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#!/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;
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