Section: (8) Updated: Jun 30, 2000


nsupdate - Dynamic DNS update utility


nsupdate [ -d ] [ [ -y keyname:secret ] [ -k keyfile ] ] [ -v ] [ filename ]


nsupdate is used to submit Dynamic DNS Update requests as defined in
RFC2136 to a name server. This 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 via 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
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.

The -d option makes nsupdate operate in debug mode. This provides
tracing information about the update requests that are made and the
replies received from the name server.

Transaction signatures can be used to authenticate the Dynamic DNS
updates. These use the TSIG resource record type described in RFC2845.
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 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. nsupdate does not
read /etc/named.conf.

nsupdate 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. With the -k option, nsupdate reads
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. When the -y option is
used, a signature is generated from keyname:secret. keyname 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. This may be visible in the output
from ps(1) or in a history file maintained by the user's shell.

By default nsupdate uses UDP to send update requests to the name server.
The -v option makes nsupdate use a TCP connection. This may be
preferable when a batch of update requests is made.


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 Dynamic DNS update request to the
name server.

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.

key name secret

Specifies that all updates are to be TSIG signed using the keyname
keysecret pair. The key command overrides any key specified on the
command line via -y or -k.

prereq nxdomain domain-name

Requires that no resource record of any type exists with name

prereq yxdomain domain-name

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

prereq nxrrset domain-name [ class ] type

Requires 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

This requires 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... ] ]

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

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

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


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.

Lines beginning with a semicolon are comments, and are ignored.


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

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

Any A records for oldhost.example.com are deleted. and an A record for
newhost.example.com it 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

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 RFC1034 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 RFC2535 to allow CNAMEs to have SIG, KEY and NXT records.)



used to identify default name server


base-64 encoding of HMAC-MD5 key created by dnssec-keygen(8).


base-64 encoding of HMAC-MD5 key created by dnssec-keygen(8).


RFC2136, RFC3007, RFC2104, RFC2845, RFC1034, RFC2535, named(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.