I have a WiFi router. It currently receives IPv6 multicast packets over a link-local address space in a WiFi network.
What I would like for it to do is to transfer those packets to an Ethernet interface it has (and vice versa) - as if the device connected to this router via Ethernet received these packets over a link-local broadcast itself.
I've tried looking up the keywords "bridging" and "relaying", and gave installing relayd a shot - all to no avail.
What should one do to achieve this goal? I control the WiFi router and the wired device, but the structure of the network traffic itself is unfortunately non-negotiable.
A switch is a layer 2 device... it simply forwards traffic from one port to another, or if it's an AP, from ethernet <-> wifi when in AP mode.
If the device is in a wifi sta (client) mode configuration, it cannot operate as a transparent bridge unless you use a method like relayd, but this is actually a bit of a routing 'hack' and it applies only to IPv4 (based on the information in the article)... IPv6 does not traverse this bridge.
Unfortunately, the underlying protocol for the wireless connection is IBSS/ad-hoc. There's no uplink per se - and I don't think WDS works with that either.
Reading between the lines, are you trying to only bridge this multicast traffic? Or is this intended to be a full bridge?
If the latter, manually adding the wireless adhoc/IBSS interface to the wired side's existing bridge device (or create a new one with both wired and wireless "ports" and move the layer 3 interface if none exists) might be worth a try. I've done that and as far as I can tell broadcast IPv6 traffic traverses the bridge - but, my wireless side is WDS.
(Note, with this I mean adding the wireless port to the br-* defice in its list of ports directly, not as part of a layer 3 "interface")
If the layer 2 bridge doesn't work... Would some macvlan craziness to spoof the IBSS MAC on the wired side work in conjunction with relayd?