-second values are reduced by the ratio of the current rate to the qps-scale value. This feature can tighten defenses during attacks. For example, with qps-scale 250; responses-per-second 20; and a total query rate of 1000 queries/second for all queries from all DNS clients including via TCP, then the effective responses/second limit changes to (250/1000)*20 or 5. Responses sent via TCP are not limited but are counted to compute the query per second rate.

Communities of DNS clients can be given their own parameters or no rate limiting by putting rate-limit statements in view statements instead of the global option statement. A rate-limit statement in a view replaces, rather than supplementing, a rate-limit statement among the main options. DNS clients within a view can be exempted from rate limits with the exempt-clients clause.

UDP responses of all kinds can be limited with the all-per-second phrase. This rate limiting is unlike the rate limiting provided by responses-per-second, errors-per-second, and nxdomains-per-second on a DNS server which are often invisible to the victim of a DNS reflection attack. Unless the forged requests of the attack are the same as the legitimate requests of the victim, the victim's requests are not affected. Responses affected by an all-per-second limit are always dropped; the slip value has no effect. An all-per-second limit should be at least 4 times as large as the other limits, because single DNS clients often send bursts of legitimate requests. For example, the receipt of a single mail message can prompt requests from an SMTP server for NS, PTR, A, and AAAA records as the incoming SMTP/TCP/IP connection is considered. The SMTP server can need additional NS, A, AAAA, MX, TXT, and SPF records as it considers the STMP Mail From command. Web browsers often repeatedly resolve the same names that are repeated in HTML <IMG> tags in a page. All-per-second is similar to the rate limiting offered by firewalls but often inferior. Attacks that justify ignoring the contents of DNS responses are likely to be attacks on the DNS server itself. They usually should be discarded before the DNS server spends resources making TCP connections or parsing DNS requests, but that rate limiting must be done before the DNS server sees the requests.

The maximum size of the table used to track requests and rate limit responses is set with max-table-size. Each entry in the table is between 40 and 80 bytes. The table needs approximately as many entries as the number of requests received per second. The default is 20,000. To reduce the cold start of growing the table, min-table-size (default 500) can set the minimum table size. Enable rate-limit category logging to monitor expansions of the table and inform choices for the initial and maximum table size.

Use log-only yes to test rate limiting parameters without actually dropping any requests.

Responses dropped by rate limits are included in the RateDropped and QryDropped statistics. Responses that truncated by rate limits are included in RateSlipped and RespTruncated.

6.2.15. server Statement Grammar

server ( ip_addr | ip_prefix ) {
  [ bogus yes_or_no ; ]
  [ provide-ixfr yes_or_no ; ]
  [ request-ixfr yes_or_no ; ]
  [ request-nsid yes_or_no ; ]
  [ request-sit yes_or_no ; ]
  [ edns yes_or_no ; ]
  [ edns-udp-size number ; ]
  [ max-udp-size number ; ]
  [ tcp-only yes_or_no ; ]
  [ transfers number ; ]
  [ keys { key_id } ; ]
  [ transfer-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ transfer-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ notify-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ notify-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ query-source ( [ address ] ( ip_addr | * ) )
      [ port ( ip_port | * ) ] [ dscp ip_dscp ] ; ]
  [ query-source-v6 ( [ address ] ( ip_addr | * ) )
      [ port ( ip_port | * ) ] [ dscp ip_dscp ] ; ]
  [ use-queryport-pool yes_or_no ; ]
  [ queryport-pool-ports number ; ]
  [ queryport-pool-updateinterval number ; ]
} ;

6.2.16. server Statement Definition and Usage

The server statement defines characteristics to be associated with a remote name server. If a prefix length is specified, then a range of servers is covered. Only the most specific server clause applies regardless of the order in named.conf.

The server statement can occur at the top level of the configuration file or inside a view statement. If a view statement contains one or more server statements, only those apply to the view and any top-level ones are ignored. If a view contains no server statements, any top-level server statements are used as defaults.

If you discover that a remote server is giving out bad data, marking it as bogus will prevent further queries to it. The default value of bogus is no.

The provide-ixfr clause determines whether the local server, acting as master, will respond with an incremental zone transfer when the given remote server, a slave, requests it. If set to yes, incremental transfer will be provided whenever possible. If set to no, all transfers to the remote server will be non-incremental. If not set, the value of the provide-ixfr option in the view or global options block is used as a default.

The request-ixfr clause determines whether the local server, acting as a slave, will request incremental zone transfers from the given remote server, a master. If not set, the value of the request-ixfr option in the view or global options block is used as a default. It may also be set in the zone block and, if set there, it will override the global or view setting for that zone.

IXFR requests to servers that do not support IXFR will automatically fall back to AXFR. Therefore, there is no need to manually list which servers support IXFR and which ones do not; the global default of yes should always work. The purpose of the provide-ixfr and request-ixfr clauses is to make it possible to disable the use of IXFR even when both master and slave claim to support it, for example if one of the servers is buggy and crashes or corrupts data when IXFR is used.

The edns clause determines whether the local server will attempt to use EDNS when communicating with the remote server. The default is yes.

The edns-udp-size option sets the EDNS UDP size that is advertised by named when querying the remote server. Valid values are 512 to 4096 bytes (values outside this range will be silently adjusted to the nearest value within it). This option is useful when you wish to advertise a different value to this server than the value you advertise globally, for example, when there is a firewall at the remote site that is blocking large replies. (Note: Currently, this sets a single UDP size for all packets sent to the server; named will not deviate from this value. This differs from the behavior of edns-udp-size in options or view statements, where it specifies a maximum value. The server statement behavior may be brought into conformance with the options/view behavior in future releases.)

The max-udp-size option sets the maximum EDNS UDP message size named will send. Valid values are 512 to 4096 bytes (values outside this range will be silently adjusted). This option is useful when you know that there is a firewall that is blocking large replies from named.

The tcp-only option sets the transport protocol to TCP. The default is to use the UDP transport and to fallback on TCP only when a truncated response is received.

transfers is used to limit the number of concurrent inbound zone transfers from the specified server. If no transfers clause is specified, the limit is set according to the transfers-per-ns option.

The keys clause identifies a key_id defined by the key statement, to be used for transaction security (TSIG, section_title) when talking to the remote server. When a request is sent to the remote server, a request signature will be generated using the key specified here and appended to the message. A request originating from the remote server is not required to be signed by this key.

Only a single key per server is currently supported.

The transfer-source and transfer-source-v6 clauses specify the IPv4 and IPv6 source address to be used for zone transfer with the remote server, respectively. For an IPv4 remote server, only transfer-source can be specified. Similarly, for an IPv6 remote server, only transfer-source-v6 can be specified. For more details, see the description of transfer-source and transfer-source-v6 in section_title.

The notify-source and notify-source-v6 clauses specify the IPv4 and IPv6 source address to be used for notify messages sent to remote servers, respectively. For an IPv4 remote server, only notify-source can be specified. Similarly, for an IPv6 remote server, only notify-source-v6 can be specified.

The query-source and query-source-v6 clauses specify the IPv4 and IPv6 source address to be used for queries sent to remote servers, respectively. For an IPv4 remote server, only query-source can be specified. Similarly, for an IPv6 remote server, only query-source-v6 can be specified.

The request-nsid clause determines whether the local server will add a NSID EDNS option to requests sent to the server. This overrides request-nsid set at the view or option level.

The request-sit clause determines whether the local server will add a SIT EDNS option to requests sent to the server. This overrides request-sit set at the view or option level. Named may determine that SIT is not supported by the remote server and not add a SIT EDNS option to requests.

6.2.17. trusted-keys Statement Grammar

trusted-keys {
  ( domain_name flags protocol algorithm key_data ; )
    ...
} ;

6.2.18. trusted-keys Statement Definition and Usage

The trusted-keys statement defines DNSSEC security roots. DNSSEC is described in section_title. A security root is defined when the public key for a non-authoritative zone is known, but cannot be securely obtained through DNS, either because it is the DNS root zone or because its parent zone is unsigned. Once a key has been configured as a trusted key, it is treated as if it had been validated and proven secure. The resolver attempts DNSSEC validation on all DNS data in subdomains of a security root.

All keys (and corresponding zones) listed in trusted-keys are deemed to exist regardless of what parent zones say. Similarly for all keys listed in trusted-keys only those keys are used to validate the DNSKEY RRset. The parent's DS RRset will not be used.

The trusted-keys statement can contain multiple key entries, each consisting of the key's domain name, flags, protocol, algorithm, and the Base64 representation of the key data. Spaces, tabs, newlines and carriage returns are ignored in the key data, so the configuration may be split up into multiple lines.

trusted-keys may be set at the top level of named.conf or within a view. If it is set in both places, they are additive: keys defined at the top level are inherited by all views, but keys defined in a view are only used within that view.

6.2.19. managed-keys Statement Grammar

managed-keys {
  ( domain_name initial_key flags protocol algorithm key_data ; )
    ...
} ;

6.2.20. managed-keys Statement Definition and Usage

The managed-keys statement, like trusted-keys, defines DNSSEC security roots. The difference is that managed-keys can be kept up to date automatically, without intervention from the resolver operator.

Suppose, for example, that a zone's key-signing key was compromised, and the zone owner had to revoke and replace the key. A resolver which had the old key in a trusted-keys statement would be unable to validate this zone any longer; it would reply with a SERVFAIL response code. This would continue until the resolver operator had updated the trusted-keys statement with the new key.

If, however, the zone were listed in a managed-keys statement instead, then the zone owner could add a "stand-by" key to the zone in advance. named would store the stand-by key, and when the original key was revoked, named would be able to transition smoothly to the new key. It would also recognize that the old key had been revoked, and cease using that key to validate answers, minimizing the damage that the compromised key could do.

A managed-keys statement contains a list of the keys to be managed, along with information about how the keys are to be initialized for the first time. The only initialization method currently supported is initial-key. This means the managed-keys statement must contain a copy of the initializing key. (Future releases may allow keys to be initialized by other methods, eliminating this requirement.)

Consequently, a managed-keys statement appears similar to a trusted-keys, differing in the presence of the second field, containing the keyword initial-key. The difference is, whereas the keys listed in a trusted-keys continue to be trusted until they are removed from named.conf, an initializing key listed in a managed-keys statement is only trusted once: for as long as it takes to load the managed key database and start the RFC 5011 key maintenance process.

The first time named runs with a managed key configured in named.conf, it fetches the DNSKEY RRset directly from the zone apex, and validates it using the key specified in the managed-keys statement. If the DNSKEY RRset is validly signed, then it is used as the basis for a new managed keys database.

From that point on, whenever named runs, it sees the managed-keys statement, checks to make sure RFC 5011 key maintenance has already been initialized for the specified domain, and if so, it simply moves on. The key specified in the managed-keys statement is not used to validate answers; it has been superseded by the key or keys stored in the managed keys database.

The next time named runs after a name has been removed from the managed-keys statement, the corresponding zone will be removed from the managed keys database, and RFC 5011 key maintenance will no longer be used for that domain.

In the current implementation, the managed keys database is stored as a master-format zone file.

On servers which do not use views, this file is named managed-keys.loop. When views are in use, there will be a separate managed keys database for each view; the filename will be a hash of the view name followed by the suffix .mkeys.

When the key database is changed, the zone is updated. As with any other dynamic zone, changes will be written into a journal file, e.g., managed-keys.loop.jnl. Changes are committed to the master file as soon as possible afterward; this will usually occur within 30 seconds. So, whenever named is using automatic key maintenance, the zone file and journal file can be expected to exist in the working directory. (For this reason among others, the working directory should be always be writable by named.)

If the dnssec-validation option is set to auto, named will automatically initialize a managed key for the root zone. The key that is used to initialize the key maintenance process is built-in and can be overridden with the dnssec-keys-file option.

6.2.21. view Statement Grammar

view view_name [ class ] {
    match-clients { address_match_list } ;
    match-destinations { address_match_list } ;
    match-recursive-only yes_or_no ;
  [ view_option ; ... ]
  [ zone_statement ; ... ]
} ;

6.2.22. view Statement Definition and Usage

The view statement is a powerful feature of Loop that lets a name server answer a DNS query differently depending on who is asking. It is particularly useful for implementing split DNS setups without having to run multiple servers.

Each view statement defines a view of the DNS namespace that will be seen by a subset of clients. A client matches a view if its source IP address matches the address_match_list of the view's match-clients clause and its destination IP address matches the address_match_list of the view's match-destinations clause. If not specified, both match-clients and match-destinations default to matching all addresses. In addition to checking IP addresses match-clients and match-destinations can also take keys which provide an mechanism for the client to select the view. A view can also be specified as match-recursive-only, which means that only recursive requests from matching clients will match that view. The order of the view statements is significant — a client request will be resolved in the context of the first view that it matches.

Zones defined within a view statement will only be accessible to clients that match the view. By defining a zone of the same name in multiple views, different zone data can be given to different clients, for example, "internal" and "external" clients in a split DNS setup.

Many of the options given in the options statement can also be used within a view statement, and then apply only when resolving queries with that view. When no view-specific value is given, the value in the options statement is used as a default. Also, zone options can have default values specified in the view statement; these view-specific defaults take precedence over those in the options statement.

Views are class specific. If no class is given, class IN is assumed. Note that all non-IN views must contain a hint zone, since only the IN class has compiled-in default hints.

If there are no view statements in the config file, a default view that matches any client is automatically created in class IN. Any zone statements specified on the top level of the configuration file are considered to be part of this default view, and the options statement will apply to the default view. If any explicit view statements are present, all zone statements must occur inside view statements.

Here is an example of a typical split DNS setup implemented using view statements:

view "internal" {
      // This should match our internal networks.
      match-clients { 10.0.0.0/8; };

      // Provide recursive service to internal
      // clients only.
      recursion yes;

      // Provide a complete view of the example.com
      // zone including addresses of internal hosts.
      zone "example.com" {
        type master;
        file "example-internal.db";
      };
};

view "external" {
      // Match all clients not matched by the
      // previous view.
      match-clients { any; };

      // Refuse recursive service to external clients.
      recursion no;

      // Provide a restricted view of the example.com
      // zone containing only publicly accessible hosts.
      zone "example.com" {
       type master;
       file "example-external.db";
      };
};

6.2.23. zone Statement Grammar

zone zone_name [ class ] {
    type master ;
  [ allow-query { address_match_list } ; ]
  [ allow-query-on { address_match_list } ; ]
  [ allow-transfer { address_match_list } ; ]
  [ allow-update { address_match_list } ; ]
  [ update-check-ksk yes_or_no ; ]
  [ dnssec-dnskey-kskonly yes_or_no ; ]
  [ dnssec-loadkeys-interval number ; ]
  [ update-policy local | { update_policy_rule ; ...  } ; ]
  [ also-notify [ port ip_port ] [ dscp ip_dscp ] {
      ( masters_list | ip_addr [ port ip_port ] ) [ key key_name ] ;
    ...
    } ; ]
  [ check-names ( warn | fail | ignore ) ; ]
  [ check-mx ( warn | fail | ignore ) ; ]
  [ check-wildcard yes_or_no ; ]
  [ check-spf ( warn | ignore ); ]
  [ check-integrity yes_or_no ; ]
  [ dialup dialup_option ; ]
  [ file string ; ]
  [ journal string ; ]
  [ max-journal-size size_spec ; ]
  [ forward ( only | first ) ; ]
  [ forwarders { [ ip_addr [ port ip_port ] [ dscp ip_dscp ] ; ... ] } ; ]
  [ ixfr-from-differences yes_or_no ; ]
  [ max-ixfr-log-size number ; ]
  [ max-transfer-idle-out number ; ]
  [ max-transfer-time-out number ; ]
  [ notify yes_or_no | explicit | master-only ; ]
  [ notify-delay seconds ; ]
  [ notify-to-soa yes_or_no ; ]
  [ notify-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ notify-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ zone-statistics ( full | terse | none ) ; ]
  [ sig-validity-interval number [ number ] ; ]
  [ sig-signing-nodes number ; ]
  [ sig-signing-signatures number ; ]
  [ sig-signing-type number ; ]
  [ database string ; ]
  [ min-refresh-time number ; ]
  [ max-refresh-time number ; ]
  [ min-retry-time number ; ]
  [ max-retry-time number ; ]
  [ key-directory path_name ; ]
  [ auto-dnssec ( allow | maintain | off ) ; ]
  [ inline-signing yes_or_no ; ]
  [ zero-no-soa-ttl yes_or_no ; ]
  [ serial-update-method ( increment | unixtime ) ; ]
  [ max-zone-ttl number ; ]
} ;

zone zone_name [ class ] {
    type slave ;
  [ allow-notify { address_match_list } ; ]
  [ allow-query { address_match_list } ; ]
  [ allow-query-on { address_match_list } ; ]
  [ allow-transfer { address_match_list } ; ]
  [ allow-update-forwarding { address_match_list } ; ]
  [ dnssec-update-mode ( maintain | no-resign ); ]
  [ update-check-ksk yes_or_no ; ]
  [ dnssec-dnskey-kskonly yes_or_no ; ]
  [ dnssec-loadkeys-interval number ; ]
  [ dnssec-secure-to-insecure yes_or_no ; ]
  [ try-tcp-refresh yes_or_no ; ]
  [ also-notify [ port ip_port ] [ dscp ip_dscp ] {
      ( masters_list | ip_addr [ port ip_port ] ) [ key key_name ] ;
    ...
    } ; ]
  [ check-names ( warn | fail | ignore ) ; ]
  [ dialup dialup_option ; ]
  [ file string ; ]
  [ journal string ; ]
  [ max-journal-size size_spec ; ]
  [ forward ( only | first ) ; ]
  [ forwarders { [ ip_addr [ port ip_port ] [ dscp ip_dscp ] ; ... } ; ]
  [ ixfr-from-differences yes_or_no ; ]
  [ request-ixfr yes_or_no ; ]
  [ masters [ port ip_port ] [ dscp ip_dscp ] {
      ( masters_list | ip_addr [ port ip_port ] ) [ key key_name ] ;
    ...
    } ; ]
  [ max-ixfr-log-size number ; ]
  [ max-transfer-idle-in number ; ]
  [ max-transfer-idle-out number ; ]
  [ max-transfer-time-in number ; ]
  [ max-transfer-time-out number ; ]
  [ notify ( yes_or_no | explicit | master-only ) ; ]
  [ notify-delay seconds ; ]
  [ notify-to-soa yes_or_no ; ]
  [ transfer-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ transfer-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ alt-transfer-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ alt-transfer-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ use-alt-transfer-source yes_or_no ; ]
  [ notify-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ notify-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ zone-statistics ( full | terse | none ) ; ]
  [ sig-validity-interval number [ number ] ; ]
  [ sig-signing-nodes number ; ]
  [ sig-signing-signatures number ; ]
  [ sig-signing-type number ; ]
  [ database string ; ]
  [ min-refresh-time number ; ]
  [ max-refresh-time number ; ]
  [ min-retry-time number ; ]
  [ max-retry-time number ; ]
  [ key-directory path_name ; ]
  [ auto-dnssec ( allow | maintain | off ) ; ]
  [ inline-signing yes_or_no ; ]
  [ multi-master yes_or_no ; ]
  [ zero-no-soa-ttl yes_or_no ; ]
} ;

zone zone_name [ class ] {
    type hint;
    file string ;
  [ delegation-only yes_or_no ; ]
  [ check-names ( warn | fail | ignore ) ; ] // Not Implemented.
} ;

zone zone_name [ class ] {
    type stub;
  [ allow-query { address_match_list } ; ]
  [ allow-query-on { address_match_list } ; ]
  [ check-names ( warn | fail | ignore ) ; ]
  [ dialup dialup_option ; ]
  [ delegation-only yes_or_no ; ]
  [ file string ; ]
  [ forward ( only | first ) ; ]
  [ forwarders { [ ip_addr [ port ip_port ] [ dscp ip_dscp ] ; ... ] } ; ]
  [ masters [ port ip_port ] [ dscp ip_dscp ] {
      ( masters_list | ip_addr [ port ip_port ] ) [ key key_name ] ;
    ...
    } ; ]
  [ max-transfer-idle-in number ; ]
  [ max-transfer-time-in number ; ]
  [ transfer-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ transfer-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ alt-transfer-source ( ip4_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ alt-transfer-source-v6 ( ip6_addr | * )
      [ port ip_port ] [ dscp ip_dscp ] ; ]
  [ use-alt-transfer-source yes_or_no ; ]
  [ zone-statistics ( full | terse | none ) ; ]
  [ database string ; ]
  [ min-refresh-time number ; ]
  [ max-refresh-time number ; ]
  [ min-retry-time number ; ]
  [ max-retry-time number ; ]
  [ multi-master yes_or_no ; ]
} ;

zone zone_name [ class ] {
    type static-stub;
  [ allow-query { address_match_list } ; ]
  [ server-addresses { [ ip_addr ; ... } ; ]
  [ server-names { [ namelist ] } ; ]
  [ zone-statistics ( full | terse | none ) ; ]
} ;

zone zone_name [ class ] {
    type forward;
  [ forward ( only | first ) ; ]
  [ forwarders { [ ip_addr [ port ip_port ] [ dscp ip_dscp ] ; ... } ; ]
  [ delegation-only yes_or_no ; ]
} ;

zone "." [ class ] {
    type redirect;
    file string ;
  [ allow-query { address_match_list } ; ]
  [ max-zone-ttl number ; ]
} ;

zone zone_name [ class ] {
    type delegation-only;
} ;

zone zone_name [ class ] {
  [ in-view string ; ]
} ;

6.2.24. zone Statement Definition and Usage

6.2.24.1. Zone Types

The type keyword is required for the zone configuration unless it is an in-view configuration. Its acceptable values include: delegation-only, forward, hint, master, redirect, slave, static-stub, and stub.

master The server has a master copy of the data for the zone and will be able to provide authoritative answers for it.
slave A slave zone is a replica of a master zone. The masters list specifies one or more IP addresses of master servers that the slave contacts to update its copy of the zone. Masters list elements can also be names of other masters lists. By default, transfers are made from port 53 on the servers; this can be changed for all servers by specifying a port number before the list of IP addresses, or on a per-server basis after the IP address. Authentication to the master can also be done with per-server TSIG keys. If a file is specified, then the replica will be written to this file whenever the zone is changed, and reloaded from this file on a server restart. Use of a file is recommended, since it often speeds server startup and eliminates a needless waste of bandwidth. Note that for large numbers (in the tens or hundreds of thousands) of zones per server, it is best to use a two-level naming scheme for zone filenames. For example, a slave server for the zone example.com might place the zone contents into a file called ex/example.com where ex/ is just the first two letters of the zone name. (Most operating systems behave very slowly if you put 100000 files into a single directory.)
stub

A stub zone is similar to a slave zone, except that it replicates only the NS records of a master zone instead of the entire zone. Stub zones are not a standard part of the DNS; they are a feature specific to the Loop implementation.

Stub zones can be used to eliminate the need for glue NS record in a parent zone at the expense of maintaining a stub zone entry and a set of name server addresses in named.conf. This usage is not recommended for new configurations, and Loop supports it only in a limited way. If a Loop master serving a parent zone has child stub zones configured, all the slave servers for the parent zone also need to have the same child stub zones configured.

Stub zones can also be used as a way of forcing the resolution of a given domain to use a particular set of authoritative servers. For example, the caching name servers on a private network using RFC1918 addressing may be configured with stub zones for 10.in-addr.arpa to use a set of internal name servers as the authoritative servers for that domain.

static-stub

A static-stub zone is similar to a stub zone with the following exceptions: the zone data is statically configured, rather than transferred from a master server; when recursion is necessary for a query that matches a static-stub zone, the locally configured data (nameserver names and glue addresses) is always used even if different authoritative information is cached.

Zone data is configured via the server-addresses and server-names zone options.

The zone data is maintained in the form of NS and (if necessary) glue A or AAAA RRs internally, which can be seen by dumping zone databases by rndc dumpdb -all. The configured RRs are considered local configuration parameters rather than public data. Non recursive queries (i.e., those with the RD bit off) to a static-stub zone are therefore prohibited and will be responded with REFUSED.

Since the data is statically configured, no zone maintenance action takes place for a static-stub zone. For example, there is no periodic refresh attempt, and an incoming notify message will be rejected with an rcode of NOTAUTH.

Each static-stub zone is configured with internally generated NS and (if necessary) glue A or AAAA RRs

forward A "forward zone" is a way to configure forwarding on a per-domain basis. A zone statement of type forward can contain a forward and/or forwarders statement, which will apply to queries within the domain given by the zone name. If no forwarders statement is present or an empty list for forwarders is given, then no forwarding will be done for the domain, canceling the effects of any forwarders in the options statement. Thus if you want to use this type of zone to change the behavior of the global forward option (that is, "forward first" to, then "forward only", or vice versa, but want to use the same servers as set globally) you need to re-specify the global forwarders.
hint The initial set of root name servers is specified using a "hint zone". When the server starts up, it uses the root hints to find a root name server and get the most recent list of root name servers. If no hint zone is specified for class IN, the server uses a compiled-in default set of root servers hints. Classes other than IN have no built-in defaults hints.
redirect

Redirect zones are used to provide answers to queries when normal resolution would result in NXDOMAIN being returned. Only one redirect zone is supported per view. allow-query can be used to restrict which clients see these answers.

If the client has requested DNSSEC records (DO=1) and the NXDOMAIN response is signed then no substitution will occur.

To redirect all NXDOMAIN responses to 100.100.100.2 and 2001:ffff:ffff::100.100.100.2, one would configure a type redirect zone named ".", with the zone file containing wildcard records that point to the desired addresses: "*. IN A 100.100.100.2" and "*. IN AAAA 2001:ffff:ffff::100 .100.100.2".

To redirect all Spanish names (under .ES) one would use similar entries but with the names ".ES." instead of ".". To redirect all commercial Spanish names (under COM.ES) one would use wildcard entries called "*.COM.ES.".

Note that the redirect zone supports all possible types; it is not limited to A and AAAA records.

Because redirect zones are not referenced directly by name, they are not kept in the zone lookup table with normal master and slave zones. Consequently, it is not currently possible to use ``rndc reload

zonename`` to

reload a redirect zone. However, when using rndc reload without specifying a zone name, redirect zones will be reloaded along with other zones.

delegation-only

This is used to enforce the delegation-only status of infrastructure zones (e.g. COM, NET, ORG). Any answer that is received without an explicit or implicit delegation in the authority section will be treated as NXDOMAIN. This does not apply to the zone apex. This should not be applied to leaf zones.

delegation-only has no effect on answers received from forwarders.

See caveats in varlistentry_title.

6.2.24.2. Class

The zone's name may optionally be followed by a class. If a class is not specified, class IN (for Internet), is assumed. This is correct for the vast majority of cases.

The hesiod class is named for an information service from MIT's Project Athena. It is used to share information about various systems databases, such as users, groups, printers and so on. The keyword HS is a synonym for hesiod.

Another MIT development is Chaosnet, a LAN protocol created in the mid-1970s. Zone data for it can be specified with the CHAOS class.

6.2.24.3. Zone Options

allow-notify
See the description of allow-notify in section_title.
allow-query
See the description of allow-query in section_title.
allow-query-on
See the description of allow-query-on in section_title.
allow-transfer
See the description of allow-transfer in section_title.
allow-update
See the description of allow-update in section_title.
update-policy
Specifies a "Simple Secure Update" policy. See section_title.
allow-update-forwarding
See the description of allow-update-forwarding in section_title.
also-notify
Only meaningful if notify is active for this zone. The set of machines that will receive a DNS NOTIFY message for this zone is made up of all the listed name servers (other than the primary master) for the zone plus any IP addresses specified with also-notify. A port may be specified with each also-notify address to send the notify messages to a port other than the default of 53. A TSIG key may also be specified to cause the NOTIFY to be signed by the given key. also-notify is not meaningful for stub zones. The default is the empty list.
check-names
This option is used to restrict the character set and syntax of certain domain names in master files and/or DNS responses received from the network. The default varies according to zone type. For master zones the default is fail. For slave zones the default is warn. It is not implemented for hint zones.
check-mx
See the description of check-mx in section_title.
check-spf
See the description of check-spf in section_title.
check-wildcard
See the description of check-wildcard in section_title.
check-integrity
See the description of check-integrity in section_title.
check-sibling
See the description of check-sibling in section_title.
zero-no-soa-ttl
See the description of zero-no-soa-ttl in section_title.
update-check-ksk
See the description of update-check-ksk in section_title.
dnssec-loadkeys-interval
See the description of dnssec-loadkeys-interval in section_title.
dnssec-update-mode
See the description of dnssec-update-mode in section_title.
dnssec-dnskey-kskonly
See the description of dnssec-dnskey-kskonly in section_title.
try-tcp-refresh
See the description of try-tcp-refresh in section_title.
database

Specify the type of database to be used for storing the zone data. The string following the database keyword is interpreted as a list of whitespace-delimited words. The first word identifies the database type, and any subsequent words are passed as arguments to the database to be interpreted in a way specific to the database type.

The default is "rbt", Loop's native in-memory red-black-tree database. This database does not take arguments.

Other values are possible if additional database drivers have been linked into the server. Some sample drivers are included with the distribution but none are linked in by default.

dialup
See the description of dialup in section_title.
delegation-only

The flag only applies to forward, hint and stub zones. If set to yes, then the zone will also be treated as if it is also a delegation-only type zone.

See caveats in varlistentry_title.

file
Set the zone's filename. In master, hint, and redirect zones which do not have masters defined, zone data is loaded from this file. In slave, stub, and redirect zones which do have masters defined, zone data is retrieved from another server and saved in this file. This option is not applicable to other zone types.
forward
Only meaningful if the zone has a forwarders list. The only value causes the lookup to fail after trying the forwarders and getting no answer, while first would allow a normal lookup to be tried.
forwarders
Used to override the list of global forwarders. If it is not specified in a zone of type forward, no forwarding is done for the zone and the global options are not used.
journal
Allow the default journal's filename to be overridden. The default is the zone's filename with ".jnl" appended. This is applicable to master and slave zones.
max-journal-size
See the description of max-journal-size in section_title.
max-records
See the description of max-records in section_title.
max-transfer-time-in
See the description of max-transfer-time-in in section_title.
max-transfer-idle-in
See the description of max-transfer-idle-in in section_title.
max-transfer-time-out
See the description of max-transfer-time-out in section_title.
max-transfer-idle-out
See the description of max-transfer-idle-out in section_title.
notify
See the description of notify in section_title.
notify-delay
See the description of notify-delay in section_title.
notify-to-soa
See the description of notify-to-soa in section_title.
zone-statistics
See the description of zone-statistics in section_title.
server-addresses

Only meaningful for static-stub zones. This is a list of IP addresses to which queries should be sent in recursive resolution for the zone. A non empty list for this option will internally configure the apex NS RR with associated glue A or AAAA RRs.

For example, if "example.com" is configured as a static-stub zone with 192.0.2.1 and 2001:db8::1234 in a server-addresses option, the following RRs will be internally configured.

example.com. NS example.com.
example.com. A 192.0.2.1
example.com. AAAA 2001:db8::1234

These records are internally used to resolve names under the static-stub zone. For instance, if the server receives a query for "www.example.com" with the RD bit on, the server will initiate recursive resolution and send queries to 192.0.2.1 and/or 2001:db8::1234.

server-names

Only meaningful for static-stub zones. This is a list of domain names of nameservers that act as authoritative servers of the static-stub zone. These names will be resolved to IP addresses when named needs to send queries to these servers. To make this supplemental resolution successful, these names must not be a subdomain of the origin name of static-stub zone. That is, when "example.net" is the origin of a static-stub zone, "ns.example" and "master.example.com" can be specified in the server-names option, but "ns.example.net" cannot, and will be rejected by the configuration parser.

A non empty list for this option will internally configure the apex NS RR with the specified names. For example, if "example.com" is configured as a static-stub zone with "ns1.example.net" and "ns2.example.net" in a server-names option, the following RRs will be internally configured.

example.com. NS ns1.example.net.
example.com. NS ns2.example.net.

These records are internally used to resolve names under the static-stub zone. For instance, if the server receives a query for "www.example.com" with the RD bit on, the server initiate recursive resolution, resolve "ns1.example.net" and/or "ns2.example.net" to IP addresses, and then send queries to (one or more of) these addresses.

sig-validity-interval
See the description of sig-validity-interval in section_title.
sig-signing-nodes
See the description of sig-signing-nodes in section_title.
sig-signing-signatures
See the description of sig-signing-signatures in section_title.
sig-signing-type
See the description of sig-signing-type in section_title.
transfer-source
See the description of transfer-source in section_title.
transfer-source-v6
See the description of transfer-source-v6 in section_title.
alt-transfer-source
See the description of alt-transfer-source in section_title.
alt-transfer-source-v6
See the description of alt-transfer-source-v6 in section_title.
use-alt-transfer-source
See the description of use-alt-transfer-source in section_title.
notify-source
See the description of notify-source in section_title.
notify-source-v6
See the description of notify-source-v6 in section_title.
min-refresh-time; max-refresh-time; min-retry-time; max-retry-time
See the description in section_title.
ixfr-from-differences
See the description of ixfr-from-differences in section_title. (Note that the ixfr-from-differences master and slave choices are not available at the zone level.)
key-directory
See the description of key-directory in section_title.
auto-dnssec
See the description of auto-dnssec in section_title.
serial-update-method
See the description of serial-update-method in section_title.
inline-signing
If yes, this enables "bump in the wire" signing of a zone, where a unsigned zone is transferred in or loaded from disk and a signed version of the zone is served, with possibly, a different serial number. This behaviour is disabled by default.
multi-master
See the description of multi-master in section_title.
max-zone-ttl
See the description of max-zone-ttl in section_title.
dnssec-secure-to-insecure
See the description of dnssec-secure-to-insecure in section_title.

6.2.24.4. Dynamic Update Policies

Loop supports two alternative methods of granting clients the right to perform dynamic updates to a zone, configured by the allow-update and update-policy option, respectively.

The allow-update clause is a simple access control list. Any client that matches the ACL is granted permission to update any record in the zone.

The update-policy clause allows more fine-grained control over what updates are allowed. It specifies a set of rules, in which each rule either grants or denies permission for one or more names in the zone to be updated by one or more identities. Identity is determined by the key that signed the update request using either TSIG or SIG(0). In most cases, update-policy rules only apply to key-based identities. There is no way to specify update permissions based on client source address.

update-policy rules are only meaningful for zones of type master, and are not allowed in any other zone type. It is a configuration error to specify both allow-update and update-policy at the same time.

A pre-defined update-policy rule can be switched on with the command update-policy local;. Using this in a zone causes named to generate a TSIG session key when starting up and store it in a file; this key can then be used by local clients to update the zone while named is running. By default, the session key is stored in the file /var/run/loop/session.key, the key name is "local-ddns", and the key algorithm is HMAC-SHA256. These values are configurable with the session-keyfile, session-keyname and session-keyalg options, respectively. A client running on the local system, if run with appropriate permissions, may read the session key from the key file and use it to sign update requests. The zone's update policy will be set to allow that key to change any record within the zone. Assuming the key name is "local-ddns", this policy is equivalent to:

update-policy { grant local-ddns zonesub any; };

...with the additional restriction that only clients connecting from the local system will be permitted to send updates.

Note that only one session key is generated by named; all zones configured to use update-policy local will accept the same key.

The command nsupdate -l implements this feature, sending requests to localhost and signing them using the key retrieved from the session key file.

Other rule definitions look like this:

( grant | deny ) identity ruletype  name   types

Each rule grants or denies privileges. Rules are checked in the order in which they are specified in the update-policy statement. Once a message has successfully matched a rule, the operation is immediately granted or denied, and no further rules are examined. There are 13 types of rules; the rule type is specified by the ruletype field, and the interpretation of other fields varies depending on the rule type.

In general, a rule is matched when the key that signed an update request matches the identity field, the name of the record to be updated matches the name field (in the manner specified by the ruletype field), and the type of the record to be updated matches the types field. Details for each rule type are described below.

The identity field must be set to a fully-qualified domain name. In most cases, this represensts the name of the TSIG or SIG(0) key that must be used to sign the update request. If the specified name is a wildcard, it is subject to DNS wildcard expansion, and the rule may apply to multiple identities. When a TKEY exchange has been used to create a shared secret, the identity of the key used to authenticate the TKEY exchange will be used as the identity of the shared secret. Some rule types use indentities matching the client's Kerberos principal (e.g, "host/machine@REALM") or Windows realm (machine$@REALM).

The name field also specifies a fully-qualified domain name. This often represents the name of the record to be updated. Interpretation of this field is dependent on rule type.

If no types are explicitly specified, then a rule matches all types except RRSIG, NS, SOA, NSEC and NSEC3. Types may be specified by name, including "ANY" (ANY matches all types except NSEC and NSEC3, which can never be updated). Note that when an attempt is made to delete all records associated with a name, the rules are checked for each existing record type.

The ruletype field has 13 values: name, subdomain, wildcard, self, selfsub, selfwild, tcp-self, 6to4-self, zonesub, and external.

name Exact-match semantics. This rule matches when the name being updated is identical to the contents of the name field.
subdomai n This rule matches when the name being updated is a subdomain of, or identical to, the contents of the name field.
``zonesub` `

This rule is similar to subdomain, except that it matches when the name being updated is a subdomain of the zone in which the update-policy statement appears. This obviates the need to type the zone name twice, and enables the use of a standard update-policy statement in multiple zones without modification.

When this rule is used, the name field is omitted.

``wildcard `` The name field is subject to DNS wildcard expansion, and this rule matches when the name being updated is a valid expansion of the wildcard.
self

This rule matches when the name of the record being updated matches the contents of the identity field. The name field is ignored. To avoid confusion, it is recommended that this field be set to the same value as the identity field or to "."

The self rule type is most useful when allowing one key per name to update, where the key has the same name as the record to be updated. In this case, the identity field can be specified as * (an asterisk).

``selfsub` ` This rule is similar to self except that subdomains of self can also be updated.
``selfwild `` This rule is similar to self except that only subdomains of self can be updated.
``tcp-self ``

This rule allows updates that have been sent via TCP and for which the standard mapping from the client's IP address into the in-addr.arpa and ip6.arpa namespaces match the name to be updated. The identity field must match that name. The name field should be set to ".". Note that, since identity is based on the client's IP address, it is not necessary for update request messages to be signed.

Note

It is theoretically possible to spoof these TCP sessions.

6to4-sel f

This allows the name matching a 6to4 IPv6 prefix, as specified in RFC 3056, to be updated by any TCP connection from either the 6to4 network or from the corresponding IPv4 address. This is intended to allow NS or DNAME RRsets to be added to the ip6.arpa reverse tree.

The identity field must match the 6to4 prefix in ip6.arpa. The name field should be set to ".". Note that, since identity is based on the client's IP address, it is not necessary for update request messages to be signed.

In addition, if specified for an ip6.arpa name outside of the 2.0.0.2.ip6.arpa namespace, the corresponding /48 reverse name can be updated. For example, TCP/IPv6 connections from 2001:DB8:ED0C::/48 can update records at C.0.D.E.8.B.D.0.1.0.0.2.ip6.arpa.

Note

It is theoretically possible to spoof these TCP sessions.

``external ``

This rule allows named to defer the decision of whether to allow a given update to an external daemon.

The method of communicating with the daemon is specified in the identity field, the format of which is "local:path", where path is the location of a UNIX-domain socket. (Currently, "local" is the only supported mechanism.)

Requests to the external daemon are sent over the UNIX-domain socket as datagrams with the following format:

Protocol version number (4 bytes, network byte o
rder, currently 1)
Request length (4 bytes, network byte order) Signer (null-terminated string) Name (null-terminated string) TCP source address (null-terminated string) Rdata type (null-terminated string) Key (null-terminated string) TKEY token length (4 bytes, network byte order) TKEY token (remainder of packet)

The daemon replies with a four-byte value in network byte order, containing either 0 or 1; 0 indicates that the specified update is not permitted, and 1 indicates that it is.

6.2.24.5. Multiple views

When multiple views are in use, a zone may be referenced by more than one of them. Often, the views will contain different zones with the same name, allowing different clients to receive different answers for the same queries. At times, however, it is desirable for multiple views to contain identical zones. The in-view zone option provides an efficient way to do this: it allows a view to reference a zone that was defined in a previously configured view. Example:

view internal {
    match-clients { 10/8; };

    zone example.com {
    type master;
    file "example-external.db";
    };
};

view external {
    match-clients { any; };

    zone example.com {
    in-view internal;
    };
};

An in-view option cannot refer to a view that is configured later in the configuration file.

A zone statement which uses the in-view option may not use any other options with the exception of forward and forwarders. (These options control the behavior of the containing view, rather than changing the zone object itself.)

Zone level acls (e.g. allow-query, allow-transfer) and other configuration details of the zone are all set in the view the referenced zone is defined in. Care need to be taken to ensure that acls are wide enough for all views referencing the zone.

An in-view zone cannot be used as a response policy zone.

An in-view zone is not intended to reference a forward zone.