What happens if port 53 for DNS is blocked?

• A. A problem arises in the first step of the DNS request process.
• B. DNS requests are directed to port 80.
• C. DNS queries will automatically retry on a different port.
• D. Name resolution continues normally via cached records

Answer: (A)

Blocking port 53 stops the DNS resolver from reaching any DNS server, so the first step of looking up a domain cannot be completed. Since DNS queries normally travel to a DNS server on port 53 (UDP is typical, with TCP used in special cases), the client cannot send the request or receive a response, causing name resolution to fail for new queries. Cached results can only help if the domain has a recently stored entry; otherwise you’ll see timeouts or errors. DNS won’t try port 80, and it doesn’t automatically retry on a different port, so the immediate outcome is a failure to initiate DNS lookups.