Strange behavior Windows 10 firewall

  1. Posts : 1

    Strange behavior Windows 10 firewall

    I have written a tcp client application which shall create a connection to a unix system. I have following strange behavior with activated firewall on windows 10:

    Thats what I can see in wireshark:
    My software on Win10 starts the TCP Connection request with a SYN.
    the unix system sends a SYN, ACK back
    My software answers with an ACK
    the unix system terminates with a FIN

    This repeats all 5 seconds and after 2-3 minutes suddenly the unix system doesnít terminate the connection but accepts it with a PSH, ACK and then the connection is ok and we can exchange data

    When I deactivate the firewall on windows 10 then the connection works immediately and doesn't need 2-3 minutes.

    can someone help me what setting in the firewall could cause this? I always thought either it blocks or it blocks not. But this looks strange to me
      My Computer

  2. Posts : 228
    Win 10 1809 LTSC

    It could be MTU/MSS negotiation, for some odd reason by default the windows firewall doesnt allow the necessary ICMP packets through.

    You need to allow the following in and out on windows firewall.

    Destination Unreachable
    Packet too big
    Time exceeded
    Parameter problem
    Echo request
    Type 129

    Not all of those are absolutely necessary but a good idea to make diagnosing problems easier.

    Packet too big
    Destination Unreachable
    Echo request
    Time exceeded
    Paramater problem

    What I think is happening the ICMP packets are failing, then its falling back to TCP based negotiation so things still work but are delayed.

    If you want it quick and simple just allow ICMP fully in/out, you still probably wont notice any ill affects of that.
      My Computer


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