path: root/extensions/
diff options
authorJan Engelhardt <>2008-02-14 03:02:55 +0100
committerPatrick McHardy <>2008-04-14 09:09:49 +0200
commite24815d8a1b2755182fe983582a56fcc47153804 (patch)
tree74521e49f0274dd0ff7831d9b71d25ef020820b5 /extensions/
parent23545c2a7a31c68c1e49c7c901b632c2f1c59968 (diff)
Combine ipt and ip6t manpages
Combine ipt and ip6t manpages
Diffstat (limited to 'extensions/')
1 files changed, 43 insertions, 0 deletions
diff --git a/extensions/ b/extensions/
new file mode 100644
index 00000000..82f93e02
--- /dev/null
+++ b/extensions/
@@ -0,0 +1,43 @@
+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
+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
+.PD 0
+.RS 0.1i
+.TP 0.3i
+Web browsers connect, then hang with no data received.
+Small mail works fine, but large emails hang.
+ssh works fine, but scp hangs after initial handshaking.
+Workaround: activate this option and add a rule to your firewall
+configuration like:
+ iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \\
+ -j TCPMSS --clamp-mss-to-pmtu
+.BI "--set-mss " "value"
+Explicitly set MSS option to specified value.
+.B "--clamp-mss-to-pmtu"
+Automatically clamp MSS value to (path_MTU - 40 for IPv4; -60 for IPv6).
+These options are mutually exclusive.