Chapter 15. Debugging Network Problems

This chapter consists of case studies in network problem analysis and debugging, ranging from Ethernet addressing problems to a machine posing as an NIS server in the wrong domain. This chapter is a bridge between the formal discussion of NFS and NIS tools and their use in performance analysis and tuning. The case studies presented here walk through debugging scenarios, but they should also give you an idea of how the various tools work together.

When debugging a network problem, it’s important to think about the potential cause of a problem, and then use that to start ruling out other factors. For example, if your attempts to bind to an NIS server are failing, you should know that you could try testing the network using ping, the health of ypserv processes using rpcinfo, and finally the binding itself with ypset. Working your way through the protocol layers ensures that you don’t miss a low-level problem that is posing as a higher-level failure. Keeping with that advice, we’ll start by looking at a network layer problem.

Duplicate ARP replies

ARP misinformation was briefly mentioned in Section 13.2.3, and this story showcases some of the baffling effects it creates. A network of two servers and ten clients suddenly began to run very slowly, with the following symptoms:

  • Some users attempting to start a document-processing application were waiting ten to 30 minutes for the application’s window to appear, while those on well-behaved machines ...

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