RA relay shouldn't relay link-local DNS #218
Description
.. as that DNS server address is unreachable from clients.
The router advertisements sent by my OPPO 5G CPE T2 sets a link-local DNS resolver. This works fine for clients connected directly to the OPPO router. It doesn't work for clients behind a second router that uses odhcpd to relay the router advertisements, as the link-local address of the OPPO router is unreachable from behind this second router.
Suggested fix:
If relaying a router advertisement that includes a link-local DNS, remove that entry from the relayed message as that will never work.
If the dns_service option is enabled, set c->always_rewrite_dns to true so we hit https://github.com/openwrt/odhcpd/blob/master/src/router.c#L971C1-L971C59