MTU settings with pppoe

  • Hi

    I have Pfsense 2.0.1 on a mini PC with 2 nics. One interface on my LAN and the other connecting to my ISP via pppoe with a modem in bridge mode.

    Recently I setup a new mailserver which has a webmail component, the webserver runs on port 3000. So I have a port forwarding rule set to forward webmail traffic to the mail server on port 3000.

    Webmail works perfectly on my LAN.

    When I test externally in IE, firefox etc, I seem to get random results. From what I can see files for the webpage like css and javascript only half download or not at all causing the page to fail.

    I'm gussing I may have a blackhole router problem. Using wireshark on the mail server and pfsense packet capture I can see a TCP handshake take place, but then after a while I see [TCP Retransmission] packets.

    ifconfig on my interfaces

    LAN = 1500 mtu
    pppoe = 1492 mtu

    I do not have the MTU or MSS set on either interface.
    I have tried disableing PF scrubbing with no change.

    The packets that are retransmitted have a 1506 byte frame size and TCP len is 1452. Don't fragment is set!

    I guess these will be dropped. But shouldnt the firewall instruct the mail server to send smaller packets? Any ideas?


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