You have installed a Windows Server 2003 DNS server, but for security reasons the server is placed behind a firewall. Clients on the Internet and on other networks must be able to send DNS queries to this server. Which port or ports must be opened on the firewall so that users can successfully access the DNS server?

Introduction / Context:
This question tests your knowledge of network ports used by the Domain Name System service on Windows Server 2003. Proper firewall configuration is critical for DNS servers that must accept queries from clients outside a local network. You must know which protocols and ports DNS uses for normal queries and for operations such as zone transfers.

Given Data / Assumptions:

• The DNS service is running on a Windows Server 2003 computer.
• The server is behind a firewall.
• External clients need to send DNS queries to this server.
• The firewall can be configured to permit specific protocol and port combinations.

Concept / Approach:
Standard DNS queries primarily use UDP port 53, because simple question and answer exchanges are small and connectionless communication is efficient. However, some DNS operations, especially zone transfers and responses that exceed a certain size, use TCP port 53. Therefore, a correctly configured firewall for a public facing DNS server must allow both UDP 53 and TCP 53 traffic to and from the DNS server. Other ports such as 42, 21, or VPN related ports are not used for standard DNS resolution.

Step-by-Step Solution:
Step 1: Recall that DNS uses port 53 as its well known port.Step 2: Distinguish between UDP 53 for regular queries and TCP 53 for tasks such as zone transfers and large responses.Step 3: Recognize that for full DNS functionality, both UDP and TCP on port 53 must be permitted through the firewall.Step 4: Choose the option that explicitly states UDP and TCP port 53 must be opened.

Verification / Alternative check:
Networking documentation for Windows Server and standard DNS RFCs confirm that port 53 is assigned to DNS for both UDP and TCP. Many firewall guides include specific examples that show rules permitting UDP 53 and TCP 53 to DNS servers while blocking unnecessary ports. This cross checks the correctness of the answer.

Why Other Options Are Wrong:
Option a refers to port 42, which is not the standard DNS port. Option c opens only TCP 53 and blocks UDP, which would break normal query behaviour for most clients. Option d mixes PPTP and FTP related ports, which have nothing to do with DNS. Option e suggests that DNS uses HTTP port 80, which is incorrect; DNS is not encapsulated in web traffic by default.

Common Pitfalls:
Some administrators remember that DNS uses port 53 but forget that both UDP and TCP are required for full functionality, leading them to open only one protocol. Others misassociate port numbers with different services, especially when many services are being configured on the firewall. Keeping a short reference list of common service ports is helpful when designing secure firewall rules.

Final Answer:
You must open UDP port 53 and TCP port 53 on the firewall so that clients can send DNS queries and perform zone transfers with the Windows Server 2003 DNS server.