DNS from Perl

Perl has gethostby* functions available in libc, and these are the ones most people use. The use of the gethostby* functions is documented in the perlipc man page. Because DNS lookups are (or can be) slow, the results are prime suspects for caching in a hash. The following code fragment looks up the IP address of the host named by the $host variable:

 use Socket; ... # Check cache first if (exists($address{$host})) { # We know the address already $thataddr=$address{$host}; } else { # Cache miss, get and remember. (my $fqdn, undef, undef, undef, $thataddr) = gethostbyname($host); # Hostname lookup failure?  Cache the miss. if (defined($fqdn)) { $address{$host}=$thataddr; $address{$fqdn}=$thataddr if $fqdn ne $host; } else { $thataddr=$address{$host}=undef; } } if ($thataddr) { print "$host resolves to ",inet_ntoa($thataddr),"\n"; } else { print "$host does not resolve\n"; } 

This code caches answers, but the caching code does not take the TTL into account. It is not available from the API, but if the program is "alive" for a relatively short time (for hours or less), it is reasonable to ignore the TTL and use the same address throughout the lifetime of the program .

The Net::DNS Module

The module Net::DNS, written by Mike Fuhr, is available on CPAN. It provides a more complete DNS interface, including DNS UPDATE. If you are unfamiliar with CPAN, see the system Perl man pages or http://www.cpan.org/ . The Net::DNS home page is at http://www.fuhr.org/~mfuhr/perldns/ .

The Net::DNS module enables you to build DNS query packets by calling object methods and then sending and later analyzing the answer. The interfaces are high-level and very powerful, and the documentation is good and filled with examples. This code example is from the man page; it looks up the available records of http://foo.bar.com and prints any A records returned:

 use Net::DNS; $res = new Net::DNS::Resolver; $query = $res->search("http://foo.bar.com"); if ($query) { foreach $rr ($query->answer) { next unless $rr->type eq "A"; print $rr->address, "\n"; } } else { print "query failed: ", $res->errorstring, "\n"; } 

This module does not use the OS resolver at all. It uses DNS directly and parses your /etc/resolv.conf to obtain the address of your nameservers. Using this library, you also can issue non-blocking, asynchronous DNS queries. See my mresolv2 script for an example (it can be found on the perldns home page).