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Chapter 13. Troubleshooting TCP/IP


Approaching a Problem
Diagnostic Tools
Testing Basic Connectivity
Troubleshooting Network Access
Checking Routing
Checking Name Service
Analyzing Protocol Problems
Protocol Case Study

Network administration tasks fall into two very different categories: configuration and troubleshooting. Configuration tasks prepare for the expected; they require detailed knowledge of command syntax, but are usually simple and predictable. Once a system is properly configured, there is rarely any reason to change it. The configuration process is repeated each time a new operating system release is installed, but with very few changes.

In contrast, network troubleshooting deals with the unexpected. Troubleshooting frequently requires knowledge that is conceptual rather than detailed. Network problems are usually unique and sometimes difficult to resolve. Troubleshooting is an important part of maintaining a stable, reliable network service.

In this chapter, we discuss the tools you will use to ensure that the network is in good running condition. However, good tools are not enough. No troubleshooting tool is effective if applied haphazardly. Effective troubleshooting requires a methodical approach to the problem, and a basic understanding of how the network works. We'll start our discussion by looking at ways to approach a network problem.

13.1. Approaching a Problem

To approach a problem properly, you need a basic understanding of TCP/IP. The first few chapters of this book discuss the basics of TCP/IP and provide enough background information to troubleshoot most network problems. Knowledge of how TCP/IP routes data through the network, between individual hosts, and between the layers in the protocol stack is important for understanding a network problem. But detailed knowledge of each protocol usually isn't necessary. When you need these details, look them up in a definitive reference -- don't try to recall them from memory.

Not all TCP/IP problems are alike, and not all problems can be approached in the same manner. But the key to solving any problem is understanding what the problem is. This is not as easy as it may seem. The "surface" problem is sometimes misleading, and the "real" problem is frequently obscured by many layers of software. Once you understand the true nature of the problem, the solution to the problem is often obvious.

First, gather detailed information about exactly what's happening. When a user reports a problem, talk to her. Find out which application failed. What is the remote host's name and IP address? What is the user's hostname and address? What error message was displayed? If possible, verify the problem by having the user run the application while you talk her through it. If possible, duplicate the problem on your own system.

Testing from the user's system, and other systems, find out:

Once you know the symptoms of the problem, visualize each protocol and device that handles the data. Visualizing the problem will help you avoid oversimplification, and keep you from assuming that you know the cause even before you start testing. Using your TCP/IP knowledge, narrow your attack to the most likely causes of the problem, but keep an open mind.

13.1.1. Troubleshooting Hints

Below are several useful troubleshooting hints. They are not part of a troubleshooting methodology -- just good ideas to keep in mind.

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