Re: [gentoo-user] How do I change MSS separately from MTU?
On Wednesday 14 January 2009, Mick wrote: On Monday 12 January 2009, Walter Dnes wrote: The only suggestion I've found via Google is iptables mangle. Does it manage to change MSS without changing MTU? If so, what is the invocation in the mangle table? It would probably be something like: iptables --insert OUTPUT --jump TCPMSS --protocol tcp --set-mss 1408 Oops! I just checked the manual: === TCPMSS 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). Of course, it can only be used in conjunction with -p tcp. It is only valid in the *mangle* table. === Then the rule can be set as follows: === iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \ -j TCPMSS --set-mss 1408 === If you have forwarding disabled on your box I would try the OUTPUT chain instead of FORWARD and see what this gets you. I think you can also set the advertised (by your machine) MSS for a network using ip route: ip route add 192.168.1.0/24 dev eth0 advmss 1408 PS. I am not sure if the above will break your connection because of dropped packets, or how it will interact with the MTU set at 1492. In my case I have just set my MTU at 1492 to cater for the PPP authentication on my ISP's ADSL network. I leave the MSS to be at what the kernel wants it to be - typically MSS = MTU - 40. Hope this helps. -- Regards, Mick signature.asc Description: This is a digitally signed message part.
Re: [gentoo-user] How do I change MSS separately from MTU?
On Monday 12 January 2009, Walter Dnes wrote: Seeing discussions about best MTU/MSS for MLPPP on my ISP's forum, I asked about the optimal settings for a standard DSL connection. Because of the way ATM cells line up, the optimal MSS is 1408. This would normally imply MTU 1448, because MTU is normally MSS + 40. The optimal setting is apparently MTU 1492 and MSS 1408. I tried forcing MSS to 1408 in my routing config (/etc/conf.d/net), but that also forces the MTU to 1448. Is there a way to do them separately? The only suggestion I've found via Google is iptables mangle. Does it manage to change MSS without changing MTU? If so, what is the invocation in the mangle table? I think it is in the OUTPUT table that you should insert a rule, but I don't have time to search for it now. It would probably be something like: iptables --insert OUTPUT --jump TCPMSS --protocol tcp --set-mss 1408 I think you can also set the advertised (by your machine) MSS for a network using ip route: ip route add 192.168.1.0/24 dev eth0 advmss 1408 PS. I am not sure if the above will break your connection because of dropped packets, or how it will interact with the MTU set at 1492. In my case I have just set my MTU at 1492 to cater for the PPP authentication on my ISP's ADSL network. I leave the MSS to be at what the kernel wants it to be - typically MSS = MTU - 40. -- Regards, Mick signature.asc Description: This is a digitally signed message part.
[gentoo-user] How do I change MSS separately from MTU?
Seeing discussions about best MTU/MSS for MLPPP on my ISP's forum, I asked about the optimal settings for a standard DSL connection. Because of the way ATM cells line up, the optimal MSS is 1408. This would normally imply MTU 1448, because MTU is normally MSS + 40. The optimal setting is apparently MTU 1492 and MSS 1408. I tried forcing MSS to 1408 in my routing config (/etc/conf.d/net), but that also forces the MTU to 1448. Is there a way to do them separately? The only suggestion I've found via Google is iptables mangle. Does it manage to change MSS without changing MTU? If so, what is the invocation in the mangle table? -- Walter Dnes waltd...@waltdnes.org
