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    SubjectRe: mss to pmtu clamping partially broken?
    Andreas Steinmetz wrote:
    > 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.


    You forgot to mention *how* you're clamping the MSS. Using
    TCPMSS? Do you have a rule for incoming packets?

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