Messages in this thread | | | Date | Fri, 29 Jun 2007 13:58:16 +0200 | From | Andreas Steinmetz <> | Subject | Re: mss to pmtu clamping partially broken? |
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Patrick McHardy wrote: > 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? >
The relevant iptables commands I do use for masquerading and clamping are:
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS \ --clamp-mss-to-pmtu
-- Andreas Steinmetz SPAMmers use robotrap@domdv.de - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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