This is a Nagios plugin to check the expiration date of a domain name.
It uses the "whois" command, which usually comes as a "whois" package.
Debian/Ubuntu: apt-get install whois

Original version had this copyright notice in it:
Copyright (c) 2005 Tomàs Núñez Lirola <tnunez@criptos.com>,
     2009-2012 Elan Ruusamäe <glen@delfi.ee>, under GPL License

Ryan found it hosted at:
http://git.pld-linux.org/gitweb.cgi/?p=packages/nagios-plugin-check_domain.git

This is a modified version, hosted at https://github.com/Elyrith/check_domain
Currently supports .com, .ca, .tv, .ee, .ru
Support for .ca and .se by Ryan Loudfoot (elyrith@gmail.com)
Support for Performance Data by Ryan Loudfoot