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Loss of TCP Connectivity

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  • Loss of TCP Connectivity

    Fresh from sorting out our Exchange 2013 problem we are straight into a problem with Server 2012.


    We are have one instance of Server 2012 running Hyper-V. This is not joined to a domain.

    Within this is another Server 2012 instance running full GUI, Active Directory, DHCP and DNS. This works fine.

    Also within Hyper-V is another Server 2012 instance. This is joined to the domain and running Exchange Server 2013 Standard.

    The problem that we have is that TCP traffic seems to have failed. UDP and ICMP works fine as does SMB. We can ping the box, nslookups work and filesharing works.

    I ran a packet sniffer and what I saw when attempting to open a website was:

    UDP DNS lookup from the failed box to the domain controller
    UDP response from DC to failed
    TCP Packet with "syn ecn cwr" flags set from failed box to external site
    TCP Response from external site to failed box with "syn ack" flags.
    Repeat of last two and nothing else.

    I'm unsure as to what could cause this. A/v was Vipre but is uninstalled now. We've stopped and disabled all Exchange services in case that caused the issue..

    Added: For reference, the firewall is off
    Last edited by beddo; 23rd January 2013, 22:58.

  • #2
    Re: Loss of TCP Connectivity

    Even more oddly, this only seems to affect outbound connections.

    The server cannot connect active directory and it cannot open websites yet from the other server I get a valid response from IIS on the failed box (all be it a 404 error as Exchange is turned off)


    • #3
      Re: Loss of TCP Connectivity

      Ok, getting somewhere - I remembered an issue from 2003 with an update that caused IPSEC to enter default block mode. Out of curiosity I turned off ipsec and guess what? It all works now.

      Not sure why IPSEC is blocking things as we haven't set up any policies..

      Edit: False Hope alas, a reboot and the server has gone again
      Last edited by beddo; 24th January 2013, 00:29.


      • #4
        Re: Loss of TCP Connectivity

        One of our guys spent most of a day on to PSS about this. After trying everything they decided to try changing the IP address of the server and this resolved the issue. How bizarre.