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