#!/usr/local/bin/perl
#
# Cross check the a host name using forward and reverse DNS 
# lookups.  If the name passed in is a "cname" (aka alias), 
# then the second hostname returned will not match the first.
# That is ok.  The biggest thing to watch for is that the addresses
# match.
#
# The gethostbyname or gethostbyaddr functions use whatever name service 
# the system you run it on is configured to use.  For example, this may be 
# local hosts files, NIS, NIS+, or of course, DNS.
# 
# Andy Welter
# www.the-welters.com
# May 16, 2001
# 

use Socket;
#
# Get the address for the name passed in.  If someone 
# passed us an address, gethostbyname will return the address anyway.
$name1=$ARGV[0];
$addr1=gethostbyname($name1);
if ( $addr1 ) {
	($a,$b,$c,$d)=unpack ('C4', $addr1);
	$addr1txt="$a.$b.$c.$d";
} else {
	die "Lookup failed for name1=$name1\n";
};
#
# Do a reverse look up to get the name that goes with this address
$name2=gethostbyaddr (inet_aton ($addr1txt), AF_INET);
if ( $name2 ) {
	$addr2=gethostbyname($name2);
} else {
	die "Lookup failed for addr1=$addr1txt\n";
};
#
# Now do a forward lookup on the address to get the name.
if ( $addr2 ) {
	($a,$b,$c,$d)=unpack ('C4', $addr2);
	$addr2txt="$a.$b.$c.$d";
} else {
	die "Lookup failed for name2=$name2\n";
};

printf ("%-32s\t%s\n", "Hostname:","Address:");
printf ("%-32s\t%s\n%-32s\t%s\n", 
	$name1, $addr1txt, $name2, $addr2txt);