Script I.1.6: Offline Reverse DNS Resolution

 #!/usr/local/bin/perl
 # file: logresolve.pl
  
 use constant TIMEOUT => 2;
 $SIG{ALRM} = sub {die "timeout"};
 
 while (<>) {
     s/^(\S+)/lookup($1)/e;
 } continue { 
     print;
 }
 
 sub lookup {
     my $ip = shift;
     return $ip unless $ip=~/\d+\.\d+\.\d+\.\d+/;
     return $CACHE{$ip} if exists $CACHE{$ip};
     my @h = eval <<'END';
     alarm(TIMEOUT);
     my @i = gethostbyaddr(pack('C4',split('\.',$ip)),2);
     alarm(0);
     @i;
 END
     $CACHE{$ip} = $h[0];
     return $CACHE{$ip} || $ip;
 }