130 lines
4.7 KiB
XML
130 lines
4.7 KiB
XML
<!-- EUG Chapter Four -->
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<!-- $Id$ -->
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<chapter id="Chap04">
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<title>Troubleshooting with <application>Ethereal</application></title>
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<section>
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<title>An approach to troubleshooting with Ethereal</title>
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<para>
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Ethereal is a very useful tool for network troubleshooting, since it
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contains a number of features that allow you to quickly focus on
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problems in your networkfor several reasons:
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<itemizedlist>
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<listitem>
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<para>
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It allows you to focus in on specific packets and protocols,
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as you can see a large amount of detail associated with
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various protocols.
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</para>
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</listitem>
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<listitem>
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<para>
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It supports a large number of protocols, and the list of
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protocols supported is growing as more people contribute
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dissectors
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</para>
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</listitem>
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<listitem>
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<para>
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By giving you a visual view of traffic in parts of your
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network, and providing tools to filter and colorize that
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information, you can get a better feel for your network
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traffic, and can understand your network better.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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The following general approach is suggested:
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<itemizedlist>
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<listitem>
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<para>
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Determine that the problem looks like a networking problem.
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There is no point in capturing packets if the problem is not
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networking related.
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</para>
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</listitem>
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<listitem>
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<para>
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Figure out where to capture packets. You will have to
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capture packets from a part of the network where you can
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actually get network traffic related to the problem. This is
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especially important in the presence of switches and routers.
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See <xref linkend="Ch04ROUSWI"/> for more details.
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</para>
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<para>
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Because Ethereal can read many capture file formats, you can
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capture using any conventient tool. One useful approach is
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to use <command>tcpdump</command> to capture on remote
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systems and then copy the capture file to your system for
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later analysis. For more details on capturing with
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<command>tcpdump</command>, see <xref linkend="Ch05tcpdump"/>.
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</para>
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</listitem>
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<listitem>
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<para>
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Once you have captured packets that you think relate to
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the problem, load them into Ethereal and look for your
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problem. Using Ethereal's filtering and colorization
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capabilities, you can quickly narrow down the capture to the
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area of interest.
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</para>
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</listitem>
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<listitem>
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<para>
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Examine the appropriate fields within the packets where
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the problem appears to be. These can often help to reveal
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the problem.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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</section>
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<section id="Ch04ROUSWI">
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<title>Capturing in the presence of switches and routers</title>
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<para>
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In the old days of Ethernet, all network traffic was spreaded over one
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"yellow" cable through the whole network. Capturing data was easy,
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as all packets from the network could be captured using the
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"promiscuous mode" at any place in the network. The only devices blocking
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network traffic, were routers. But as routers were extremely expensive,
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they were not widely used.
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</para>
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<para>
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Then Ethernet wiring using hubs become the state of the art. As the hubs
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still spreaded the packets all over the network, things regarding
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capturing didn't changed.
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</para>
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<para>
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At the next stage, Ethernet switches became widely available. This
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complicated things a lot. When capturing traffic on a computer connected
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to a switch, usually the switch will only forward packets to the computer,
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which are directed to it, or to all computers (broadcast's). It's much the
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same like deactivating the promiscuous mode of the capturing network card.
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</para>
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<para>
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There are some ways to circumvent this.
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</para>
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<para>
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Many vendor's switches support a feature known as "port spanning"
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or "port mirroring" in which all of the traffic to and from port A
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are also sent out port B. An excellent reference on the
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"port spanning" feature of Cisco switches can be found at
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<ulink url="http://www.cisco.com/warp/public/473/41.html">
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Configuring the Catalyst Switched Port Analyzer (SPAN) Feature
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</ulink>
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</para>
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</section>
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<section>
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<title>Examples of troubleshooting</title>
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<para>
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Troubleshooting often requires a reasonable knowledge of the
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protocols in question. However, as Ethereal will often give you some
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good hints, you might get an idea of what is going wrong simply by
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looking in the packets being exchanged.
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</para>
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</section>
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</chapter>
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<!-- End of EUG Chapter 4 -->
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