Long time reader, first time writing in...
> The big question: Is there any DOC for OpenBSD about this? What pf rules
> needed to redirect any DNS server (ex.: 8.8.8.8 or 1.1.1.1) requests to
the
> DNS server running on the ROUTER, coming from the CLIENTS?
You can use rdr-to[0] with pf to redirect all DNS queries to the DNS
resolver running on the router. A rule in pf.conf would look something like:
pass in on $int_if proto { udp , tcp } from any to any port domain \
rdr-to $dns_server port domain
Ted Unangst has short write-up about turning your network inside out to do
just this[1].
[0]: https://man.openbsd.org/pf.conf.5#rdr-to
[1]:
https://flak.tedunangst.com/post/turn-your-network-inside-out-with-one-pfconf-trick
