Diagnose and fix network problems yourself
A recent and typical case of Linux network failure was the friend who rang up to say his "network had stopped". As error reports go, this is on a par with the classic Apollo 13 line "Houston, we've had a problem", though a little less life-threatening. Luckily, Linux has a goodly collection of network tools to help us figure out exactly what had gone wrong. (To eliminate any stress-inducing suspense, let me reveal that we eventually discovered that he had been disconnected by his ISP as a result of forgetting to renew his subscription.)
So, follow along with us now as we review some of the network diagnostic tools in Linux and see how to use them to get answers to the question "what's wrong with my network?"
The most important thing, when you're trouble-shooting something, is to have some idea how it's supposed to work in the first place. Does your machine have a static IP address, and if so, what should it be? Does it use DHCP, and if so, where is the DHCP server, and what range of IP addresses is it expected to allocate? Do you have a broadband modem directly connected to your machine, or do you have a separate broadband router to which you connect via ethernet or wireless?
Our methodology in this tutorial is to take a "bottom up" approach. We start by checking the really low-level stuff first, then gradually work our way up to higher levels. The sequence of tests we'll perform is summarised (approximately) in Figure 1, below. This is a good, systematic approach for network connections that have never worked. On the other hand, if it was working fine yesterday, it's generally faster to start at the top and work your way down.
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Don
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