[lxc-devel] Cloud instance with reduced MTU, container uses 1500, connections stall

[Andreas Hasenack]

> > Bridge:
> > # brctl show lxcbr0
> > bridge name    bridge id        STP enabled    interfaces
> > lxcbr0        8000.feee96ca7280    no        veth13ME9N
> >
> > I'm not sure how different MTUs are handled on a bridge.
>
> Right so then perhaps /etc/init/lxc-net.conf should be setting the
> mtu on lxcbr0 to match eth0's.  Of course eth0's mtu could be changed
> later and we can't catch that.
>
> Can you check whether dropping lxcbr0's mtu fixes your problem by
> itself?
>


Interesting. The moment I start the container, the lxcbr0 MTU is back to
1500. I guess creating the veth device with a MTU of 1500 and adding it to
the lxcbr0 bridge triggers that:
(...)
3: lxcbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1454 qdisc noqueue state
DOWN group default
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
    inet 10.0.3.1/24 brd 10.0.3.255 scope global lxcbr0
       valid_lft forever preferred_lft forever
    inet6 fe80::38b8:13ff:feac:a6b0/64 scope link
       valid_lft forever preferred_lft forever

root at juju-fakestack-machine-3:~# lxc-start -n juju-trusty-template -d

root at juju-fakestack-machine-3:~# ip addr
(...)
3: lxcbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state
UP group default
    link/ether fe:22:eb:2c:3a:78 brd ff:ff:ff:ff:ff:ff
    inet 10.0.3.1/24 brd 10.0.3.255 scope global lxcbr0
       valid_lft forever preferred_lft forever
    inet6 fe80::38b8:13ff:feac:a6b0/64 scope link
       valid_lft forever preferred_lft forever
11: vethNH178P: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
master lxcbr0 state UP group default qlen 1000
    link/ether fe:22:eb:2c:3a:78 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::fc22:ebff:fe2c:3a78/64 scope link
       valid_lft forever preferred_lft forever


But ok, I put it back at 1454 and login on the container, which has 1500,
and things don't work. What matters is the MTU of the container interface
I'm afraid.
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