This PHP script has run quite some time as a nagios check plugin. It allows you to check if all nameservers that host a DNS zone are up-to-date and deliver the same serial in the SOA-record of the zone. It was written 2014 by Stephan Schwarz (@stearz)

check-dns-serial.php <ip-of-primary-dns> <domain.tld>

The script uses exit codes and an output that Nagios can understand

0 - OK
1 - Warning
2 - Critical
3 - Unknown

The following threshold variables can be set inside the script to control when Nagios should raise a WARNING or a CRITICAL:

# WARNING threshold if serials differ
$warn_threshold_delta  = 5;

# CRITICAL threshold if serials differ
$crit_threshold_delta  = 10;

The script requires Net/DNS2.php (PHP Resolver library used to communicate with a DNS server). Install it with sudo apt-get install php-net-dns2 on Debian based systems.