[Bridge] regression in 4.20.0: physical to VETH (LXC) network bridge (unsure if cgroups is related)

Michael Evans mjevans1983 at gmail.com
Fri Jan 11 23:04:51 UTC 2019

I have since reverted to the working LTS kernel image offered by Arch Linux
(4.19.13), but am willing to re-test / gather data additional data on a
couple lower-use time periods during the week.

After updating to Linux 4.20.0 (along with a full system update otherwise)
my BRIDGED network connections to some LXC containers ceased working.

Attempting to troubleshoot this issue also produced extremely odd results,
which I think offhand MIGHT have caused network packets to fill up some
kind of memory buffer instead of being relaid or dropped; there are some
additional details at the serverfault and LXC bugs that I filed, as it was
initially (and still is) unclear where the actual issue is.


At this time I am unsure if it is related to netdev (bridge, veth),
cgroups, or some changed default that should now be configured in a way
that is different to previous defaults.



Also now filed: https://bugzilla.kernel.org/show_bug.cgi?id=202235

* It is NOT related to IP forwarding.  This is a BRIDGED connection, not a
routed one, and it works on older kernels without that enabled.

* physical network to bridge works (and will stay connected for a few min
after later troubleshooting steps, even if ARP caches / ping flake out and
stop responding)

* VETH (within LXC) can ping the the host IP on the bridge (but not the
gateway, the host can before this step) if manually assigned a static
address.  Doing this seems to cause general instability and a timed out SSH
session.  This lead me to reboot between each round of testing to ensure I
had a clean slate to start with.

I went over the major settings that I did check in the other two bug
reports, but I'm open to checking other values and/or performing different
kinds of tests occasionally over a given week.  Responses won't be
immediate but I'll try to check on this frequently over the next two weeks.
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