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debuser
  Post  Post subject: Tcpdump: How to capture first two packets of tcp conversation (SYN/SYN+ACK)  |  Posted: Wed Jun 13, 2012 5:32 am

Joined: Thu Aug 06, 2009 2:48 am
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Tcpdump: How to capture first two packets of tcp conversation (SYN/SYN+ACK)

Capturing only SYN (fist packet) of tcp conversation:
Code:
$ sudo tcpdump -nni eth0 port 80 and tcp[13] == 2

09:26:23.044132 IP 181.210.53.36.24495 > 10.0.0.2.80: Flags [S], seq 4179562228, win 65535, options [mss 1280,nop,wscale 3,nop,nop,TS val 290512683 ecr 0,sackOK,eol], length 0


How to capture only SYN packet with tcpdump:


Code:
$ sudo tcpdump -nni eth0 port 80 and "tcp[13] == 2"

Capturing both SYN and SYN+ACK (fist two packet) of tcp conversation with tcpdump:


Code:
$ sudo tcpdump -nni eth0 port 80 and 'tcp[13] & 2 == 2'
09:24:47.052301 IP 181.210.53.36.4896 > 10.0.0.2.80: Flags [S], seq 1906016236, win 65535, options [mss 1280,nop,wscale 3,nop,nop,TS val 290511723 ecr 0,sackOK,eol], length 0
09:24:47.052347 IP 10.0.0.2.80 > 181.210.53.36.4896: Flags [S.], seq 1199622835, ack 1906016237, win 5792, options [mss 1460,sackOK,TS val 724817121 ecr 290511723,nop,wscale 6], length 0


By using this tcpdump command, you can see the TCP options like MSS, selective ACK, window scale, timestamps.

Why tcp[13] ? Because the tcp flags byte is the 13th byte in the tcp header:

Code:
        0                            15                              31
       -----------------------------------------------------------------
       |          source port          |       destination port        |
       -----------------------------------------------------------------
       |                        sequence number                        |
       -----------------------------------------------------------------
       |                     acknowledgment number                     |
       -----------------------------------------------------------------
       |  HL   | rsvd  |C|E|U|A|P|R|S|F|        window size            |
       -----------------------------------------------------------------
       |         TCP checksum          |       urgent pointer          |
       -----------------------------------------------------------------



Starting from right to left (little endian) the SYN flag is 2^1:
Code:
                       |               |
                       |---------------|
                       |C|E|U|A|P|R|S|F|
                       |---------------|
                       |7   5   3     0|





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