Re^2: Retrieving all hostnames for an IP address...

There is one and only one PTR record for a given IP

Not on this planet.

use strict;
use Socket;

my $addr = shift || '172.17.0.1';
my ($name,$aliases) = gethostbyaddr(inet_aton($addr), AF_INET);

print "address $addr has the following names:\n";
print "\t$_\n" for ($name, split / /, $aliases);
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When run against my DNS server, produces:

address 172.17.0.1 has the following names:
        host1.example.com
        host2.example.com
        host3.example.com
        host4.example.com
        host5.example.com
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In other words, a PTR lookup can return a result set of more than one record. Many buggy applications make the invalid assumption that no more than one result will be returned. But of course, no Perlmonk ever would :)

gethostbyaddr is a bit naughty by referring to one as the name, and the others, aliases. According the the RFC (if I remember correctly, it's been a while), they are all equally "at the same level". It's up to the application to sift through them and find the one that suits its needs.

• another intruder with the mooring in the heart of the Perl

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Re^3: Retrieving all hostnames for an IP address... by Fletch (Bishop) on Dec 13, 2006 at 20:07 UTC
Technically yes DNS allows multiple PTR RRs to be returned for a single query. However the General Consensus from the DNS types (search an archive of comp.protocols.dns.bind, for example here or here; look enough and you'll see threats to put together a "Best Practices" comdemnation of it) is still that you shouldn't do that because pretty much everything is written with the expectation that PTRs map to a single A record. There's also the problem that it's possible (with a sufficiently hostname-ful IP) to have more PTR RRs than can be returned in a single UDP packet. And I can't say that I've personally ever seen it happen in the wild in 15+ years of varying degrees of sysadmining. Sure, you can do it; you can also use a pickaxe to poke speed holes in your car's hood. Neither of them really are going to be of much practical benefit.	[reply]

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Re^3: Retrieving all hostnames for an IP address...
by Fletch (Bishop) on Dec 13, 2006 at 20:07 UTC

Technically yes DNS allows multiple PTR RRs to be returned for a single query. However the General Consensus from the DNS types (search an archive of comp.protocols.dns.bind, for example here or here; look enough and you'll see threats to put together a "Best Practices" comdemnation of it) is still that you shouldn't do that because pretty much everything is written with the expectation that PTRs map to a single A record. There's also the problem that it's possible (with a sufficiently hostname-ful IP) to have more PTR RRs than can be returned in a single UDP packet. And I can't say that I've personally ever seen it happen in the wild in 15+ years of varying degrees of sysadmining.

Sure, you can do it; you can also use a pickaxe to poke speed holes in your car's hood. Neither of them really are going to be of much practical benefit.

[reply]