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 interface's MTU minus 40 for IPv4 or 60 for IPv6, respectively).
Of course, it can only be used
in conjunction with
.BR "-p tcp" .
It is only valid in the
.BR mangle
table.
.br
This target is used to overcome criminally braindead ISPs or servers
which block "ICMP Fragmentation Needed" or "ICMPv6 Packet Too Big"
packets.  The symptoms of this
problem are that everything works fine from your Linux
firewall/router, but machines behind it can never exchange large
packets:
.PD 0
.RS 0.1i
.TP 0.3i
1)
Web browsers connect, then hang with no data received.
.TP
2)
Small mail works fine, but large emails hang.
.TP
3)
ssh works fine, but scp hangs after initial handshaking.
.RE
.PD
Workaround: activate this option and add a rule to your firewall
configuration like:
.nf
 iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \\
             -j TCPMSS --clamp-mss-to-pmtu
.fi
.TP
.BI "--set-mss " "value"
Explicitly set MSS option to specified value.
.TP
.B "--clamp-mss-to-pmtu"
Automatically clamp MSS value to (path_MTU - 40 for IPv4; -60 for IPv6).
.TP
These options are mutually exclusive.