Perl - Net::DNS to create PTR record - perl

Question

First, the relevant xkcd comic: http://xkcd.com/979/
Next, the 10-year old thread on PerlMonks: http://www.perlmonks.org/?node_id=210422
Basically, I'm failing in my attempts to use Net::DNS::Update to create a PTR record and I'd like to see how others have manged this.
Below is what I'm trying. $hst is the hostname that I already have an A record for. $rev is the backwards IP address in-addr.arpa thingy.
# Create the update packet:
my $update = Net::DNS::Update->new($OURDOMAIN);
# Add the PTR record:
$update->push(update => rr_add("$rev 3600 PTR $hst"));
# Send the update to the zone's primary master.
my $res = Net::DNS::Resolver->new;
$res->nameservers("$OURNMSERV");

Answer 1

If $OURDOMAIN is your main domain name, you need to know that you can't put PTR records into your own domain, they have to be put in the right .in-addr.arpa zone.
That zone is most likely being run by your ISP, and they're unlikely to support dynamic updates from end users.

Answer 2

Another ten years later, http://xkcd.com/979/ is still relevant. Having read the question and answer and also the linked PerlMonks thread, it was still unclear to me how to properly add PTR records using Net::DNS.
Well, 'when in Rome do as the Romans do' the saying goes. Having noticed that nsupdate can add PTR records to our internal DNS system just fine without me specifying the reverse zone, I was trying to find out what it does to send valid updates. Turned out it actually sends a SOA query first to figure out what the reverse zone is. Copying this behavior seems to be a somewhat robust way to add PTR records via Net::DNS:
use Net::DNS;
my $hostname = 'somehost.example.com.';
my $ip = '10.11.12.13';
my $primary_ns = 'primary.example.com';
# First, construct the reverse name based on the IP address
my #octets = split /\./, $ip;
my #rev_octs = reverse(#octets);
my $rev_name = join(".", #rev_octs) . '.in-addr.arpa.';
# Then, query the SOA record for the constructed reverse name
my $resolver = Net::DNS::Resolver->new();
$resolver->nameservers($primary_ns);
# Note: Need to use 'send' rather than 'query' since we are interested
# in the 'authority section' of the reply only.
my $packet = $resolver->send($rev_name, 'SOA');
my $zone;
if ($packet) {
# We expect only one authority record
my $len = $packet->authority;
if ($len == 0) {
# Server might not be authorative for reverse lookup zone.
warn "No authority section in reply.\n";
exit 1;
}
my $auth = ($packet->authority)[0];
$zone = $auth->name;
} else {
warn 'query failed: ', $resolver->errorstring, "\n";
exit 1;
}
# Armed with this information, we can finally add the PTR records
my $update = Net::DNS::Update->new($zone);
# Make sure that the PTR record does not already exist
$update->push( pre => nxrrset("$rev_name PTR") );
$update->push( update => rr_add("$rev_name 1200 PTR $hostname") );
my $reply = $resolver->send($update);
if ($reply) {
if ($reply->header->rcode eq 'NOERROR') {
print "Successfully added PTR record!\n";
} else {
print 'Update failed: ', $reply->header->rcode, "\n";
}
} else {
print 'Update failed: ', $resolver->errorstring, "\n";
}