Re: [gentoo-user] NAT problem

[Mick]

On Saturday 11 Jan 2014 08:49:15 the wrote:
> On 01/11/14 03:24, Mick wrote:

> > Is there a router somewhere (your ISP?) that does not play nice
> > with PMTU Discovery?  What happens if you set your ifaces to have
> > an mtu of 1492 (needed to accomodate your PPPoE headers) or even
> > lower like 1440, or 1380?
>
> Thanks you Kerin, Mick! It works like a charm. Indeed:
>
> ppp0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>  mtu 1492
>
> So do I understand correctly that field of size 1500 - 1492 is
> reserved for pppoe stuff?
> Will it also work if I set a smaller mtu in my wlan like
> 1400 (assuming that the smallest mtu on the path is not less than 1400)?

In PPPoE the PPP frames are encapsulated inside Ethernet frames, so that they 
can travel over Ethernet.  The Ethernet frames use an MTU of 1500 bytes, but 2 
of these are consumed by the PPP header and 6 by the PPPoE header.  As a 
result the packet payload is smaller.  PMTU Discovery is used to allow end 
routers to communicate across the Internet (using ICMP) and discover what is 
the MTU accepted by the route in question.  Some badly implemented 
routers/firewalls do not respond as they should dropping all PMTUD packets and 
therefore the end router will respond with larger than the MTU size packets 
accepted throughout the route.  We don't know where the borked router is in 
your set up.  I have found that setting the interface to a smaller MTU cures 
such problems and if it works is a more efficient solution than MSS Clamping 
which requires every packet to be processed and its header changed in flight.

So as a first test I would suggest you try setting the interface of your PC to 
an MTU of 1492 (using ip or ifconfig) and walk down from there until you get a 
responsive connection to the servers that you were previously having problems 
with.

You can go below and MTU of 1400 if you wish.

-- 
Regards,
Mick