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The excellent open-source monitoring software Nagios is set up to use plugins to do the actual work of checking services. Here are some useful plugins from the official plugin site and an unofficial plugin collection.
check_apt: Checks for
aptsoftware updates. The main advantage over
cron-aptis that you can centralize your package monitoring: easier to refer to, and more obvious if a machine is missing.
check_jabber: The most useful aspect of this is the
-Doption, which warns when your jabber server certificate is close to expiring. Command-line options mean you can fine-tune what you send to the server and what you expect back.
check_mysql: By default this will check the local database anonymously. You can specify username, hostname, and so on, but be warned: Any password will be sent clear-text! A very useful feature is the ability to check whether the slave server is keeping up with the master server.
- check_krb5 (homepage): Keep tabs on your Kerberos server. The plugin takes hostname, realm, principal and keytab options, so is appropriately secure.
- check_ldap: Will check ldaps as well as ldap, but you do have to provide a login ID and password in plaintext, or just check anonymous bind.
More generally: Roll your own quickly with check_generic (homepage). This lets you define a command and a set of expectations/thresholds for the return data, thus saving you much of the work involved in writing a plugin. The homepage has thorough documentation and examples.