firehol-tcpmss man page

firehol-tcpmss — set the MSS of TCP SYN packets for routers


tcpmss { mss | auto } [if-list]


The tcpmss helper command sets the MSS (Maximum Segment Size) of TCP SYN packets routed through the firewall. This can be used to overcome situations where Path MTU Discovery is not working and packet fragmentation is not possible.

A numeric mss will set MSS of TCP connections to the value given. Using the word auto will set the MSS to the MTU of the outgoing interface minus 40 (clamp-mss-to-pmtu).

If used within a router or interface definition the MSS will be applied to outgoing traffic on the outface(s) of the router or interface.

If used before any router or interface definitions it will be applied to all traffic passing through the firewall. If if-list is given, the MSS will be applied only to those interfaces.


tcpmss auto

tcpmss 500

tcpmss 500 "eth1 eth2 eth3"

See Also

firehol(1) - FireHOL program
firehol.conf(5) - FireHOL configuration
firehol-interface(5) - interface definition
firehol-router(5) - router definition
FireHOL Website (
FireHOL Online PDF Manual (
FireHOL Online HTML Manual (
TCPMSS target in the iptables tutorial (…)


FireHOL Team.

Referenced By

firehol-conf(5), firehol-router(5).

Built 15 Feb 2015 FireHOL Reference 2.0.1