11.18. TCPMSS target
The TCPMSS target can be used to alter the MSS (Maximum Segment Size) value of TCP SYN packets that the firewall sees. The MSS value is used to control the maximum size of packets for specific connections. Under normal circumstances, this means the size of the MTU (Maximum Transfer Unit) value, minus 40 bytes. This is used to overcome some ISP's and servers that block ICMP fragmentation needed packets, which can result in really weird problems which can mainly be described such that everything works perfectly from your firewall/router, but your local hosts behind the firewall can't exchange large packets. This could mean such things as mail servers being able to send small mails, but not large ones, web browsers that connect but then hang with no data received, and ssh connecting properly, but scp hangs after the initial handshake. In other words, everything that uses any large packets will be unable to work.
The TCPMSS target is able to solve these problems, by changing the size of the packets going out through a connection. Please note that we only need to set the MSS on the SYN packet since the hosts take care of the MSS after that. The target takes two arguments.
Table 11-13. TCPMSS target options
| Option | --set-mss |
| Example | iptables -t mangle -A POSTROUTING -p tcp --tcp-flags SYN,RST SYN -o eth0 -j TCPMSS --set-mss 1460 |
| Explanation | The --set-mss argument explicitly sets a specific MSS value of all outgoing packets. In the example above, we set the MSS of all SYN packets going out over the eth0 interface to 1460 bytes -- normal MTU for ethernet is 1500 bytes, minus 40 bytes is 1460 bytes. MSS only has to be set properly in the SYN packet, and then the peer hosts take care of the MSS automatically. |
| Option | --clamp-mss-to-pmtu |
| Example | iptables -t mangle -A POSTROUTING -p tcp --tcp-flags SYN,RST SYN -o eth0 -j TCPMSS --clamp-mss-to-pmtu |
| Explanation | The --clamp-mss-to-pmtu automatically sets the MSS to the proper value, hence you don't need to explicitly set it. It is automatically set to PMTU (Path Maximum Transfer Unit) minus 40 bytes, which should be a reasonable value for most applications. |
 | Works under Linux kernel 2.5 and 2.6. |