#!/usr/bin/perl
#
#	1.1	10/Jun/2003	Finally got around to placing on website (and slight tidy)
#	1.0	03/Dec/2002	Completed
#
#	DNS Resolving NameServer checker
#	Written by Kul - http://akakul.co.uk/
#

use strict;
use Socket qw(AF_INET PF_INET SOCK_STREAM);
use Fcntl qw(:DEFAULT :flock);

my $timeout	= 3;
my $port	= 53;

if (! @ARGV)	{
	my $resolver	= q(/etc/resolv.conf);
	if (-e $resolver)	{
		sysopen (RESOLVER, $resolver, O_RDONLY)	or die qq(Could not SYSOPEN file $resolver for READ: $!);
		flock (RESOLVER, LOCK_SH)	or die qq(Could not SH-LOCK $resolver file: $!);
		while (<RESOLVER>)	{
			chomp;
			if (/^\s*nameserver\s+(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})\s*$/i)	{
				print netutil_test_dns_server($1) . "\n";
			}
		}
		flock (RESOLVER, LOCK_UN)	or die qq(Could not UN-LOCK $resolver file: $!);
		close (RESOLVER)	or die qq(Could NOT CLOSE $resolver: $!);
	}
}
else	{
	while (my $this	= shift @ARGV)	{
		if ($this =~ /^\s*(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})\s*$/i)	{
			print netutil_test_dns_server($this) . "\n";
		}
		else	{	print "Not an IP: $this\n";	}
	}
}



sub netutil_test_dns_server	{
my ($ip)	= @_;
local (*SSOCK);

	my $packedip	= pack('S n a4 x8', AF_INET(), $port, pack ('C4', split (/\./o, $ip) ) );
	$SIG{'ALRM'}	= sub	{ close SSOCK; return ("Failed $ip"); } ;
	alarm ($timeout);
	unless ( socket (SSOCK, PF_INET(), SOCK_STREAM(), getprotobyname('tcp') ) )	{
		close (SSOCK);
		return ("Failed: $ip");
	}
	unless ( connect (SSOCK, $packedip) )	{
		close (SSOCK);
		return ("Failed: $ip");
	}
	alarm (0);
	$SIG{'ALRM'}	= 'DEFAULT';
	close (SSOCK);
	return ("OK: $ip");
}

__END__