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newf5learner's avatar
newf5learner
Icon for Nimbostratus rankNimbostratus
Aug 19, 2016

TCPDUMP with SNAT and '-p' usage

Hi,

I was reading about the tcpdump and various filters we can use in capturing the appropriate traffic.

came across the post @ https://devcentral.f5.com/questions/long-term-tcpdump

Assuming that I can capture the traffic from my server even if I have SNAT enabled, I have used option '-p' this is capture filter I have used.

tcpdump -ni 0.0:nnn -s0 -v -p host 10.20.20.29 and port 80 -w /var/tmp/test_sw.bin

I used wireshark to read the content of the file, but I couldn't find any details traffic of the traffic from my backend server. Is it possible to see the traffic from my backend server with the option -p

Am i missing something here ? or It will not show the traffic from my backend server as the IP is not included in the filter?

application delivery
LTM
tcpdump
TMSH
  • Stanislas_Piro2's avatar
    Stanislas_Piro2
    Icon for Cumulonimbus rankCumulonimbus
    Aug 19, 2016

    Hi,

    I never saw this thread and I learned something interesting...

    In this thread, the p option is not 

    -p
    but append 
    p
    in the string 
    nnn
    :

    tcpdump -ni 0.0:nnnp -s0 -v host 10.20.20.29 and port 80 -w /var/tmp/test_sw.bin
  • IanB's avatar
    IanB
    Icon for Employee rankEmployee
    Aug 19, 2016

    You may wish to review SOL13637: Capturing internal TMM information with tcpdump which documents the interface modifiers, such as n,p, and h

     

    Please note the warning about using the 'p' modifier:

     

    Note: This modifier will continue to produce flow information for the life of the connection. Subsequent tcpdump captures will reveal flow information from previous tcpdump captures using the :p modifier if the connection is still active. To clear flow information from previous use, run the tcpdump command without the :p modifier using a filter that matches no information in the flow and ensure some traffic has been received by the BIG-IP system for the flow.