Allowing custom ports in name server forwarders would enable forwarding to alternative internalt name servers (unbound, stubby, dnscrypt-proxy etc.).

This would also allow using DNS over TLS in PowerDNS Recursor 4.6 onwards (pdns doesn't support certificate check for validity yet) by enabling dot-to-port-853. This is automatically set by default if compiled in with DoT support.

See: https://doc.powerdns.com/recursor/settings.html#dot-to-port-853

This indirectly implements part of T921, T2195 (DoT without certificate check).