Re-enabling interface-mtu in dhcp3

Matt Zimmerman mdz at ubuntu.com
Mon Dec 10 15:59:36 GMT 2007


On Sun, Dec 09, 2007 at 05:19:14PM +0000, Colin Watson wrote:
> Back in the mists of time (Hoary), we tried to tell dhclient to fetch
> the interface-mtu from the DHCP server by default, but this broke
> systems behind insane routers that suggested an MTU of 64
> (https://bugs.launchpad.net/ubuntu/+source/dhcp3/+bug/14575).
> 
> More recently, Andrew Pollock changed dhcp3 to ignore interface-mtu
> option values less than 576 (which is also something that Matt Zimmerman
> suggested in
> https://bugs.launchpad.net/ubuntu/+source/dhcp3/+bug/61989):
> [...]
> Unfortunately, there appear to be DHCP servers that set inappropriate
> MTUs of 576
> (https://bugs.launchpad.net/ubuntu/+source/dhcp3/+bug/65787). This is
> legal and I understand used for old analogue modems. At what point can
> we reasonably declare that the bug lies in the DHCP server and not in
> Ubuntu? I think that bug 65787 is such a case (server declares legal MTU
> value and we honour it), and that in that case the answer is really "fix
> your DHCP server or ask your vendor to do so", but I would like to check
> that I'm not on my own here. Does that make sense to others?

I think the answer is more like "to work around this, edit the configuration
file".  It's difficult for us to assess when we're in the right; can someone
tell us what Windows does in this situations, so at least we can be
bug-compatible?

One would think that broken uplinks like this would cause just as much
trouble for Windows users if Windows honored the MTU from DHCP.

I'm surprised that using a *lower* MTU than necessary caused problems for
the user in bug 65787.

-- 
 - mdz



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