libxt_TCPMSS.man revision 22bdd6966f2c3ccded984a37ba0b97470bcf9323 
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\fB\-p tcp\fP.
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.IP
33 iptables \-t mangle \-A FORWARD \-p tcp \-\-tcp\-flags SYN,RST SYN
34             \-j TCPMSS \-\-clamp\-mss\-to\-pmtu
35.TP
36\fB\-\-set\-mss\fP \fIvalue\fP
37Explicitly sets MSS option to specified value. If the MSS of the packet is
38already lower than \fIvalue\fP, it will \fBnot\fP be increased (from Linux
392.6.25 onwards) to avoid more problems with hosts relying on a proper MSS.
40.TP
41\fB\-\-clamp\-mss\-to\-pmtu\fP
42Automatically clamp MSS value to (path_MTU \- 40 for IPv4; \-60 for IPv6).
43This may not function as desired where asymmetric routes with differing
44path MTU exist \(em the kernel uses the path MTU which it would use to send
45packets from itself to the source and destination IP addresses. Prior to
46Linux 2.6.25, only the path MTU to the destination IP address was
47considered by this option; subsequent kernels also consider the path MTU
48to the source IP address.
49.PP
50These options are mutually exclusive.
51