How to ask a DNS resolver listening on a different port than the tcp/udp 53

[Laurent SALIN]

Hello,
I wondering how i can send queries to a dns resolver listening on a
different port than the normaly 53 tcp/udp ?

The situation:
I've got a vps who running NSD as a autoritative nameserver, listening
on tcp/udp 53 and unbound as personnal resolver, listening on a
different tcp/udp port. It work very well on his own or with my OpenBSD
gateway at home as DNS cache.

Recently i've got a new FreeBSD VPS and I want to use the first VPS as
DNS nameserver for the second VPS but FreeBSD is unable to send queries
to nameserver on a different port as the normal one (tcp/udp 53).

I've got a "bad" solution, use unbound on the second VPS and maybe tell
him to ask the 1rst VPS on the unusual tcp/udp port, but I wonder myself
if is it possible with Packet Filter to change the destination port of
the queries forwarded to my 1rst VPS from tcp/udp 53 to tcp/udp 5353 for
exemple ?

Or maybe anybody got a other solution ?

I hope you'll understand me :-/

Laurent SALIN