Simple perl script to do inline reverse DNS lookups of IPs. It processes stdin.. so just 'cat somefile | ./ip2name.pl'

ip2name.pl

#!/usr/bin/perl

# TODO: make command line options possible use $endline as selector
###$endline=true;
# TODO: allow a comment character in the mode that prints at the end of a line.  user passable comment character.
# TODO: make it work with ipv6.. for now its ipv4 only

use Socket;

my %dnscache;
my $commentchar = "#";

$FILE = "-";
open(FILE) or die("Could not open file.");
foreach $line (<FILE>) {
    my $tmpline = $line;
    my $namelist;

    # Loop through the current line and find all IP addresses and process each one.
    while ($tmpline =~ /((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?))/o) {

	    my $value = $1;
	    ## Test to see if we have already looked for this entry before
	    if (!$dnscache{$value}) {
		    $dnscache{$value} = "";

		    ## Do DNS lookup for the names
		    my $ipaddrs = gethostbyaddr(inet_aton($value), AF_INET);
		    if (!$ipaddrs) { $ipaddrs = "UNKNOWN_IP"; }
		    my $tmp = $ipaddrs;

		    #print "DEBUG => Found the following IP addresses for $value: $ipaddrs\n";

		    ## If a name was found, put it in dnscache hash
		    if ($tmp) {
		        $dnscache{$value} = $tmp;
		    }
	    }

	    # If the user wants an end of line list, build the list
      if ($endline) {
	      $tmpline =~ s/$value//;
	      if (!$namelist) {
	        $namelist = $dnscache{$value}
	      } else {
	        $namelist = $namelist.", ".$dnscache{$value};
        }
      } else {
	      # If the user wants in line replacement of names
	      # Remove the current IP from the temporary line so we can find the next IP
	      $tmpline =~ s/$value//g;

	      ## Replace the IP with the DNS lookup results
	      $line =~ s/$value/$value \($dnscache{$value}\)/g;
      }

    }

    # If there is a list of names, add it to the end of the line
    if ($namelist) { $line =~ s/$/\t\t$commentchar ($namelist)/; }

    # Print the line
    print $line;
}