Re: NM 0.9.9 ignores DHCP interface mtu?

[Dan Williams <dcbw redhat com>]

On Fri, 2014-01-03 at 09:41 -0500, Derek Atkins wrote:
> Hi all,
>
> Happy New Year!
>
> I just started playing with Fedora 20 which has NM 0.9.9 and I noticed
> that my F20 hosts do not honor the DHCP interface-mtu option.  This was
> not the case with previous versions (including F19, F18, all the way
> back to at least F15 if not earlier) so this is clearly a new regression
> in F20.  The only difference I can see is the change in NM version from
> 0.9.8 to 0.9.9.  The dhclient versions are 4.2.5 on both system, so it's
> unlikely that's the culprit (although I could be wrong).
>
> Has anyone else seen this behavior?  It's pretty easy to test.  On your
> dhcp server (assuming you run ISC DHCPD) add something like "option
> interface-mtu 1476;" to part of your DHCP config and then grab a lease
> from that config.  My F19 host will set the interface MTU to 1476, but
> F20 keeps it at 1500.
>
> The client is clearly requesting the option because it shows up in the
> lease file on the client.  It's just not being processed and honored
> anymore.
>
> Any known fixes or workarounds?

It appears to be a bug in the reworked IP4 config code.  I've filed
https://bugzilla.gnome.org/show_bug.cgi?id=721420 to track the issue.
Until it's resolved, you could set the MTU for your device via the
device specific tab of nm-connection-editor, or directly via the
configuration files.  For Fedora, you'd put "MTU=xxxx" into
the /etc/sysconfig/network-scripts/ifcfg-xxxx file corresponding to that
network connection.  That *should* work; only automatic MTUs get ignored
according to my code inspection.

Dan