3. Operation

In this chapter we provide some suggested configurations along with guidelines for their use. We suggest reasonable values for certain option settings.

3.1. Sample Configurations

3.1.1. A Caching-only Name Server

The following sample configuration is appropriate for a caching-only name server for use by clients internal to a corporation. All queries from outside clients are refused using the allow-query option. Alternatively, the same effect could be achieved using suitable firewall rules.

// Two corporate subnets we wish to allow queries from.
acl corpnets { 192.168.4.0/24; 192.168.7.0/24; };
options {
     // Working directory
     directory "/etc/namedb";

     allow-query { corpnets; };
};
// Provide a reverse mapping for the loopback
// address 127.0.0.1
zone "0.0.127.in-addr.arpa" {
     type master;
     file "localhost.rev";
     notify no;
};

3.1.2. An Authoritative-only Name Server

This sample configuration is for an authoritative-only server that is the master server for "example.com" and a slave for the subdomain "eng.example.com".

options {
     // Working directory
     directory "/etc/namedb";
     // Do not allow access to cache
     allow-query-cache { none; };
     // This is the default
     allow-query { any; };
     // Do not provide recursive service
     recursion no;
};

// Provide a reverse mapping for the loopback
// address 127.0.0.1
zone "0.0.127.in-addr.arpa" {
     type master;
     file "localhost.rev";
     notify no;
};
// We are the master server for example.com
zone "example.com" {
     type master;
     file "example.com.db";
     // IP addresses of slave servers allowed to
     // transfer example.com
     allow-transfer {
      192.168.4.14;
      192.168.5.53;
     };
};
// We are a slave server for eng.example.com
zone "eng.example.com" {
     type slave;
     file "eng.example.com.bk";
     // IP address of eng.example.com master server
     masters { 192.168.4.12; };
};

3.2. Load Balancing

A primitive form of load balancing can be achieved in the DNS by using multiple records (such as multiple A records) for one name.

For example, if you have three WWW servers with network addresses of 10.0.0.1, 10.0.0.2 and 10.0.0.3, a set of records such as the following means that clients will connect to each machine one third of the time:

Name TTL CLASS TYPE Resource Record (RR) Data
www 60 0 IN A 10.0.0.1
  60 0 IN A 10.0.0.2
  60 0 IN A 10.0.0.3

When a resolver queries for these records, Loop will rotate them and respond to the query with the records in a different order. In the example above, clients will randomly receive records in the order 1, 2, 3; 2, 3, 1; and 3, 1, 2. Most clients will use the first record returned and discard the rest.

For more detail on ordering responses, check the rrset-order sub-statement in the options statement, see RRset Ordering.

3.3. Name Server Operations

3.3.1. Tools for Use With the Name Server Daemon

This section describes several indispensable diagnostic, administrative and monitoring tools available to the system administrator for controlling and debugging the name server daemon.

3.3.1.1. Diagnostic Tools

The dig and host programs are command line tools for manually querying name servers. They differ in style and output format.

dig

dig is the most versatile and complete of these lookup tools. It has two modes: simple interactive mode for a single query, and batch mode which executes a query for each in a list of several query lines. All query options are accessible from the command line.

dig @ server domain query-type query-class + query-option - dig-option % comment The usual simple use of dig will take the form

dig @server domain query-type query-class

For more information and a list of available commands and options, see the dig man page.

host

The host utility emphasizes simplicity and ease of use. By default, it converts between host names and Internet addresses, but its functionality can be extended with the use of options.

host -aCdlnrsTwv -c class -N ndots -t type -W timeout -R retries -m flag -4 -6 hostname server For more information and a list of available commands and options, see the host man page.

3.3.1.2. Administrative Tools

Administrative tools play an integral part in the management of a server.

named-checkconf

The named-checkconf program checks the syntax of a named.conf file.

named-checkconf -jvz -t directory filename

named-checkzone

The named-checkzone program checks a master file for syntax and consistency.

named-checkzone -djqvD -c class -o output -t directory -w directory -k (ignore|warn|fail) -n (ignore|warn|fail) -W (ignore|warn) zone filename

named-compilezone
Similar to named-checkzone, but it always dumps the zone content to a specified file (typically in a different format).
rndc

The remote name daemon control (rndc) program allows the system administrator to control the operation of a name server. If you run rndc without any options it will display a usage message as follows:

rndc -c config -s server -p port -y key command command See ??? for details of the available rndc commands.

rndc requires a configuration file, since all communication with the server is authenticated with digital signatures that rely on a shared secret, and there is no way to provide that secret other than with a configuration file. The default location for the rndc configuration file is /etc/loop/rndc.conf, but an alternate location can be specified with the -c option. If the configuration file is not found, rndc will also look in /etc/loop/rndc.key. The rndc.key file is generated by running rndc-confgen -a as described in section_title.

The format of the configuration file is similar to that of named.conf, but limited to only four statements, the options, key, server and include statements. These statements are what associate the secret keys to the servers with which they are meant to be shared. The order of statements is not significant.

The options statement has three clauses: default-server, default-key, and default-port. default-server takes a host name or address argument and represents the server that will be contacted if no -s option is provided on the command line. default-key takes the name of a key as its argument, as defined by a key statement. default-port specifies the port to which rndc should connect if no port is given on the command line or in a server statement.

The key statement defines a key to be used by rndc when authenticating with named. Its syntax is identical to the key statement in named.conf. The keyword key is followed by a key name, which must be a valid domain name, though it need not actually be hierarchical; thus, a string like "rndc_key" is a valid name. The key statement has two clauses: algorithm and secret. While the configuration parser will accept any string as the argument to algorithm, currently only the strings "hmac-md5", "hmac-sha1", "hmac-sha224", "hmac-sha256", "hmac-sha384" and "hmac-sha512" have any meaning. The secret is a Base64 encoded string as specified in RFC 3548.

The server statement associates a key defined using the key statement with a server. The keyword server is followed by a host name or address. The server statement has two clauses: key and port. The key clause specifies the name of the key to be used when communicating with this server, and the port clause can be used to specify the port rndc should connect to on the server.

A sample minimal configuration file is as follows:

key rndc_key {
     algorithm "hmac-sha256";
     secret
       "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K";
};
options {
     default-server 127.0.0.1;
     default-key    rndc_key;
};

This file, if installed as /etc/loop/rndc.conf, would allow the command:

$rndc reload

to connect to 127.0.0.1 port 953 and cause the name server to reload, if a name server on the local machine were running with following controls statements:

controls {
    inet 127.0.0.1
        allow { localhost; } keys { rndc_key; };
};

and it had an identical key statement for rndc_key.

Running the rndc-confgen program will conveniently create a rndc.conf file for you, and also display the corresponding controls statement that you need to add to named.conf. Alternatively, you can run rndc-confgen -a to set up a rndc.key file and not modify named.conf at all.

3.3.2. Signals

Certain UNIX signals cause the name server to take specific actions, as described in the following table. These signals can be sent using the kill command.

SIGHUP Causes the server to read named.conf and reload the database.
SIGTERM Causes the server to clean up and exit.
SIGINT Causes the server to clean up and exit.