Man pages sections > man5 > firehol-tcpmss

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

firehol-tcpmss(5) 3.1.5 firehol-tcpmss(5)


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"


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 Documentation (
TCPMSS target in the iptables tutorial (


FireHOL Team.
Built 20 Sep 2017 FireHOL Reference