Re: [gentoo-user] How do I change MSS separately from MTU?

[Mick]

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