Hi, this script queries the defined list of DNS servers and grabs the serial numbers for the listed zones. It then compares them against each other and logs if they match or not. It may not be pretty, but it works !

But, the bit I'm struggling with now is how best to do the serial number comparison if the number of DNS server in the list changes - that is if I add in more or remove DNS server IPs to the list, I don't have to manually go and add/remove the serial number references for the check - I want to make the serial compare "dynamic", based on the number of DNS servers.

I'm okay with just comparing all other serials to the first serial number, rather than comparing all possible combinations.

fyi - I'm writing all the results out to a file for reference, in addition to sending errors only to STDOUT. Thanks

#!/usr/bin/env perl
use strict;
use warnings;
use Net::DNS;
use 5.010;

my @nameservers = qw(8.8.8.8 4.2.2.2);
my @errors;

open(my $resultsfh, '>', "results-serials.csv")
    or die "cannot open > results-serials.csv: $!";

print $resultsfh "Result,Zone";
foreach (@nameservers) {
    print $resultsfh ",Serial for: $_";
}
print $resultsfh "\n";

while (<DATA>) {
    my $fqdn = $_;
    chomp $fqdn;

    my @serials;

    my $res = Net::DNS::Resolver->new;
    $res->tcp_timeout(2);
    $res->udp_timeout(2);

    foreach my $ns (@nameservers) {
        $res->nameservers($ns);
        my $reply = $res->query("$fqdn", "SOA");
        if ($reply) {
                push(@serials, ($reply->answer)[0]->serial);
        } else {
                push(@serials, $res->errorstring);
         }
     }
    # Compare Serials
    no warnings 'numeric';
    if ($serials[0] == $serials[1]) {
          say $resultsfh "MATCH,$fqdn,$serials[0],$serials[1]";
    } else {
          say $resultsfh "MISMATCH,$fqdn,$serials[0],$serials[1]";
          push @errors,  "MISMATCH.$fqdn,$serials[0],$serials[1]";
    }
}

if (@errors) {
    say "ERRORS:";
    foreach (@errors) {
      say "$_";
    }
}

__DATA__
google.com
apple.com
[download]