Net::DNS(3)

NAME
Net::DNS - Perl interface to the Domain Name System

SYNOPSIS
    use Net::DNS;

DESCRIPTION
Net::DNS 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 somewhat familiar with the format of a DNS packet and its various sections. See RFC 1035 or DNS and BIND (Albitz & Liu) for details.

  Resolver Objects
A resolver object is an instance of the Net::DNS::Resolver class. A program can 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 queries return Net::DNS::Packet objects. Packet objects have five sections:

•   The header section, a Net::DNS::Header object.

•   The question section, a list of Net::DNS::Question objects.

•   The answer section, a list of Net::DNS::RR objects.

•   The authority section, a list of Net::DNS::RR objects.

•   The additional section, a list of Net::DNS::RR objects.

  Update Objects
The Net::DNS::Update package is a subclass of Net::DNS::Packet for creating packet objects to be used in dynamic updates.

  Header Objects
Net::DNS::Header objects represent the header section of a DNS packet.

  Question Objects
Net::DNS::Question objects represent the question section of a DNS packet.

  RR Objects
Net::DNS::RR is the base class for DNS resource record (RR) objects in the answer, authority, and additional sections of a DNS packet.

Don't assume that RR objects will be of the type you requested -- always check an RR object's type before calling any of its methods.

METHODS
See the manual pages listed above for other class-specific methods.

  version
    print Net::DNS->version, "\n";

Returns the version of Net::DNS.

  mx
    # Use a default resolver -- can't get an error string this way.
    use Net::DNS;
    my @mx = mx("example.com");

    # Use your own resolver object.
    use Net::DNS;
    my $res = Net::DNS::Resolver->new;
    my  @mx = mx($res, "example.com");

Returns a list of Net::DNS::RR::MX 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 records were found.

This method does not look up A records -- it only performs MX queries.

See "EXAMPLES" for a more complete example.

  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 A"));

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

    # RRset exists (value-dependent)
    $packet->push(pre => yxrrset("host.example.com A 10.1.2.3"));

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

Returns a "Net::DNS::RR" object or "undef" if the object couldn't be created.

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

    $packet->push(pre => nxrrset("host.example.com A"));

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

Returns a "Net::DNS::RR" object or "undef" if the object couldn't be created.

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

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

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

Returns a "Net::DNS::RR" object or "undef" if the object couldn't be created.

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

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

Meaning: No RR with the specified name can exist.

Returns a "Net::DNS::RR" object or "undef" if the object couldn't be created.

  rr_add
Use this method to add RRs to a zone.

    $packet->push(update => rr_add("host.example.com A 10.1.2.3"));

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" object or "undef" if the object couldn't be created.

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

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

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

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

Meaning: Delete all RRs having the specified name.

    # Delete an RR.
    $packet->push(update => rr_del("host.example.com A 10.1.2.3"));

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

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

Returns a "Net::DNS::RR" object or "undef" if the object couldn't be created.

Zone Serial Number Management
The Net::DNS module provides auxiliary functions which support policy-driven zone serial numbering regimes.

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

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

  Time Encoded
    $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.

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

The 32 bit value returned by the auxiliary 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
As of version 0.55 there is functionality to help you sort RR arrays. 'rrsort()' is the function that is available to do the sorting. In most cases rrsort will give you the answer that you want but you can specify your own sorting method by using the Net::DNS::RR::FOO->set_rrsort_func() class method. See Net::DNS::RR for details.

rrsort()

   use Net::DNS qw(rrsort);

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

rrsort() selects all RRs from the input array that are of the type that are defined in 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 specifically defined for certain attributes. If such sorting function is defined in the code (it can be set or overwritten using the set_rrsort_func() class method) that function is used.

For instance:
   my @prioritysorted=rrsort("SRV","priority",@rr_array); returns the SRV records sorted from lowest to heighest priority and for equal priorities from heighes to lowes 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 does not exist for a certain RR than the RRs are sorted on string comparrisson of the rdata.

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

rrsort() returns a sorted array with only elements of the specified RR type or undef.

rrsort() returns undef when arguments are incorrect.

EXAMPLES
The following examples show how to use the "Net::DNS" modules. See the other manual pages and the demo scripts included with the source code for additional examples.

See the "Net::DNS::Update" manual page for an example of performing dynamic updates.

  Look up a host's addresses.
  use Net::DNS;
  my $res   = Net::DNS::Resolver->new;
  my $query = $res->search("host.example.com");

  if ($query) {
      foreach my $rr ($query->answer) {
          next unless $rr->type eq "A";
          print $rr->address, "\n";
      }
  } else {
      warn "query failed: ", $res->errorstring, "\n";
  }

  Find the nameservers for a domain.
  use Net::DNS;
  my $res   = Net::DNS::Resolver->new;
  my $query = $res->query("example.com", "NS");

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

  Find the MX records for a domain.
  use Net::DNS;
  my $name = "example.com";
  my $res  = Net::DNS::Resolver->new;
  my @mx   = mx($res, $name);

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

  Print a domain's SOA record in zone file format.
  use Net::DNS;
  my $res   = Net::DNS::Resolver->new;
  my $query = $res->query("example.com", "SOA");

  if ($query) {
      ($query->answer)[0]->print;
  } else {
      print "query failed: ", $res->errorstring, "\n";
  }

  Perform a zone transfer and print all the records.
  use Net::DNS;
  my $res  = Net::DNS::Resolver->new;
  $res->nameservers("ns.example.com");

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

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

  Perform a background query and do some other work while waiting for the answer.
  use Net::DNS;
  my $res    = Net::DNS::Resolver->new;
  my $socket = $res->bgsend("host.example.com");

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

  my $packet = $res->bgread($socket);
  $packet->print;

  Send a background query and use select to determine when the answer has arrived.
  use Net::DNS;
  use IO::Select;

  my $timeout = 5;
  my $res     = Net::DNS::Resolver->new;
  my $bgsock  = $res->bgsend("host.example.com");
  my $sel     = IO::Select->new($bgsock);

  # Add more sockets to $sel if desired.
  my @ready = $sel->can_read($timeout);
  if (@ready) {
      foreach my $sock (@ready) {
          if ($sock == $bgsock) {
              my $packet = $res->bgread($bgsock);
              $packet->print;
              $bgsock = undef;
          }
          # Check for the other sockets.
          $sel->remove($sock);
          $sock = undef;
      }
  } else {
      warn "timed out after $timeout seconds\n";
  }

BUGS
"Net::DNS" 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)1997-2002 Michael Fuhr. Portions Copyright(c)2002-2004 Chris Reinhardt. Portions Copyright(c)2005 Olaf Kolkman (RIPE NCC) Portions Copyright(c)2006 Olaf Kolkman (NLnet Labs)

All rights reserved.

This program is free software; you may redistribute it and/or modify it under the same terms as Perl itself.

AUTHOR INFORMATION
Net::DNS is currently maintained at NLnet Labs (www.nlnetlabs.nl) by:
        Olaf Kolkman
    olaf@net-dns.org

Between 2002 and 2004 Net::DNS was maintained by:
       Chris Reinhardt

Net::DNS was created by:
    Michael Fuhr
    mike@fuhr.org

For more information see:
    http://www.net-dns.org/

Stay tuned and syndicate:
    http://www.net-dns.org/blog/

SEE ALSO
perl, Net::DNS::Resolver, Net::DNS::Packet, Net::DNS::Update, Net::DNS::Header, Net::DNS::Question, Net::DNS::RR, RFC 1035, DNS and BIND by Paul Albitz & Cricket Liu