# phpman > perldoc > Net::DNS

## NAME
    [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown) - Perl Interface to the Domain Name System

## SYNOPSIS
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);

## DESCRIPTION
    [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown) is a collection of Perl modules that act as a Domain Name System (DNS) resolver. It
    allows the programmer to perform DNS queries that are beyond the capabilities of "gethostbyname"
    and "gethostbyaddr".

    The programmer should be familiar with the structure of a DNS packet. See RFC 1035 or DNS and
    BIND (Albitz & Liu) for details.

### Resolver Objects
    A resolver object is an instance of the [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown) class. A program may have multiple
    resolver objects, each maintaining its own state information such as the nameservers to be
    queried, whether recursion is desired, etc.

### Packet Objects
    [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown) queries return [Net::DNS::Packet](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3APacket/markdown) objects. A packet object has five sections:

    *  header, represented by a [Net::DNS::Header](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AHeader/markdown) object

    *  question, a list of no more than one [Net::DNS::Question](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AQuestion/markdown) object

    *  answer, a list of [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) objects

    *  authority, a list of [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) objects

    *  additional, a list of [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) objects

### Update Objects
    [Net::DNS::Update](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AUpdate/markdown) is a subclass of [Net::DNS::Packet](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3APacket/markdown) useful for creating dynamic update requests.

### Header Object
    The [Net::DNS::Header](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AHeader/markdown) object mediates access to the header data which resides within the
    corresponding [Net::DNS::Packet](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3APacket/markdown).

### Question Object
    The [Net::DNS::Question](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AQuestion/markdown) object represents the content of the question section of the DNS packet.

  RR Objects
    [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) is the base class for DNS resource record (RR) objects in the answer, authority,
    and additional sections of a DNS packet.

    Do not assume that RR objects will be of the type requested. The type of an RR object must be
    checked before calling any methods.

## METHODS
    [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown) exports methods and auxilliary functions to support DNS updates, zone serial number
    management, and simple DNS queries.

  version
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);
        print [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown)->version, "\n";

    Returns the version of [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown).

  rr
        # Use a default resolver -- can not get an error string this way.
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);
        my @rr = rr("example.com");
        my @rr = rr("example.com", "AAAA");
        my @rr = rr("example.com", "AAAA", "IN");

        # Use your own resolver object.
        my $res = [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown)->new;
        my @rr  = rr($res, "example.com" ... );

        my ($ptr) = rr("2001:[DB8::dead](https://www.chedong.com/phpMan.php/perldoc/DB8%3A%3Adead/markdown):beef");

    The "rr()" method provides simple RR lookup for scenarios where the full flexibility of [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown)
    is not required.

    Returns a list of [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) objects for the specified name or an empty list if the query
    failed or no record was found.

    See "EXAMPLES" for more complete examples.

  mx
        # Use a default resolver -- can not get an error string this way.
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);
        my @mx = mx("example.com");

        # Use your own resolver object.
        my $res = [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown)->new;
        my @mx  = mx($res, "example.com");

    Returns a list of [Net::DNS::RR::MX](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR%3A%3AMX/markdown) objects representing the MX records for the specified name.
    The list will be sorted by preference. Returns an empty list if the query failed or no MX record
    was found.

    This method does not look up address records; it resolves MX only.

## Dynamic DNS Update Support
    The [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown) module provides auxilliary functions which support dynamic DNS update requests.

        $update = [Net::DNS::Update](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AUpdate/markdown)->new( 'example.com' );

        $update->push( prereq => nxrrset('example.com. AAAA') );
        $update->push( update => rr_add('example.com. 86400 AAAA [2001::DB8::F00](https://www.chedong.com/phpMan.php/perldoc/2001%3A%3ADB8%3A%3AF00/markdown)') );

  yxrrset
    Use this method to add an "RRset exists" prerequisite to a dynamic update packet. There are two
    forms, value-independent and value-dependent:

        # RRset exists (value-independent)
        $update->push( pre => yxrrset("host.example.com AAAA") );

    Meaning: At least one RR with the specified name and type must exist.

        # RRset exists (value-dependent)
        $update->push( pre => yxrrset("host.example.com AAAA 2001:[DB8::1](https://www.chedong.com/phpMan.php/perldoc/DB8%3A%3A1/markdown)") );

    Meaning: At least one RR with the specified name and type must exist and must have matching
    data.

    Returns a [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) object or "undef" if the object could not be created.

  nxrrset
    Use this method to add an "RRset does not exist" prerequisite to a dynamic update packet.

        $update->push( pre => nxrrset("host.example.com AAAA") );

    Meaning: No RRs with the specified name and type can exist.

    Returns a [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) object or "undef" if the object could not be created.

  yxdomain
    Use this method to add a "name is in use" prerequisite to a dynamic update packet.

        $update->push( pre => yxdomain("host.example.com") );

    Meaning: At least one RR with the specified name must exist.

    Returns a [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) object or "undef" if the object could not be created.

  nxdomain
    Use this method to add a "name is not in use" prerequisite to a dynamic update packet.

        $update->push( pre => nxdomain("host.example.com") );

    Meaning: No RR with the specified name can exist.

    Returns a [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) object or "undef" if the object could not be created.

  rr_add
    Use this method to add RRs to a zone.

        $update->push( update => rr_add("host.example.com AAAA 2001:[DB8::c001](https://www.chedong.com/phpMan.php/perldoc/DB8%3A%3Ac001/markdown):a1e") );

    Meaning: Add this RR to the zone.

    RR objects created by this method should be added to the "update" section of a dynamic update
    packet. The TTL defaults to 86400 seconds (24 hours) if not specified.

    Returns a [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) object or "undef" if the object could not be created.

  rr_del
    Use this method to delete RRs from a zone. There are three forms: delete all RRsets, delete an
    RRset, and delete a specific RR.

        # Delete all RRsets.
        $update->push( update => rr_del("host.example.com") );

    Meaning: Delete all RRs having the specified name.

        # Delete an RRset.
        $update->push( update => rr_del("host.example.com AAAA") );

    Meaning: Delete all RRs having the specified name and type.

        # Delete a specific RR.
        $update->push( update => rr_del("host.example.com AAAA 2001:[DB8::dead](https://www.chedong.com/phpMan.php/perldoc/DB8%3A%3Adead/markdown):beef") );

    Meaning: Delete the RR which matches the specified argument.

    RR objects created by this method should be added to the "update" section of a dynamic update
    packet.

    Returns a [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) object or "undef" if the object could not be created.

## Zone Serial Number Management
    The [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown) module provides auxilliary functions which support policy-driven zone serial
    numbering regimes.

        $soa->serial(SEQUENTIAL);
        $soa->serial(YYYMMDDxx);

  SEQUENTIAL
        $successor = $soa->serial( SEQUENTIAL );

    The existing serial number is incremented modulo 2**32.

  UNIXTIME
        $successor = $soa->serial( UNIXTIME );

    The Unix time scale will be used as the basis for zone serial numbering. The serial number will
    be incremented if the time elapsed since the previous update is less than one second.

  YYYYMMDDxx
        $successor = $soa->serial( YYYYMMDDxx );

    The 32 bit value returned by the auxilliary "YYYYMMDDxx()" function will be used as the base for
    the date-coded zone serial number. Serial number increments must be limited to 100 per day for
    the date information to remain useful.

## Sorting of RR arrays
    "rrsort()" provides functionality to help you sort RR arrays. In most cases this will give you
    the result that you expect, but you can specify your own sorting method by using the
    "[Net::DNS::RR::FOO](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR%3A%3AFOO/markdown)->set_rrsort_func()" class method. See [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown) for details.

  rrsort
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);

        my @sorted = rrsort( $rrtype, $attribute, @rr_array );

    "rrsort()" selects all RRs from the input array that are of the type defined by the first
    argument. Those RRs are sorted based on the attribute that is specified as second argument.

    There are a number of RRs for which the sorting function is defined in the code.

    For instance:

        my @prioritysorted = rrsort( "SRV", "priority", @rr_array );

    returns the SRV records sorted from lowest to highest priority and for equal priorities from
    highest to lowest weight.

    If the function does not exist then a numerical sort on the attribute value is performed.

        my @portsorted = rrsort( "SRV", "port", @rr_array );

    If the attribute is not defined then either the "default_sort()" function or "canonical sorting"
    (as defined by DNSSEC) will be used.

    "rrsort()" returns a sorted array containing only elements of the specified RR type. Any other
    RR types are silently discarded.

    "rrsort()" returns an empty list when arguments are incorrect.

## EXAMPLES
    The following brief examples illustrate some of the features of [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown). The documentation for
    individual modules and the demo scripts included with the distribution provide more extensive
    examples.

    See [Net::DNS::Update](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AUpdate/markdown) for an example of performing dynamic updates.

### Look up host addresses.
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);
        my $res   = [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown)->new;
        my $reply = $res->search("www.example.com", "AAAA");

        if ($reply) {
            foreach my $rr ($reply->answer) {
                print $rr->address, "\n" if $rr->can("address");
            }
        } else {
            warn "query failed: ", $res->errorstring, "\n";
        }

### Find the nameservers for a domain.
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);
        my $res   = [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown)->new;
        my $reply = $res->query("example.com", "NS");

        if ($reply) {
            foreach $rr (grep { $_->type eq "NS" } $reply->answer) {
                print $rr->nsdname, "\n";
            }
        } else {
            warn "query failed: ", $res->errorstring, "\n";
        }

### Find the MX records for a domain.
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);
        my $name = "example.com";
        my $res  = [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown)->new;
        my @mx   = mx($res, $name);

        if (@mx) {
            foreach $rr (@mx) {
                print $rr->preference, "\t", $rr->exchange, "\n";
            }
        } else {
            warn "Can not find MX records for $name: ", $res->errorstring, "\n";
        }

### Print domain SOA record in zone file format.
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);
        my $res   = [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown)->new;
        my $reply = $res->query("example.com", "SOA");

        if ($reply) {
            foreach my $rr ($reply->answer) {
                $rr->print;
            }
        } else {
            warn "query failed: ", $res->errorstring, "\n";
        }

### Perform a zone transfer and print all the records.
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);
        my $res  = [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown)->new;
        $res->[tcp_timeout(20)](https://www.chedong.com/phpMan.php/man/tcptimeout/20/markdown);
        $res->nameservers("ns.example.com");

        my @zone = $res->axfr("example.com");

        foreach $rr (@zone) {
            $rr->print;
        }

        warn $res->errorstring if $res->errorstring;

### Perform a background query and print the reply.
        use [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown);
        my $res    = [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown)->new;
        $res->[udp_timeout(10)](https://www.chedong.com/phpMan.php/man/udptimeout/10/markdown);
        $res->[tcp_timeout(20)](https://www.chedong.com/phpMan.php/man/tcptimeout/20/markdown);
        my $socket = $res->bgsend("host.example.com", "AAAA");

        while ( $res->bgbusy($socket) ) {
            # do some work here while waiting for the response
            # ...and some more here
        }

        my $packet = $res->bgread($socket);
        if ($packet) {
            $packet->print;
        } else {
            warn "query failed: ", $res->errorstring, "\n";
        }

## BUGS
    [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown) is slow.

    For other items to be fixed, or if you discover a bug in this distribution please use the CPAN
    bug reporting system.

## COPYRIGHT
    Copyright (c)1997-2000 Michael Fuhr.

    Portions Copyright (c)2002,2003 Chris Reinhardt.

    Portions Copyright (c)2005 Olaf Kolkman (RIPE NCC)

    Portions Copyright (c)2006 Olaf Kolkman (NLnet Labs)

    Portions Copyright (c)2014 Dick Franks

    All rights reserved.

## LICENSE
    Permission to use, copy, modify, and distribute this software and its documentation for any
    purpose and without fee is hereby granted, provided that the original copyright notices appear
    in all copies and that both copyright notice and this permission notice appear in supporting
    documentation, and that the name of the author not be used in advertising or publicity
    pertaining to distribution of the software without specific prior written permission.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING
    BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
    NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
    DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

## AUTHOR INFORMATION
    [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown) is maintained at NLnet Labs (www.nlnetlabs.nl) by Willem Toorop.

    Between 2005 and 2012 [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown) was maintained by Olaf Kolkman.

    Between 2002 and 2004 [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown) was maintained by Chris Reinhardt.

    [Net::DNS](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS/markdown) was created in 1997 by Michael Fuhr.

## SEE ALSO
    perl, [Net::DNS::Resolver](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AResolver/markdown), [Net::DNS::Question](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AQuestion/markdown), [Net::DNS::RR](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3ARR/markdown), [Net::DNS::Packet](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3APacket/markdown), [Net::DNS::Update](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3ADNS%3A%3AUpdate/markdown),
    RFC1035, <<http://www.net-dns.org/>>, *DNS and BIND* by Paul Albitz & Cricket Liu