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====== tcpdump ======
<color #db5f0b>Jan 2017 (Updated MApr 2017)</color>
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tcpdump is a tool to capture IP Packets at a command line level, its similar to the PCAP used by Wireshark (Wireshark is just a GUI to control PCAP). I say just a GUI, its rather good and saying it is 'just' a GUI is a little harsh.
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Originally, I was using <color #ed1c24>tcpdump</color> to capture IP traffic and write it to a file, and if this is what you wish to do, then go to the section on this page called <color #ed1c24>File Capture</color>. However if you just wish to view the <color #ed1c24>tcpdump</color> output on screen, then look at the section entitled <color #ed1c24>Screen Capture</color>.
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==== Screen Caputre ====
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To show the output of tcpdump on the screen, use the follwoing syntax.
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    tcpdump -i eth2 -nn
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The <color #ed1c24>-i eth2</color> specifies the interface you wish to monitor. the <color #ed1c24>-nn</color> tells tcpdump not to resolve port numbers to names (so won't display the word 'ftp' where it sees port 21 for example).
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This results in the following style output.
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<file>
05:58:06.235597 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.235646 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.236112 IP 10.0.20.1.10000 > 239.0.200.1.10002: UDP, length 1316
05:58:06.236238 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.236422 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.236521 IP 10.0.20.100.1234 > 239.0.100.1.1234: UDP, length 1316
05:58:06.236595 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.236818 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.237060 IP 10.0.20.1.10000 > 239.0.200.1.10001: UDP, length 1316
05:58:06.237211 IP 10.0.20.1.10000 > 239.0.200.1.10003: UDP, length 1316
05:58:06.237402 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.237425 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.237524 IP 10.0.20.100.1234 > 239.0.100.1.1234: UDP, length 1316
05:58:06.237598 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.237985 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.238063 IP 10.0.20.1.10000 > 239.0.200.1.10001: UDP, length 1316
05:58:06.238213 IP 10.0.20.1.10000 > 239.0.200.1.10002: UDP, length 1316
05:58:06.238425 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.238572 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.238599 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.239174 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.239423 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.239521 IP 10.0.20.100.1234 > 239.0.100.1.1234: UDP, length 1316
05:58:06.239597 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.239741 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.240065 IP 10.0.20.1.10000 > 239.0.200.1.10001: UDP, length 1316
05:58:06.240215 IP 10.0.20.1.10000 > 239.0.200.1.10002: UDP, length 1316
05:58:06.240325 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.240423 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.240521 IP 10.0.20.100.1234 > 239.0.100.1.1234: UDP, length 1316
05:58:06.240597 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.240916 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.241166 IP 10.0.20.1.10000 > 239.0.200.1.10003: UDP, length 1316
05:58:06.241304 IP 10.0.20.1.10000 > 239.0.200.1.10004: UDP, length 1316
05:58:06.241423 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328
05:58:06.241507 IP 10.0.85.1.60817 > 239.0.85.1.1234: UDP, length 1316
05:58:06.241596 IP 10.0.20.1.1234 > 239.1.2.3.1234: UDP, length 1328

18909 packets captured
18913 packets received by filter
0 packets dropped by kernel

</file>
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==== File Capture ====
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On the MFEL, we can use tcpdump to capture either the <color red>IP Input</color> or the <color red>IP Output</color>. There are many commands available for tcpdump, and I will list some later on this page, but first lets just look at a working example, because this might be all you need for now.
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To capture an IP Output (ensure you have started your service) use the following example. The following example is for eth2, where a multicast exists on 239.0.12.1 port 1234.
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    The format for tcpdump is - tcpdump -i <interface> -s 65535 -w <some-file>
    tcpdump -i eth2 -B 64000 dst host 239.0.12.1 and port 1234 and multicast -w /home/ts_capture_test.pcap
    
    if the syntax is correct, the tcpdump will start:
    tcpdump: listening on eth2, link-type EN10MB (Ethernet), capture size 65535 bytes
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running the previous command will start the capture, and it will capture until stopped (using <color red>CTRL-C</color>)
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Pressing <color red>CTRL-C</color> stops the output, and you will see something like the following:
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<file>
119658 packets captured
119663 packets received by filter
0 packets dropped by kernel
</file>
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The capture is a <color red>pcap</color> file, the same format that would have been captured if you were using Wireshark, to get the video from this you will have to extract it first.
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