MSS Clamping - distinguish between IPv4 / IPv6



  • Hi there,

    I'm running a dual stack over PPPoE.
    tl;dr; I've had problem with some HTTPS sites because the MSS value in the TCP header was too large, the packets got dropped and the ICMP error message has been filtered by the transfer network. Not the pfsense's fault...

    Okay, luckily we can adjust the MSS value in pfsense.
    And here comes my issue:

    The explanation says:
    If a value is entered in this field, then MSS clamping for TCP connections to the value entered above minus 40 (TCP/IP header size) will be in effect.
    That's true for IPv4, but not for IPv6.
    In case of an TCP/IPv6, the header size is 60 (40 IPv6 + 20 TCP).

    In my case, having a MTU of 1492, I have to use a value of 1472 to avoid problems.


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