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

Sjoerd Simons sjoerd at spring.luon.net
Tue Apr 17 09:55:26 UTC 2007


On Tue, Apr 17, 2007 at 11:17:01AM +0200, Michel Dänzer wrote:
> 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've got a broadcom card (Airport Extreme) in my machine and that also doesn't
show the behaviour your describing.

> 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?

Not very specifically i'm afraid. When your network interfaces comes up
networkmanager runs the scripts in /etc/NetworkManager/dispatcher.d, which on
debian should only  contain one to trigger the ifupdown scripts. 

So the first suspects would be the scripts in /etc/network/if-*.d/. But i guess
you've already looked at those. The only other thing that comes to mind is the
dhcp client. It can set the mtu if instructed to do so by the DHCP server.

But for these things it shouldn't matter if you use the normal manual/ifupdown
way of configuring interfaces or NetworkManager... Could you please
double-check that it really only happens when using NetwokManager?

> >   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.

Ok, that's one factor less then :)

  Sjoerd
-- 
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