Thank you very much @arne_f !
Thank you for the background on the pseudo interface and why the return code is displayed when udev rules are applied.
The term “ifb” is described clearly here:
The Intermediate Functional Block (ifb) pseudo network interface acts as a QoS concentrator for multiple different sources of traffic. Packets from or to other interfaces have to be redirected to it using the mirred action in order to be handled, regularly routed traffic will be dropped. This way, a single stack of qdiscs, classes and filters can be shared between multiple interfaces.
And this seems like a good write-up on QoS and filters (from 2016):
It references Linux Advanced Routing & Traffic Control HOWTO which is too much for me now, but which I’d reference when trying to understand a specific part of this in more detail.
One of the reasons I had originally set up QoS on my IPFire system was to take advantage of the fq_codel qdisc to combat “bufferbloat”. I didn’t realise when I saw all interfaces had that qdisc in use that it is now applied by default to all links when QoS is not enabled. That’s a great improvement to the default for users. Thanks guys!