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Daniel_Ao_10370's avatar
Daniel_Ao_10370
Icon for Nimbostratus rankNimbostratus
May 29, 2017

Client stuck in TCP SYN sometimes

Hello all, After upgraded LTM from 3600 (TMOS version 10.2) to 2000 (TMOS version 11.5.4). I found that sometime Client may stuck in TCP SYN status. Below is my scenario.

 

  1. I created 1 Virtual Server 2.2.2.2:80, enabled Performance L4, SNAT and Source Port "Changed".
  2. 2 Clients try to telnet 2.2.2.2 port 80 at the same time
  3. 1 of the client may got stuck in TCP SYN status. Server did not response SYN ACK or RST

But this case did not happen in TMOS version 10.2 before. To workaround this issue, I changed Performance L4 to standard and the problem resolve. Any idea on this case? please advise. Thanks.

 

application delivery
BIG-IP

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  • dragonflymr's avatar
    dragonflymr
    Icon for Cirrostratus rankCirrostratus
    May 29, 2017

    Hi,

    Best way to find out is:

    • Do tcpdump on BIG-IP - something like 
      tcpdump -nni -s0 0.0:p host [VS IP]
      - not sure if this will work, you can try 
      tcpdump -nni -s0 0.0:p 'host [client 1 IP] or host [client 2 IP]'
    • Issue 
      watch -n 1 tmsh show sys connection cs-server-addr [VS IP]

    Then you should see more or less what happens on BIG-IP when two clients are connecting to VS. Sure best option is do that in test environment where you are not processing thousands of connections.

    But seems to be kind of bug, because you should get RST after Idle Timeout set TCP profile - except if you have Reset on Timeout disabled in TCP profile.

    Piotr