You can specify the port in the query policies FORWARD do you want
version
Knot DNS Resolver, version 1.1.0
OS
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04 LTS
Release: 16.04
Codename: xenial
$ uname -rm
3.14.73-odroidc2 aarch64
issue
Living in FORWARD but no IP address is the authoritative Server launches in the 10053, so specify.
My best regards.