libxt_TCPMSS.man revision e24815d8a1b2755182fe983582a56fcc47153804
1This target allows to alter the MSS value of TCP SYN packets, to control 2the maximum size for that connection (usually limiting it to your 3outgoing interface's MTU minus 40 for IPv4 or 60 for IPv6, respectively). 4Of course, it can only be used 5in conjunction with 6.BR "-p tcp" . 7It is only valid in the 8.BR mangle 9table. 10.br 11This target is used to overcome criminally braindead ISPs or servers 12which block "ICMP Fragmentation Needed" or "ICMPv6 Packet Too Big" 13packets. The symptoms of this 14problem are that everything works fine from your Linux 15firewall/router, but machines behind it can never exchange large 16packets: 17.PD 0 18.RS 0.1i 19.TP 0.3i 201) 21Web browsers connect, then hang with no data received. 22.TP 232) 24Small mail works fine, but large emails hang. 25.TP 263) 27ssh works fine, but scp hangs after initial handshaking. 28.RE 29.PD 30Workaround: activate this option and add a rule to your firewall 31configuration like: 32.nf 33 iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \\ 34 -j TCPMSS --clamp-mss-to-pmtu 35.fi 36.TP 37.BI "--set-mss " "value" 38Explicitly set MSS option to specified value. 39.TP 40.B "--clamp-mss-to-pmtu" 41Automatically clamp MSS value to (path_MTU - 40 for IPv4; -60 for IPv6). 42.TP 43These options are mutually exclusive. 44