[Pkg-utopia-maintainers] Bug#418023: Interfaces managed by NetworkManager have MTU 576

[Michel Dänzer]

On Tue, 2007-04-17 at 10:25 +0200, Sjoerd Simons wrote:
> On Fri, Apr 06, 2007 at 12:38:34PM +0200, Michel Dänzer wrote:
> > I noticed that all interfaces managed by NetworkManager seem to
> > have an MTU of 576 bytes (the minimum). This seems to have a negative
> > effect on download speed, or certainly not a positive one. As
> > nm_system_get_mtu() in src/backends/NetworkManagerDebian.c returns 0,
> > NetworkManager doesn't seem to set the MTU explicitly though, so I'm not
> > sure where the default is coming from. I verified that hacking the above
> > function to return 1500 results in interfaces managed by NetworkManager
> > having an MTU of 1500, with a line like
> > 
> > Apr  6 12:18:52 thor NetworkManager: <information>^ISetting MTU of
> > interface 'sungem' to 1500 in syslog.
> 
>   I've personally never seen this behaviour. I've also got a powerbook with a
>   sungem network card and network-manager keeps it nicely at mtu 1500. 

FWIW the same thing happens with the WiFi interface.

I'm not ruling out a local configuration issue, but I can't seem to find
anything related in /etc. Any suggestions what to look for?

>   When nm_system_get_mtu() returns 0, NM should just leave the mtu at the
>   default value.. Looking at the code, if the configured mtu == 0, the set_mtu
>   function just exits. So that all seems fine.

Right.

>   So it seems we have ourselves a little mystery here :) I noticed your running
>   kernel 2.6.21-rc5, could you try debian's 2.6.20-1 kernel to see if that
>   changes the behaviour ? I know that that works fine on my powerpc with a
>   sungem, so it allows us to easily see if the .21-rc kernel is doing strange
>   things.

I only recently switched to running a self-built kernel for unrelated
reasons. I previously used the standard Debian kernel for years and have
been seeing this problem for a long time, possibly since I first started
using network-manager.


-- 
Earthling Michel Dänzer           |          http://tungstengraphics.com
Libre software enthusiast         |          Debian, X and DRI developer