VLAN interface with MTU over 1500 may be left unconfigured on boot if created on bond

[Alexey Dushechkin]

Ubuntu 14.04.1 (also spotted on 12.04), ifenslave 2.4ubuntu1.

When VLAN interface with MTU > 1500 is created on a bonded interface (also with MTU > 1500) via /etc/network/interfaces, sometimes that VLAN interface will be left unconfigured after boot, with message "Waiting for newtork configuration" during the process.

In /var/log/upstart/network*.log:

  RTNETLINK answers: Numerical result out of range

Reproduction steps

On production servers such VLAN interfaces are left unconfigured after boot maybe in 10% of cases, but I was able to 100% reproduce the described behaviour on a VirtualBox VM with the attached "/etc/netwok/interfaces" config and "/etc/network/if-pre-up.d/zzz-bond-sleep" script that makes "sleep 2" for any bond* interface (else "exit 0" immediately). Removing the "mtu" setting from a VLAN interface leads to correct behaviour in any case (with/without zzz-bond-sleep script).

Workaround

if-pre-up.d script which sleeps several seconds for any vlan* interface workarounds the issue.

Comment

"Unconfigured" means with IP address assigned, but in DOWN state and with default MTU.

I suppose that it has something to do with udev, namely with /lib/udev/vlan-network-interface hook called from /etc/network/if-pre-up.d/ifenslave, because of attempt to set MTU on a VLAN interface before MTU is set on a bond interface itself.