Monitoring Unix clients is very easy with the check_by_ssh plugin. The only prerequisite is public-key-based access and installation of some plugins on the remote side. Then, running a check is as easy as:

<br />
check_by_ssh –host 10.177.3.39 –logname nagios \<br />
    –command “lib/nagios/plugins/check_swap -w 15% -c 8%”<br />

The drawback of this method is extra load on the nagios server. With every check, a ssh process is forked which has to do a complete handshake with the remote side. With newer ssh implementations it is possible to have a persistent connection which requires only one handshake at startup. All the following ssh connects use the already established connection, which saves a lot of cpu cycles.
Here are the instructions to combine check_by_ssh with such a persistent tunnel.

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The well-known plugin check_by_ssh is a wrapper around the ssh client program. Unfortunately the path to ssh is defined at compile-time and remains hard-coded in the check_by_ssh binary. Usually this is /usr/bin/ssh. If you want to use features which are not implemented in your distribution’s ssh, but in an alternative ssh binary, you have to recompile check_by_ssh. Here is a patch which makes it easy to switch between multiple ssh binaries using a command line parameter.

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