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I don't think posting to the linux-netdev mailing list is a terrible idea. For example, here's someone who did post a question: https://marc.info/?l=linux-netdev&m=170628444014400&w=2
But well, you might also be running into a bug or something that could potentially be exploited, or maybe just into a lack of documentation (which is also a bug). Either way, some devs might be interested in knowing about this.
I don't have any previous knowledge of this at all, but from reading the docs, nothing you're describing sounds wrong.
A u32 selector will match 4 bytes (u32 meaning unsigned 32bit presumably, which is 4 bytes).
It makes sense that you'd only be able to configure the matches on 4 byte intervals, because keeping them aligned may make the implementation simpler and more efficient. You can still match any set of bits this way.
Perhaps you could describe what you're trying to match exactly and the selectors you tried.
Edit: also if you look at 'raw payload expressions' in nft: https://netfilter.org/projects/nftables/manpage.html
That seems like it would do what you want, and you can actually access the ethernet header in a documented way. You have to switch to nft though.
I really appreciate this, thank you. I think I had confused myself by playing with 'u16' and 'u8' and somehow coming to the conclusion that they were matching the right side of a 32-bit string. (Which may still be true, but, I'm just masking u32s now).
This is what I ended up with, which is working the way I'd expect:
tc filter add dev wlan0 protocol ip parent 1: prio 1 u32 \
match u32 0x30d6 0x0000ffff at -16 \
match u32 0xc92d1905 0xffffffff at -12 flowid 1:20
This sends Ethernet frames destined for 30:d6:c9:2d:19:05 to flow 1:20, and it doesn't seem to match a second device I tested. So, all good! Thank you again.
IProut 2
The french version of fart.