Discussion:
UseMTU should not be enabled by default in systemd 235
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Andrew Jeddeloh
2017-09-30 00:00:49 UTC
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I saw in https://github.com/systemd/systemd/pull/6936 that networkd's
UseMTU will be enabled by default in 235. There is an bug
https://github.com/systemd/systemd/issues/6593 that causes networkd to
loop continually setting and unsetting the MTU on most real hardware
if UseMTU is set and the the DHCP lease specifies a larger value than
what the driver is currently using.

This can be reproduced on with the e1000 ethernet driver as well as
ixgbe. See https://github.com/coreos/bugs/issues/1827#issuecomment-321129028
for reproduction setups using a Container Linux image and dnsmasq
locally.

This bug should be fixed before UseMTU is enabled by default.

-Andrew Jeddeloh
Susant Sahani
2017-10-06 12:02:32 UTC
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It's been reverted.

commit 95ab9eff1e24cf996cd4111a93e3a8b29ff0f524
Author: Andrew Jeddeloh <***@gmail.com>
Date: Thu Oct 5 03:58:02 2017 -0700

Revert "networkd: change UseMTU default to true. (#6837)" (#6950)

This reverts commit 22043e4317ecd2bc7834b48a6d364de76bb26d91.
UseMTU is broken on real hardware and should not be enabled by default.
Post by Andrew Jeddeloh
I saw in https://github.com/systemd/systemd/pull/6936 that networkd's
UseMTU will be enabled by default in 235. There is an bug
https://github.com/systemd/systemd/issues/6593 that causes networkd to
loop continually setting and unsetting the MTU on most real hardware
if UseMTU is set and the the DHCP lease specifies a larger value than
what the driver is currently using.
This can be reproduced on with the e1000 ethernet driver as well as
ixgbe. See https://github.com/coreos/bugs/issues/1827#issuecomment-
321129028
for reproduction setups using a Container Linux image and dnsmasq
locally.
This bug should be fixed before UseMTU is enabled by default.
-Andrew Jeddeloh
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