forked from osmocom/wireshark
80 lines
3.4 KiB
Plaintext
80 lines
3.4 KiB
Plaintext
// WSUG Chapter Four
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[#Chap04]
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== Troubleshooting with Wireshark
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=== An approach to troubleshooting with Wireshark
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Wireshark is a very useful tool for network troubleshooting, since it contains a
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number of features that allow you to quickly focus on problems in your network
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for several reasons:
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* It allows you to focus in on specific packets and protocols, as you can see a
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large amount of detail associated with various protocols.
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* It supports a large number of protocols, and the list of protocols supported
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is growing as more people contribute dissectors
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* By giving you a visual view of traffic in parts of your network, and providing
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tools to filter and colorize that information, you can get a better feel for
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your network traffic, and can understand your network better.
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The following general approach is suggested:
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* Determine that the problem looks like a networking problem. There is no point
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in capturing packets if the problem is not networking related.
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* Figure out where to capture packets. You will have to capture packets from a
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part of the network where you can actually get network traffic related to the
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problem. This is especially important in the presence of switches and routers.
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See <<Ch04ROUSWI>> for more details.
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+
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Because Wireshark can read many capture file formats, you can capture using any
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convenient tool. One useful approach is to use _tcpdump_ to capture on remote
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systems and then copy the capture file to your system for later analysis. For
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more details on capturing with _tcpdump_, see <<Ch05tcpdump>>.
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* Once you have captured packets that you think relate to the problem, load them
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into Wireshark and look for your problem. Using Wireshark’s filtering and
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colorization capabilities, you can quickly narrow down the capture to the area
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of interest.
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* Examine the appropriate fields within the packets where the problem appears to
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be. These can often help to reveal the problem.
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[#Ch04ROUSWI]
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=== Capturing in the presence of switches and routers
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In the old days of Ethernet, all network traffic was spread over one “yellow”
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cable through the whole network. Capturing data was easy, as all packets from
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the network could be captured using the “promiscuous mode” at any place in the
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network. The only devices blocking network traffic, were routers. But as routers
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were extremely expensive, they were not widely used.
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Then Ethernet wiring using hubs become the state of the art. As the hubs still
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spaded the packets all over the network, things regarding capturing didn’t
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change.
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At the next stage, Ethernet switches became widely available. This complicated
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things a lot. When capturing traffic on a computer connected to a switch,
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usually the switch will only forward packets to the computer, which are directed
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to it, or to all computers (broadcasts). It’s much the same like deactivating
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the promiscuous mode of the capturing network card.
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There are some ways to circumvent this.
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Many vendor’s switches support a feature known as “port spanning” or “port
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mirroring” in which all of the traffic to and from port A are also sent out
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port B.
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=== Examples of troubleshooting
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Troubleshooting often requires a reasonable knowledge of the protocols in
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question. However, as Wireshark will often give you some good hints, you might
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get an idea of what is going wrong simply by looking in the packets being
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exchanged.
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// End of WSUG Chapter 4
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