force nameserver(bind) information exchanges with clients via tcp only

[Donika Mirdita]

Hello,

I have set up a nameserver and I would like to force all future client 
requests to TCP only.
Essentially, one scenario would be for all UDP requests to be countered 
with a packet that has the TC bit set so the connection
is retried via TCP. I want this rule to be applicable to all incoming 
request, no actual data exchange
via UDPs, even for a simple dig request. I tried achieving this with the 
following 2 strategies but with no success:

1. set split value to 1 (in the rate-limit argument in named.conf.options)

2. I also tried to setup a response policy zone. I added the following 
in named.conf.options

         response-policy {
                 zone "rpz.example.com" policy tcp-only;
         };

      and the appropriate CNAME record for rpz-tcp-only. in rpz.example.com.

Neither worked out.

I know this scenario is not compliant to standard DNS, it is only an 
experimental setup.
I am using bind 9.16.1 and the OS is Ubuntu 20.04.
If anyone has ideas on how to achieve this with bind, it would be very 
helpful.

Best Regards,

Donika Mirdita