14.3.1 Resolver Objects

Before making any queries, you must first create a resolver object:

$res = new Net::DNS::Resolver;

Resolver objects are initialized from your resolv.conf file, but you can change the default settings by making calls to the object's methods. Many of the methods described in the Net::DNS::Resolver manual page correspond to fields and options of the _res structure described earlier in the C programming section. For example, if you want to set the number of times the resolver tries each query before timing out, you can call the $res->retry method:

$res->retry(2);

To make a query, call one of the following methods:

$res->search
$res->query
$res->send

These methods behave like the res_search, res_query, and res_send library functions described in the C programming section, though they take fewer arguments. You must provide a name, and you can optionally provide a DNS record type and class (the default behavior is to query for A records in the IN class). These methods return Net::DNS::Packet objects, which we'll describe shortly. Here are a few examples:

$packet = $res->search("terminator");
$packet = $res->query("movie.edu", "MX");
$packet = $res->send("version.bind", "TXT", "CH");

14.3.2 Packet Objects

Resolver queries return Net::DNS::Packet objects, whose methods you can use to access the header, question, answer, authority, and additional sections of a DNS packet:

$header     = $packet->header;
@question   = $packet->question;
@answer     = $packet->answer;
@authority  = $packet->authority;
@additional = $packet->additional;

14.3.3 Header Objects

DNS packet headers are returned as Net::DNS::Header objects. The methods described in the Net::DNS::Header manual page correspond to the header fields described in RFC 1035 and in the HEADER structure used in C programs. For example, if you want to find out if this is an authoritative answer, you would call the $header->aa method:

if ($header->aa) {
    print "answer is authoritative\n";
} else {
    print "answer is not authoritative\n";
}

14.3.4 Question Objects

The question section of a DNS packet is returned as a list of Net::DNS::Question objects. You can find the name, type, and class of a question object with the following methods:

$question->qname
$question->qtype
$question->qclass

14.3.5 Resource Record Objects

The answer, authority, and additional sections of a DNS packet are returned as lists of Net::DNS::RR objects. You can find the name, type, class, and TTL of an RR object with the following methods:

$rr->name
$rr->type
$rr->class
$rr->ttl

Each record type is a subclass of Net::DNS::RR and has its own type-specific methods. Here's an example that shows how to get the preference and mail exchange out of an MX record:

$preference = $rr->preference;
$exchange   = $rr->exchange;

14.3.6 A Perl Version of check_soa

Now that we've described the objects Net::DNS uses, let's look at how to use them in a complete program. We've rewritten check_soa in Perl:

#!/usr/local/bin/perl -w

use Net::DNS;

#----------------------------------------------------------------------
# Get the domain from the command line.
#----------------------------------------------------------------------

die "Usage:  check_soa domain\n" unless @ARGV == 1;
$domain = $ARGV[0];

#----------------------------------------------------------------------
# Find all the nameservers for the domain.
#----------------------------------------------------------------------

$res = new Net::DNS::Resolver;

$res->defnames(0);
$res->retry(2);

$ns_req = $res->query($domain, "NS");
die "No nameservers found for $domain: ", $res->errorstring, "\n"
    unless defined($ns_req) and ($ns_req->header->ancount > 0);

@nameservers = grep { $_->type eq "NS" } $ns_req->answer;

#----------------------------------------------------------------------
# Check the SOA record on each nameserver.
#----------------------------------------------------------------------

$| = 1;
$res->recurse(0);

foreach $nsrr (@nameservers) {

  #------------------------------------------------------------------
  # Set the resolver to query this nameserver.
  #------------------------------------------------------------------

  $ns = $nsrr->nsdname;
  print "$ns ";

  unless ($res->nameservers($ns)) {
      warn ": can't find address: ", $res->errorstring, "\n";
      next;
  }

  #------------------------------------------------------------------
  # Get the SOA record.
  #------------------------------------------------------------------

  $soa_req = $res->send($domain, "SOA");
  unless (defined($soa_req)) {
      warn ": ", $res->errorstring, "\n";
      next;
  }

  #------------------------------------------------------------------
  # Is this nameserver authoritative for the domain?
  #------------------------------------------------------------------

  unless ($soa_req->header->aa) {
      warn "is not authoritative for $domain\n";
      next;
  }

  #------------------------------------------------------------------
  # We should have received exactly one answer.
  #------------------------------------------------------------------

  unless ($soa_req->header->ancount == 1) {
      warn ": expected 1 answer, got ",
            $soa_req->header->ancount, "\n";
      next;
  }

  #------------------------------------------------------------------
  # Did we receive an SOA record?
  #------------------------------------------------------------------

   unless (($soa_req->answer)[0]->type eq "SOA") {
       warn ": expected SOA, got ",
            ($soa_req->answer)[0]->type, "\n";
       next;
  }

  #------------------------------------------------------------------
  # Print the serial number.
  #------------------------------------------------------------------

  print "has serial number ", ($soa_req->answer)[0]->serial, "\n";
}

Now that you've seen how to write a DNS program using a shell script, a Perl script, and C code, you should be able to write one on your own using the language that best fits your situation.