In the context of configuring a DNS proxy, which statement correctly describes setting the TC bit and rcode NOERROR?

• A. Create a DNS Request with TC bit set and rcode NOERROR
• B. Create a DNS Response with TC bit set and rcode NOERROR
• C. Set TC bit and rcode NOERROR in the DNS Request from Client
• D. Set TC bit and rcode NOERROR in the DNS Response from DNS server

Answer: (B)

The TC bit is a flag in the DNS message header that only matters in responses. It signals that the information won’t fit in a single UDP packet and that the client should retry over TCP to get the full answer. The rcode NOERROR means the query was processed without an error.

In a DNS proxy scenario, when you can’t return the full answer in one UDP reply, you inform the client by sending back a DNS Response that has the TC bit set and the rcode NOERROR. This clearly communicates “we had a successful response, but it’s truncated due to size; please retry over TCP for the complete data.” Setting the TC bit in a request would carry no meaningful meaning, and making the proxy reflect the server’s response or modify a request wouldn’t convey the truncation intent to the client as effectively as a crafted response to the client would.