Re-enabling interface-mtu in dhcp3

[Colin Watson]

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):

dhcp3 (3.0.4-10) unstable; urgency=low

  * debian/control: make Priority: optional for dhcp3-{server,relay,dev} and
    udeb
  * debian/dhclient-script.*: ignore insane MTU values
  * debian/control: dhcp3-client suggests avahi-autoipd (closes: #391925)
  * Added patch to dhclient to not tie up the current working directory
    (closes: #387723)
  * debian/dhcp3-{server,relay}.prerm: only try to call init script if it
    exists (closes: #387667)

 -- Andrew Pollock <apollock at debian.org>  Tue, 17 Oct 2006 22:08:54 -0700

Debian now requests interface-mtu by default:

dhcp3 (3.0.4-6) unstable; urgency=high

  * The "For crying out loud, actually apply the patch" release
  * debian/patches/00list: really apply the 64-bit patch (closes: #368302)
  * debian/dhclient.conf: also request the interface-mtu setting (closes:
    #372689)
  * Updated Brazilian Portuguese debconf template translation (closes:
    #374031)

 -- Andrew Pollock <apollock at debian.org>  Tue, 20 Jun 2006 11:11:59 -0700

However, this apparently wasn't taken into account in the merge of
3.0.4-10, and we still don't request interface-mtu from the DHCP server.
This has been annoying me for a long time, since my home network
requires an MTU of 1380 and the default of 1500 causes occasional
communication problems which are a pain to track down, so I'd like to
change the default here.

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?

Thanks,

-- 
Colin Watson                                       [cjwatson at ubuntu.com]