I am constantly looking for new open source tools that can make my life easier or allow me to assess client network environments. Here are some of my favourites, including a few lesser known treasures.
Not everyone can afford Solarwinds Orion Network Configuration Management (NCM) or CiscoWorks LAN Management Solution (LMS) to manage the changes to their network device configurations. That is why Really Awesome New Cisco confIg Differ (RANCID) continues to be a favourite tool among network engineers. RANCID's real benefit to network administrators is its ability to backup network device configurations and help you investigate changes to your environment.
I like using Backtrack 4 to help me perform security assessments. It contains a large repository of tools that a penetration tester would need such as Metasploit Framework 2 & 3, SNORT and many others. Backtrack 4 organises the tools into categories that relate to the security assessment methodology. It would take you a long time to put together such an extensive collection of tools on your own.
Multicast can be one of the most elusive types of traffic to test and troubleshoot. Plus, multicast applications do not typically have good diagnostic capabilities so you need a simple multicast-capable source and receiver to test your end-to-end multicast reachability. VideoLANClient (VLC) is a great multicast client/server/media player that can use an extremely wide array of stream sources and protocols.
It is important to know that your network is able to operate at its peak potential. However, it can be difficult to artificially simulate a large amount of traffic to validate the throughput ceiling. Enter iPerf and its updated version with a Java GUI, JPerf. That latest version of JPerf 2.0.2 allows you to easily adjust the buffer/MSS/TCP window size, and navigate all the lesser known IPerf CLI options. As a bonus this tool will also work with an IPv6-capable client and server.
Whenever users are blaming the network for an application's performance issues you may need to exonerate the network with an application performance tool like JMeter. JMeter is a simple Java application that can perform load tests on a wide variety of web-based applications, FTP and other protocol traffic. JMeter can be configured for multiple threads and can really generate a lot of traffic and help you determine how many connections per second your systems are capable of serving.
No list of FOSS network management tools would be complete without Wireshark. For the IT shops that can't afford dedicated hardware-based protocol analysers or RMON probes for the organisation's many network segments, it's a must have. Many of us know how great Wireshark is and the vast number of protocols that it supports. Wireshark can even monitor the Virtual Switch Link (VSL) header (VSH) communications on a Cisco Virtual Switching System (VSS). It also has full IPv6 capabilities.
Many network administrators do not have access to a suitable lab that really mimics the production network. However, it is often useful to configure a simple little scenario to validate an idea or prototype a solution. Dynamips is a system that allows you to emulate Cisco IOS image files and run them in a configurable environment. You can use Dynagen 1.11.7 or Graphical Network Simulator (GNS3) 0.7.2 front ends that utilise Dynamips underlying capabilities to make it easy to configure a virtual lab of Cisco routers joined together.