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man pages section 1M: System Administration Commands

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Updated: July 2017



nsupdate - Dynamic DNS update utility


nsupdate [-dv] [-y keyname:secret | -k keyfile] [-t timeout]
     [-u udptimeout] [-r udpretries] [filename]


The nsupdate utility submits Dynamic DNS Update requests as defined in RFC 2136 to a name server. This utility allows resource records to be added or removed from a zone without manually editing the zone file. A single update request can contain requests to add or remove more than one resource record.

Zones that are under dynamic control with nsupdate or a DHCP server 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 must be in the same zone. Requests are sent to the zone's master servers identified by the MNAME field of the zone's SOA record.

Transaction signatures can be used to authenticate the Dynamic DNS updates using the TSIG resource record type described in RFC 2845. The signatures rely on a shared secret that should only be known to nsupdate and the name server. Currently, the only supported encryption algorithm for TSIG is HMAC-MD5, which is defined in RFC 2104. Once other algorithms are defined for TSIG, applications will need to ensure that they select the appropriate algorithm as well as the key when authenticating each other. For instance, suitable key and server statements would be added to /etc/named.conf 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. The nsupdate utility does not read /etc/named.conf.

The nsupdate utility uses the –y or –k option to provide the shared secret needed to generate a TSIG record for authenticating Dynamic DNS update requests. These options are mutually exclusive. See OPTIONS.


The following options are supported:


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

–k keyfile

Read the shared secret from the file keyfile, whose name is of the form K{name}.+157.+{random}.private. For historical reasons, the file K{name}.+157.+{ random}.key must also be present.

–r udpretries

Set the number of UDP retries. The default is 3 retries. If udpretries is set to zero, only one update request is made.

–t timeout

Set timeout interval in seconds before update is aborted. The default is 300 seconds. A setting of zero disables the timeout.

–u udptimeout

Set interval in seconds between UDP retires, the default is 3 seconds. A setting of zero causes the interval to be calculated based on the timeout (–t) and the number of UDP retries (–r).


Use a TCP connection. Using a TCP connection could be preferable when a batch of update requests is made. By default, nsupdate uses UDP to send update requests to the name server.

–y keyname:secret

Generate a signature from keyname:secret, wherekeyname is the name of the key and secret is the base64 encoded shared secret.

Use of the –y option is discouraged because the shared secret is supplied as a command line argument in clear text and could be visible in the output from ps(1) or in a history file maintained by the user's shell.


The nsupdate utility reads input from filename or the 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 condition 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 Dynamic DNS update request to the name server.

The command formats and their meaning are as follows:

server servername [ port ]

Send all dynamic update requests to the name server servername. When no server statement is provided, nsupdate sends updates to the master server of the correct zone. The MNAME field of that zone's SOA record identifies the master server for that zone. The port argument 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 ]

Send all dynamic update requests using the local address. When no local statement is provided, nsupdate sends updates using an address and port chosen by the system. The port argument can also be used to make requests come from a specific port. If no port number is specified, the system assigns one.

zone zonename

Specify that all updates are to be made to the zone zonename. If no zone statement is provided, nsupdate attempts to 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.

key name secret

Specify that all updates are to be TSIG signed using the name secret pair. The key command overrides any key specified on the command line with –y or –k.

prereq nxdomain domain-name

Require that no resource record of any type exists withthe name domain-name.

prereq yxdomain domain-name

Require that domain-name exists (has as at least one resource record, of any type).

prereq nxrrset domain-name [ class ] type

Require that no resource record exists of the specified type, class and domain-name. If class is omitted, IN (internet) is assumed.

prereq yxrrset domain-name [ class ] type

Require that a resource record of the specified type, class and domain-name must exist. If class is omitted, IN (internet) is assumed.

prereq yxrrset domain-name [ class ] type data...

The data from each set of prerequisites of this form sharing a common type, class, and domain-name are combined to form a set of RRs. This set of RRs must exactly match the set of RRs existing in the zone at the given type, class, and domain-name. The data are written in the standard text representation of the resource record's RDATA.

update delete domain-name [ ttl ] [ class ] [ type [ data... ] ]

Delete any resource records named domain-name. If type and data are provided, only matching resource records are removed. The internet class is assumed if class is not supplied. The ttl is ignored, and is only provided for compatibility.

update add domain-name ttl [ class ] type data...

Add a new resource record with the specified ttl, class and data.


Display 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.

Lines beginning with a semicolon are comments and are ignored.


Example 1 Inserting and Deleting Resource Records from the Zone

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. An A record for newhost.example.com with IP address is added. The newly-added record has a 1 day TTL (86400 seconds).

Example 2 Adding CNAME Only If No Records Exist

The following command adds a CNAME only if no records already exist for it.

# 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 action 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 4035 to allow CNAMEs to have RSIG, DNSKEY, and NSEC records.)



used to identify default name server


base-64 encoding of HMAC-MD5 key created by dnssec-keygen(1M).


base-64 encoding of HMAC-MD5 key created by dnssec-keygen(1M)


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 could change in future releases.


See attributes(5) for descriptions of the following attributes:

Interface Stability

See Also

named(1M), dnssec-keygen(1M), attributes(5)

RFC 2136, RFC 3007, RFC 2104, RFC 2845, RFC 1034, RFC 2535, RFC 2931, RFC 4035