Re: Setting MTU size for ppp0

[Manfred Sindhoff <m.sindhoff@gmx.net>]

Sven Nielsen wrote:
> This perhaps needs a little more in-depth knowledge:
>
> I am trying to limit MTU size on ppp0 to below 1492 (that is, I want 
> 1412) because my (new) linux machine has to masquerade for my roommate 
> (winXP home). I set up pppoe with pppoeconf which configures MTU for 
> ppp0 to 1492.
>
> I did manage to set up masquerading:
>
> #iptables -t nat -A POSTROUTING -j MASQUERADE
> #echo 1 > /proc/sys/net/ipv4/ip_forward
>
> Masquerading works fine.
> No other rules configured in no other tables and/or chains.
> (I haven´t secured this setup yet, so no other rules present.)
>
> PROBLEM IS: my attempts to limit MTU to 1412 are ignored.
> tried adding "mtu 1412" to
> /etc/ppp/peers/dsl-provider
> /etc/ppp/options
>
> still, after reboot or after poff -> ifdown -> ifup -> pon dsl-provider, 
> ifconfig always reports ppp0 with MTU: 1492. Also tried #pppd noauth mtu 
> 1412 while ppp0 is already active, no effect.
>
> Is some other script overriding my attempts ?
>
> The reason I want to limit MTU is also a special one. My rommmate gets 
> access to every page, EXCEPT ebay!, and I (randomly) guess this to be 
> connected with MTU size. If I am wrong I will have to dig for some other 
> reason, but this is another problem to be solved later.
>
> Anybody any clue why setting MTU is ignored ??
Is it really ignored? I'm not shure - on my router ifconfig 
reports the same as yours, but ps shows "/usr/sbin/pppoe -I eth1 
-T 80 -m 1452".

What you should try is calling iptables with this line:

#iptables  -I FORWARD  -p tcp  --tcp-flags SYN,RST SYN \
-j TCPMSS --clamp-mss-to-pmtu

This is called MSS-clamping and influences the amount of data per 
TCP packet.

Might help in the situation you describe.

fred