Can you recommend any open source network-monitoring programs?

Try Nagios (www.nagios.org), an open source network-monitoring application that runs on Linux and other Unix-based systems, and which has a large user base and active online community. Nagios is included with the operating system in several Linux distributions, as well as with OpenBSD, so you may already have a copy.

You can download the latest version and the required plug-ins from the download section of the Nagios Web site. It takes some time and effort to configure, build, install and set up Nagios, so reviewing the documentation before you start is recommended. Instructions are included with the distribution download, also available online at the Nagios site.

Nagios runs best using an Apache Web server, and requires the GD Graphics Library, available at www.boutell.com. You will need a working C compiler to build the software.

Follow the instructions for creating a Nagios user and group and the required command-file group, and identify the Apache user before you launch the Nagios configure script. After running configure and issuing the make-all and make-install commands, install the Nagios plug-ins, modify your Apache configuration, and define the items you want Nagios to monitor. In the end you'll have an outstanding network-monitoring system that is well worth the installation effort.