---
name: "firehol-tcpmss"
section: "5"
platform: "linux"
package: "firehol-doc"
version: "3.1.8+ds-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/firehol-tcpmss"
---

# firehol-tcpmss(5)

set the MSS of TCP SYN packets for routers

## NAME

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

## SYNOPSIS

tcpmss { *mss* \| auto } \[*if-list*\]

## DESCRIPTION

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.

## EXAMPLES

-

  ```
  tcpmss auto
  tcpmss 500
  tcpmss 500 "eth1 eth2 eth3"
  ```

## SEE ALSO

- •

  [firehol(1)](/firehol/1.md) - FireHOL program

- •

  [firehol.conf(5)](/firehol.conf/5.md) - FireHOL configuration

- •

  [firehol-interface(5)](/firehol-interface/5.md) - interface definition

- •

  [firehol-router(5)](/firehol-router/5.md) - router definition

- •

  FireHOL Website ⟨http://firehol.org/⟩\\c

- •

  FireHOL Online PDF Manual ⟨http://firehol.org/firehol-manual.pdf⟩\\c

- •

  FireHOL Online Documentation ⟨http://firehol.org/documentation/⟩\\c

- •

  https://www.frozentux.net/iptables-tutorial/iptables-tutorial.html#TCPMSSTARGET TCPMSS target in the iptables tutorial ⟨⟩\\c

## AUTHORS

FireHOL Team.