Which of the following indicates EDNS0 support in a DNS message?

• A. The rcode NXDOMAIN
• B. An OPT pseudo-record in the DNS message indicating EDNS0
• C. A Transfer-Encoding: chunked flag in HTTP
• D. A TCP Window Scale option in the handshake

Answer: (B)

EDNS0 support is signaled by the presence of an OPT pseudo-record in the DNS message. This OPT record is a special extension (type 41) that lives in the Additional section and carries EDNS0-related parameters, such as the UDP payload size the responder can handle, the EDNS version, and flags (including the DNSSEC OK bit). Seeing this OPT pseudo-record shows that the message is using EDNS0 to extend DNS capabilities, like allowing larger UDP messages and additional options.

The other options don’t indicate EDNS0. The NXDOMAIN rcode is just a negative response code for a non-existent domain, unrelated to EDNS0. The HTTP Transfer-Encoding: chunked flag and the TCP Window Scale option pertain to HTTP and TCP, not DNS.