1:1 NAT and DUP pings

  • I have a problem that's been bugging me for a couple of weeks.

    I recently started using 1:1 NAT with aliased virtual IP addresses for my public-facing servers.  This is working well, except for one thing.  When I'm outside my LAN, and I'm using something other than Windows, all my 1:1 NAT addresses respond with duplicate pings.

    For example:
    % ping x.x.x.x
    PING x.x.x.x (x.x.x.x): 56 data bytes
    64 bytes from x.x.x.x: icmp_seq=0 ttl=116 time=35.964 ms
    64 bytes from x.x.x.x: icmp_seq=0 DUP! ttl=116 time=1021.083 ms
    64 bytes from x.x.x.x: icmp_seq=1 ttl=116 time=33.490 ms
    64 bytes from x.x.x.x: icmp_seq=2 ttl=116 time=34.728 ms
    64 bytes from x.x.x.x: icmp_seq=1 DUP! ttl=116 time=1672.580 ms

    This only happens with addresses involved in a 1:1 NAT.  I can set up a virtual IP on pfSense without 1:1 NAT and ping responses are normal.

    Other than the duplicate pings, I haven't noticed any issues.

    Is this normal behavior for 1:1 NAT, or is something inside my LAN causing this to happen?

  • I finally figured this out.  Turns out it had nothing to do with pfSense; the duplicate ping responses were caused by a D-Link DGS-3612G L3 switch with buggy old firmware.  Updating the firmware fixed the problem.

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