4.22. nsupdate - DNS dynamic update utility

4.22.1. Synopsis

nsupdate [-d] [-D] [-i] [-L level] [[-g] | [-o] | [-l] | [-y [hmac:]keyname:secret] | [-k keyfile]] [-t timeout] [-u <udp-timeout>] [-r <udp-retries>] [-v] [-T] [-P] [-V] [filename]

4.22.2. Description

nsupdate is used to submit DNS UPDATE requests (RFC 2136) to a nameserver. DNS UPDATE allows resource records to be added or removed from a zone dynamically without manually editing the zone file. A single DNS UPDATE request can contain requests to add or remove more than one resource record.

Master files of zones that are under dynamic control via nsupdate should not be edited by hand. Manual edits could conflict with dynamic updates and cause data to be lost.

The resource records that are dynamically added or removed with nsupdate have to be in the same zone. Requests are sent to the zone's master server. This is identified by the MNAME field of the zone's SOA record.

Transaction signatures can be used to authenticate the DNS UPDATE messages. These use TSIG signatures (RFC 2845) or SIG(0) signatures (RFC 2535 and RFC 2931).

TSIG relies on a shared secret that should only be known to nsupdate and the name server. For instance, suitable key and server statements would be added to named.conf(5) so that the name server can associate the appropriate secret key and algorithm with the IP address of the client application that will be using TSIG authentication. You can use ddns-confgen(1) to generate suitable configuration fragments. nsupdate uses the -y or -k options to provide the TSIG shared secret. These options are mutually exclusive.

SIG(0) uses public key cryptography. To use a SIG(0) key, the public key must be stored in a KEY record in a zone served by the name server.

4.22.3. Options


Debug mode. This provides tracing information about the update requests that are made and the replies received from the name server.


Extra debug mode.


Force interactive mode, even when standard input is not a terminal.

-k <keyfile>

The file containing the TSIG authentication key. Keyfiles may be in two formats: a single file containing a named.conf(5) format key statement, which may be generated automatically by ddns-confgen(1), or a pair of files whose names are of the format Kname.+157.+random.key and Kname.+157.+random.private, which can be generated by dnssec-keygen(1). The -k may also be used to specify a SIG(0) key used to authenticate Dynamic DNS update requests. In this case, the key specified is not an HMAC-MD5 key.


Local-host only mode. This sets the server address to localhost (disabling the server so that the server address cannot be overridden). Connections to the local server will use a TSIG key found in /var/run/loop/session.key, which is automatically generated by named(8) if any local master zone has set update-policy to local. The location of this key file can be overridden with the -k option.

-L <level>

Set the logging debug level. If <level> is 0, logging is disabled.

-p <port>

Set the port to use for connections to a name server. The default is 53.


Print the list of private Loop-specific resource record types whose format is understood by nsupdate. See also the -T option.

-r <udp-retries>

The number of UDP retries. The default is 3. If <udp-retries> is 0, only 1 update request will be made.

-t <timeout>

The maximum time an update request can take before it is aborted. The default is 300 seconds. 0 can be used to disable the timeout.


Print the list of IANA standard resource record types whose format is understood by nsupdate. nsupdate will exit after the lists are printed. The -T option can be combined with the -P option.

Other types can be entered using the "TYPE<XXXXX>" syntax where "XXXXX" is the decimal value of the type with no leading zeros. The rdata, if present, will be parsed using the unknown RDATA format (RFC 3597).

-u <udp-timeout>

The UDP retry interval. The default is 3 seconds. If 0 is passed, the interval will be computed from the timeout interval and number of UDP retries.


Use TCP even for small update requests. By default, nsupdate uses UDP to send update requests to the name server unless they are too large to fit in a UDP request in which case TCP will be used. TCP may be preferable when a batch of update requests is made.


Print the program's version number and exit.

-y [<hmac>:]<keyname>:<secret>

Sign queries using TSIG with the authentication key specified directly as an argument. <keyname> is the name of the key, and <secret> is the Base64 encoded shared secret. <hmac> is the name of the key algorithm; valid choices are hmac-md5, hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384, or hmac-sha512. If <hmac> is not specified, the default is hmac-sha256.


You should use the -k option and avoid the -y option, because with -y the shared secret is supplied as a command line argument in clear text. This may be visible in the history file maintained by the user's shell and by other means.


TODOMUKS: Check the supported HMAC algorithms.

4.22.4. Input commands

nsupdate reads input from <filename> or standard input. Each command is supplied on exactly one line of input. Some commands are for administrative purposes. The others are either update instructions or prerequisite checks on the contents of the zone. These checks set conditions that some name or set of resource records (RRset) either exists or is absent from the zone. These conditions must be met if the entire update request is to succeed. Updates will be rejected if the tests for the prerequisite conditions fail.

Every update request consists of zero or more prerequisites and zero or more updates. This allows a suitably authenticated update request to proceed if some specified resource records are present or missing from the zone. A blank input line (or the send command) causes the accumulated commands to be sent as one DNS UPDATE request to the name server.

Lines beginning with a semicolon are comments and are ignored.

The command formats and their meaning are as follows:

server <servername> [<port>]

Sends all dynamic update requests to the name server <servername>. When no server statement is provided, nsupdate will send updates to the master server of the correct zone. The MNAME field of that zone's SOA record will identify the master server for that zone. <port> is the port number on <servername> where the dynamic update requests get sent. If no port number is specified, the default DNS port number of 53 is used.

local <address> [<port>]

Sends all dynamic update requests using the local <address>. When no local statement is provided, nsupdate will send updates using an address and port chosen by the system. <port> can additionally be used to make requests come from a specific port. If no port number is specified, the system will assign one.

zone <zonename>

Specifies that all updates are to be made to the zone <zonename>. If no zone statement is provided, nsupdate will attempt determine the correct zone to update based on the rest of the input.

class <classname>

Specify the default class. If no <class> is specified, the default class is IN.

ttl <seconds>

Specify the default TTL for records to be added. The value none will clear the default TTL.

key [<hmac>:]<keyname> <secret>

Specifies that all updates are to be TSIG-signed using the <keyname> <secret> pair. If <hmac> is specified, then it sets the signing algorithm to use. The default is hmac-sha256. The key statement overrides any key specified on the command line via -y or -k.

prereq nxdomain <owner-name>

Requires that no resource record of any type exists with name <owner-name>.

prereq yxdomain <owner-name>

Requires that <owner-name> exists (has as at least one resource record, of any type).

prereq nxrrset <owner-name> [<class>] <type>

Requires that a resource record does not exist of the specified <type>, <class> and <owner-name>. If <class> is omitted, IN is assumed.

prereq yxrrset <owner-name> [<class>] <type> [<rdata>]

If <rdata> is not specified, this statement requires that a resource record does exist of the specified <type>, <class> and <owner-name>. If <class> is omitted, IN is assumed.

If <rdata> is specified, the <data> field from each set of prerequisites of this form sharing a common <type>, <class>, and <owner-name> are combined to form an RRSet. This RRSet must exactly match the RRSet existing in the zone for the given <type>, <class>, and <owner-name>. The <rdata> fields are written in the presentation format of the resource record's RDATA.

update del[ete] <owner-name> [<ttl>] [<class>] [<type> [<rdata>]]

Deletes any resource records named <owner-name>. If <type> and/or <rdata> are provided, only matching resource records will be removed. If <class> is not supplied, it defaults to IN. The <rdata> fields are written in the presentation format of the resource record's RDATA. The <ttl> argument is ignored, and is only allowed for compatibility.

update add <owner-name> <ttl> [<class>] <type> [<rdata>]

Adds a new resource record with the specified <owner-name>, <ttl>, <class> and <rdata>. If <class> is not supplied, it defaults to IN. The <rdata> fields are written in the presentation format of the resource record's RDATA.


Displays the current message, containing all of the prerequisites and updates specified since the last send.


Sends the current message. This is equivalent to entering a blank line.


Displays the answer.


Turn on debugging.


Print the program's version number.


Print a list of supported commands.

4.22.5. Examples

The examples below show how nsupdate could be used to insert and delete resource records from the example.com zone. Notice that the input in each example contains a trailing blank line so that a group of commands are sent as one dynamic update request to the master name server for example.com.

# nsupdate
> update delete oldhost.example.com A
> update add newhost.example.com 86400 A
> send

Any A records for oldhost.example.com are deleted. And an A record for newhost.example.com with IP address is added. The newly-added record has a 1 day TTL (86400 seconds).

# nsupdate
> prereq nxdomain nickname.example.com
> update add nickname.example.com 86400 CNAME somehost.example.com
> send

The prerequisite condition gets the name server to check that there are no resource records of any type for nickname.example.com. If there are, the update request fails. If this name does not exist, a CNAME for it is added. This ensures that when the CNAME is added, it cannot conflict with the long-standing rule in RFC 1034 that a name must not exist as any other record type if it exists as a CNAME. (The rule has been updated for DNSSEC in RFC 2535 to allow CNAMEs to have RRSIG, DNSKEY and NSEC records.)

4.22.6. Files

/etc/resolv.conf:: used to identify default name server

/var/run/loop/session.key:: sets the default TSIG key for use in local-only mode

Kname.+157.+random.key:: Base64 encoding of HMAC-MD5 key created by dnssec-keygen(8).

Kname.+157.+random.private:: Base64 encoding of HMAC-MD5 key created by dnssec-keygen(8).

The TSIG key is redundantly stored in two separate files. This is a consequence of nsupdate using the DST library for its cryptographic operations, and may change in future releases.


TODOMUKS: Some of the description about files needs to be revisited.

4.22.7. See also

ddns-confgen(1), named.conf(5)