tailieunhanh - Chapter-23-Network debugging

Recall from Chapter 16 that network software and hardware operate on at least four layers. If one layer doesn’t work, the ones above won’t either. When solving problems, it obviously makes sense to start at the bottom and work up. | 2 April 2003 17 00 47 The Complete FreeBSD page 401 In this chapter How to approach network problems Link layer problems Network layer problems traceroute tcpdump Transport and application layers Ethereal Network debugging The chances are quite good that you ll have some problems somewhere when you set up your network. FreeBSD gives you a large number of tools with which to find and solve the problem. In this chapter we ll consider a methodology of debugging network problems. In the process we ll look at the programs that help debugging. It will help to have your finger in Chapter 16 while reading this section. How to approach network problems Recall from Chapter 16 that network software and hardware operate on at least four layers. If one layer doesn t work the ones above won t either. When solving problems it obviously makes sense to start at the bottom and work up. Most people understand this up to a point. Nobody expects a PPP connection to the Internet to work if the modem can t dial the ISP. On the other hand a large number of messages to the FreeBSD-questions mailing list show that many people seem to think that once this connection has been established everything else will work automatically. If it doesn t they re puzzled. Unfortunately the Net isn t that simple. In fact it s too complicated to give a hard-andfast methodology at all. Much network debugging can look more like magic than anything rational. Nevertheless a surprising number of network problems can be solved by using the steps below. Even if they don t solve your problem read through them. They might give you some ideas about where to look. v 2003 04 02 03 23 15 401 2 April 2003 17 00 47 The Complete FreeBSD . tools page 402 How to approach network problems 402 Link layer problems To test your link layer start with ping. ping is a relatively simple program that sends an ICMP echo packet to a specific IP address and checks the reply. ICMP is the Internet .