nsupdate(1)

NAME

nsupdate — Dynamic DNS update utility

SYNOPSIS

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

DESCRIPTION

nsupdate submits Dynamic DNS update requests to a name server, as defined in RFC 2136. This allows resource records to be added to 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.

Transaction signatures can be used to authenticate the Dynamic DNS updates. These use the TSIG resource record type described in RFC 2845 or the SIG(0) record described in RFC 2535 and RFC 2931. TSIG relies 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. 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. nsupdate does not read /etc/named.conf.

Options

-d

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

Provide the shared secret needed to generate a TSIG record for authenticating Dynamic DNS update requests. With this option, nsupdate reads the shared secret from the file keyfile, whose name is of the form

Kname.+157.+random.private

For historical reasons, the file Kname.+157.+random.key must also be present. This option is mutually exclusive with the -y option. -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.

-r udpretries

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

-t timeout

Set the maximum time in seconds a update request can take before it is aborted. The default is 300 seconds. Zero can be used to disable the timeout.

-u udptimeout

Set the UDP retry interval in seconds. The default is 3 seconds. If set to zero the interval will be computed from the timeout interval and number of UDP retries.

-v

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

-y keyname:secret

Generate a signature from keyname and secret. keyname is the name of the key, and secret is the base-64-encoded shared secret. The 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.

Operands

filename: A file of nsupdate commands, as described in the Input Format section. The default is standard input.

Input Format

nsupdate reads commands from filename or standard input. Each command is supplied on exactly one line of input. Some commands are for administrative purposes; others are either update instructions or prerequisite checks on the contents of the zone. The 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. The send command or a blank input line causes the accumulated commands to be sent as one Dynamic DNS update request to the name server.

The Commands

The command formats and their meaning are as follows:

;: Lines beginning with a semicolon are comments and are ignored.
server servername [port]: Sends 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. port is the port number on servername where the dynamic update requests are 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 sends 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 assigns one.
zone zonename: Specifies 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: Specifies the default class. If no class is specified, the default class is IN.
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 with -y or -k.
prereq nxdomain domain-name: Requires that no resource record of any type exists with name domain-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: 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 (resource records). 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 class type data...: Deletes any resource records named domain-name. If type and data are provided, only matching resource records will be removed. The Internet class (IN) is assumed if class is not supplied.
update add domain-name ttl class type data...: Adds a new resource record with the specified ttl, class and data.
show: Displays the current message, containing all the prerequisites and updates specified since the last send operation.
send: Sends the current message. This is equivalent to entering a blank line.
answer: Displays the answer.

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.

Example 1

#  nsupdate
> update delete oldhost.example.com A
> update add newhost.example.com 86400 A 172.16.1.1
>

Any A records for oldhost.example.com are deleted. An A record for newhost.example.com with IP address 172.16.1.1 is added. The newly-added record has a 1 day TTL (86400 seconds)

Example 2

# nsupdate
> prereq nxdomain nickname.example.com
> update add nickname.example.com 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 record 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 CNAME records to have RRSIG, DNSKEY, and NSEC records.)

WARNINGS

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.

AUTHOR

nsupdate was developed by the Internet Systems Consortium (ISC).

FILES

/etc/resolv.conf: Used to identify default name server.
Kname.+157.+random.key: Base-64 encoding of HMAC-MD5 key created by dnssec-keygen.
Kname.+157.+random.private: Base-64 encoding of HMAC-MD5 key created by dnssec-keygen.