1 <!-- EUG Chapter Four -->
5 <title>Troubleshooting with <application>Ethereal</application></title>
7 <title>An approach to troubleshooting with Ethereal</title>
9 Ethereal is a very useful tool for network troubleshooting, since it
10 contains a number of features that allow you to quickly focus on
11 problems in your networkfor several reasons:
15 It allows you to focus in on specific packets and protocols,
16 as you can see a large amount of detail associated with
22 It supports a large number of protocols, and the list of
23 protocols supported is growing as more people contribute
29 By giving you a visual view of traffic in parts of your
30 network, and providing tools to filter and colorize that
31 information, you can get a better feel for your network
32 traffic, and can understand your network better.
38 The following general approach is suggested:
42 Determine that the problem looks like a networking problem.
43 There is no point in capturing packets if the problem is not
49 Figure out where to capture packets. You will have to
50 capture packets from a part of the network where you can
51 actually get network traffic related to the problem. This is
52 especially important in the presence of switches and routers.
53 See <xref linkend="Ch04ROUSWI"/> for more details.
56 Because Ethereal can read many capture file formats, you can
57 capture using any conventient tool. One useful approach is
58 to use <command>tcpdump</command> to capture on remote
59 systems and then copy the capture file to your system for
60 later analysis. For more details on capturing with
61 <command>tcpdump</command>, see <xref linkend="Ch05tcpdump"/>.
66 Once you have captured packets that you think relate to
67 the problem, load them into Ethereal and look for your
68 problem. Using Ethereal's filtering and colorization
69 capabilities, you can quickly narrow down the capture to the
75 Examine the appropriate fields within the packets where
76 the problem appears to be. These can often help to reveal
83 <section id="Ch04ROUSWI">
84 <title>Capturing in the presence of switches and routers</title>
86 In the old days of Ethernet, all network traffic was spreaded over one
87 "yellow" cable through the whole network. Capturing data was easy,
88 as all packets from the network could be captured using the
89 "promiscuous mode" at any place in the network. The only devices blocking
90 network traffic, were routers. But as routers were extremely expensive,
91 they were not widely used.
94 Then Ethernet wiring using hubs become the state of the art. As the hubs
95 still spreaded the packets all over the network, things regarding
96 capturing didn't changed.
99 At the next stage, Ethernet switches became widely available. This
100 complicated things a lot. When capturing traffic on a computer connected
101 to a switch, usually the switch will only forward packets to the computer,
102 which are directed to it, or to all computers (broadcast's). It's much the
103 same like deactivating the promiscuous mode of the capturing network card.
106 There are some ways to circumvent this.
109 Many vendor's switches support a feature known as "port spanning"
110 or "port mirroring" in which all of the traffic to and from port A
111 are also sent out port B. An excellent reference on the
112 "port spanning" feature of Cisco switches can be found at
113 <ulink url="http://www.cisco.com/warp/public/473/41.html">
114 Configuring the Catalyst Switched Port Analyzer (SPAN) Feature
119 <title>Examples of troubleshooting</title>
121 Troubleshooting often requires a reasonable knowledge of the
122 protocols in question. However, as Ethereal will often give you some
123 good hints, you might get an idea of what is going wrong simply by
124 looking in the packets being exchanged.
128 <!-- End of EUG Chapter 4 -->