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    Subjectmss to pmtu clamping partially broken?
    There seems to be a problem with mss to pmtu clamping for incoming syn
    packets on reply to an outgoing connection on a ppp interface. The mss
    of the outgoing syn packets is always always clamped to the pmtu, I did
    check this with a target host I do have access to. The incoming syn
    reply to such a packet, however, is mss clamped only sometimes and this
    seems to depend on the DSL line used.

    The kernels tested were 2.6.20.1, 2.6.20.3 and 2.6.22rc6.

    Test setup: Two DSL lines, otherwise identical setup (same masquerading
    linux gateway, same DSL account, same DSL modem, same DSL line provider,
    same target host, request from and tcpdump on the same client).

    Linux Client<->Masquerading Linux Gateway<->DSL Modem<->DSL Line<->...

    DSL line 1, working:

    22:26:39.319281 IP (tos 0x0, ttl 64, id 22377, offset 0, flags [DF],
    length: 48
    ) 192.168.0.253.1164 > 64.34.165.170.80: S [tcp sum ok]
    1465827859:1465827859(0)
    win 5840 <mss 1460,nop,nop,sackOK>
    22:26:39.459314 IP (tos 0x0, ttl 51, id 0, offset 0, flags [DF],
    length: 48) 64
    .34.165.170.80 > 192.168.0.253.1164: S [tcp sum ok]
    3667852791:3667852791(0) ack
    1465827860 win 5840 <mss 1452,nop,nop,sackOK>

    The tcpdump on the client shows that the mss of the incoming syn reply
    packet is clamped to the ppp interface mtu.

    DSL line 2, not working:

    22:03:57.725998 IP (tos 0x0, ttl 64, id 55984, offset 0, flags [DF],
    length: 48
    ) 192.168.0.253.1600 > 64.34.165.170.80: S [tcp sum ok]
    36968258:36968258(0) win
    5840 <mss 1460,nop,nop,sackOK>
    22:03:57.866966 IP (tos 0x0, ttl 51, id 0, offset 0, flags [DF],
    length: 48) 64
    .34.165.170.80 > 192.168.0.253.1600: S [tcp sum ok]
    2226854208:2226854208(0) ack
    36968259 win 5840 <mss 1460,nop,nop,sackOK>

    The tcpdump on the client shows that the mss of the incoming syn reply
    packet is *NOT* clamped to the ppp interface mtu.

    --
    Andreas Steinmetz SPAMmers use robotrap@domdv.de

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