#!/usr/bin/perl
# $Id: check_soa 1815 2020-10-14 21:55:18Z willem $

=head1 NAME

check_soa - Check a domain's nameservers

=head1 SYNOPSIS

B<check_soa> I<domain>

=head1 DESCRIPTION

B<check_soa> queries each of a domain's nameservers for the Start
of Authority (SOA) record and prints the serial number.  Errors
are printed for nameservers that couldn't be reached or didn't
answer authoritatively.

=head1 AUTHOR

The original Bourne Shell and C versions were printed in
I<DNS and BIND> by Paul Albitz & Cricket Liu.

This Perl version was written by Michael Fuhr <mike@fuhr.org>.

=head1 SEE ALSO

L<perl(1)>, L<axfr>, L<check_zone>, L<mresolv>, L<mx>, L<perldig>, L<Net::DNS>

=cut

use strict;
use warnings;
use File::Basename;
use Net::DNS;

#------------------------------------------------------------------------------
# Get the domain from the command line.
#------------------------------------------------------------------------------

die "Usage: ", basename($0), " domain\n" unless @ARGV == 1;

my ($domain) = @ARGV;

#------------------------------------------------------------------------------
# Find all the nameservers for the domain.
#------------------------------------------------------------------------------

my $res = Net::DNS::Resolver->new();

$res->defnames(0);
$res->retry(2);

my $ns_req = $res->query($domain, "NS");
die "No nameservers found for $domain: ", $res->errorstring, "\n"
	unless defined($ns_req) and ($ns_req->header->ancount > 0);


# Send out non-recursive queries
$res->recurse(0);
# Do not buffer standard out
local $| = 1;


#------------------------------------------------------------------------------
# Check the SOA record on each nameserver.
#------------------------------------------------------------------------------

foreach my $nsrr (grep {$_->type eq "NS" } $ns_req->answer) {
	
	#----------------------------------------------------------------------
	# Set the resolver to query this nameserver.
	#----------------------------------------------------------------------
	my $ns = $nsrr->nsdname;
	
	# In order to lookup the IP(s) of the nameserver, we need a Resolver
	# object that is set to our local, recursive nameserver.  So we create
	# a new object just to do that.
	
	my $local_res = Net::DNS::Resolver->new();
	
	my $a_req = $local_res->query($ns, 'A');


	unless ($a_req) {
		warn "Can not find address for $ns: ", $res->errorstring, "\n";
		next;
	}
	
	foreach my $ip (map { $_->address } grep { $_->type eq 'A' } $a_req->answer) {
		#----------------------------------------------------------------------
		# Ask this IP.
		#----------------------------------------------------------------------
		$res->nameservers($ip);	
	
		print "$ns ($ip): ";

		#----------------------------------------------------------------------
		# Get the SOA record.
		#----------------------------------------------------------------------
	
		my $soa_req = $res->send($domain, 'SOA', 'IN');
		
		unless (defined($soa_req)) {
			warn $res->errorstring, "\n";
			next;
		}

		#----------------------------------------------------------------------
		# Is this nameserver authoritative for the domain?
		#----------------------------------------------------------------------

		unless ($soa_req->header->aa) {
			warn "isn't authoritative for $domain\n";
			next;
		}

		#----------------------------------------------------------------------
		# We should have received exactly one answer.
		#----------------------------------------------------------------------

		unless ($soa_req->header->ancount == 1) {
			warn "expected 1 answer, got ", $soa_req->header->ancount, "\n";
			next;
		}
		
		#----------------------------------------------------------------------
		# Did we receive an SOA record?
		#----------------------------------------------------------------------
	
		unless (($soa_req->answer)[0]->type eq "SOA") {
			warn "expected SOA, got ", ($soa_req->answer)[0]->type, "\n";
			next;
		}
		
		#----------------------------------------------------------------------
		# Print the serial number.
		#----------------------------------------------------------------------

		print "has serial number ", ($soa_req->answer)[0]->serial, "\n";
	}
}

0;