1 This target allows to alter the MSS value of TCP SYN packets, to control 2 the maximum size for that connection (usually limiting it to your 3 outgoing interface's MTU minus 40). Of course, it can only be used 4 in conjunction with 5 .BR "-p tcp" . 6 It is only valid in the 7 .BR mangle 8 table. 9 .br 10 This target is used to overcome criminally braindead ISPs or servers 11 which block ICMP Fragmentation Needed packets. The symptoms of this 12 problem are that everything works fine from your Linux 13 firewall/router, but machines behind it can never exchange large 14 packets: 15 .PD 0 16 .RS 0.1i 17 .TP 0.3i 18 1) 19 Web browsers connect, then hang with no data received. 20 .TP 21 2) 22 Small mail works fine, but large emails hang. 23 .TP 24 3) 25 ssh works fine, but scp hangs after initial handshaking. 26 .RE 27 .PD 28 Workaround: activate this option and add a rule to your firewall 29 configuration like: 30 .nf 31 iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \\ 32 -j TCPMSS --clamp-mss-to-pmtu 33 .fi 34 .TP 35 .BI "--set-mss " "value" 36 Explicitly set MSS option to specified value. 37 .TP 38 .B "--clamp-mss-to-pmtu" 39 Automatically clamp MSS value to (path_MTU - 40). 40 .TP 41 These options are mutually exclusive. 42
