From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andreas Steinmetz Subject: mss to pmtu clamping partially broken? Date: Fri, 29 Jun 2007 13:09:07 +0200 Message-ID: <4684E853.20900@domdv.de> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit To: Linux Kernel Mailinglist , netfilter-devel@lists.netfilter.org Return-path: Sender: linux-kernel-owner@vger.kernel.org List-Id: netfilter-devel.vger.kernel.org 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 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 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 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 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