MTU bug in Fedora 30

There appears to be a bug in Fedora 30 (KDE) that sets MTU-values above the 1500 default when connecting to a network.

I noticed this ever since I installed Fedora 30 (KDE) on my laptop. My router runs OpenWrt, and it would crash not long after my laptop connected to it. Fortunately, the logs gave a clue as to why this was happening. It turns out that the switch driver causes the kernel to panic whenever it receives frames with MTU-values above the default 1500 (OpenWrt forum post). I tried to work around this by manually setting the MTU to 1500 in KDE settings on my laptop, but the issue would show up again when reconnecting to the network after waking up from suspend.

When I was using Fedora 29 I never experienced this, and I would like to help with debugging this issue to get it fixed. Does anybody have any suggestions as to how I should debug this?

how do you set your MTU ?
if set via gnome setting (KDE don’t know) => network => … => tab " Identity" ?
then it shouldn’t change during suspend cause it’s fixed set in /etc/sysconfig/network-scripts/…

you could find your needed MTU via command tracepath/tracepath6.
for me in germany and VDSL my MTU is 1492 !

a line “/usr/sbin/ifconfig mtu 1492” in an rc.local

This is how I set it in KDE: System Settings --> Network --> Connections --> (my network in the WiFi section) --> Wi-Fi tab --> MTU = 1500

My MTU should be 1500, as was the case in Fedora 29, but clearly something has been changed in Fedora 30 which introduced a regression. The only reason I noticed it was because of the bug in the switch driver of my router.

Turns out that there’s no MTU bug in Fedora. My router crapped itself because Fedora 30 made some DNS requests with DNSSec enabled which causes a fragmented UDP packet response. This happens because the response is too large to fit into one packet. The bug in the switch driver of the router has been patched in a newer kernel which I’m currently running. I’ve had no issues since the upgrade.

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