From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mart Frauenlob Subject: TCPMSS - valid only in mangle table? Date: Sat, 07 Nov 2009 13:33:04 +0100 Message-ID: <4AF56900.9040501@chello.at> Reply-To: netfilter@vger.kernel.org Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: Sender: netfilter-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: netfilter@vger.kernel.org Hello, the man page of iptables says: TCPMSS This target allows to alter the MSS value of TCP SYN packets, to control the maximum size for that connection (usually limiting it to your outgoing inter- face's MTU minus 40 for IPv4 or 60 for IPv6, respectively). Of course, it can only be used in conjunction with -p tcp. It is only valid in the mangle table. but when i put a TCPMSS rule into the filter table: eris:~# iptables -I FORWARD -o eth2 -p tcp --syn -j TCPMSS --set-mss 1440 it shows up inserted: eris:~# iptables -vnL FORWARD Chain FORWARD (policy DROP 0 packets, 0 bytes) pkts bytes target prot opt in out source destination 2 104 TCPMSS tcp -- * eth2 0.0.0.0/0 0.0.0.0/0 tcp flags:0x17/0x02 TCPMSS set 1440 10758 6553K ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0 state ESTABLISHED 0 0 GLOBAL_INVALID_STATE all -- * * 0.0.0.0/0 0.0.0.0/0 state INVALID 341 17732 BAD_TCP_PACKETS tcp -- * * 0.0.0.0/0 0.0.0.0/0 0 0 eth2_to_eth1 all -- eth2 eth1 0.0.0.0/0 0.0.0.0/0 341 17732 eth1_to_eth2 all -- eth1 eth2 0.0.0.0/0 0.0.0.0/0 0 0 ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0 state RELATED 0 0 LOG all -- * * 0.0.0.0/0 0.0.0.0/0 limit: avg 1/sec burst 3 LOG flags 0 level 6 prefix `IPT_DROP_FORWARD ' eris:~# and it actually seems to work: eris:~# tshark -n -i eth2 tcp port 80 273.366753 62.178.111.127 -> 193.99.144.85 TCP 1989 > 80 [SYN] Seq=0 Len=0 MSS=1440 WS=0 So is the man page wrong? From what i know from the netfilter concept, packet alteration should be done in the mangle table, so the man page is right from theory, but practically the target extension does not care? eris:~# uname -a Linux eris 2.6.24-etchnhalf.1-686 #1 SMP Sat Aug 15 16:51:49 UTC 2009 i686 GNU/Linux eris:~# iptables --version iptables v1.4.5 Thank you for clarification, regards Mart