N10-009 Question 58
Single answerDNS: 53A company migrated several internal applications to new servers over the weekend. On Monday, users report they can reach the applications by IP address, but they cannot connect using the applications' hostnames. A network technician confirms the clients have correct IP settings and can ping the DNS server. Firewall logs show DNS traffic is being allowed. Which of the following is the MOST likely cause of the problem?
- A
Port 53 is blocked between the clients and the default gateway
- B
The DNS records for the application hostnames were not updated after the server migration
- C
DHCP is assigning duplicate IP addresses to the application servers
- D
The clients are missing a default gateway configuration
Show answer and explanation
Correct answer: B
Explanation
The key troubleshooting clue is that users can access the applications by IP address but not by hostname. That pattern usually indicates a DNS resolution issue rather than a routing or basic connectivity issue. In real environments, server migrations often require updating DNS resource records, especially A records for IPv4 and AAAA records for IPv6. DNS queries and responses typically use port 53, most commonly over UDP, with TCP 53 also used in cases such as zone transfers or large responses. Because the scenario confirms DNS traffic is allowed and the DNS server is reachable, the most likely remaining issue is stale or missing DNS records. This aligns with standard troubleshooting guidance from DNS best practices and vendor documentation, which recommends verifying name records after host moves, IP changes, or service migrations.
- A. Incorrect.
This is incorrect. The scenario states the clients can ping the DNS server and firewall logs show DNS traffic is being allowed. If port 53 were blocked in the path, name resolution requests would likely fail because queries could not reach the DNS server. The evidence provided points away from a transport path issue and toward a DNS data issue instead.
- B. Correct.
This is correct. If users can reach the applications by IP address but not by hostname, connectivity to the destination exists, but name resolution is failing or returning outdated information. After a server migration, one of the most common causes is that the A or AAAA records for the application hostnames still point to the old server IP addresses or were never created for the new servers. DNS uses port 53 for name queries, but successful transport on port 53 does not guarantee the records themselves are correct.
- C. Incorrect.
This is incorrect. Duplicate IP addresses can cause intermittent connectivity, ARP conflicts, or inconsistent access, but they do not specifically explain why hostname-based access fails while direct IP-based access works. The symptom strongly indicates a name resolution problem rather than an addressing conflict on the application servers.
- D. Incorrect.
This is incorrect. A missing default gateway would affect communication outside the local subnet, including reaching a remote DNS server or remote application server by either hostname or IP address. Since the clients can ping the DNS server and reach the applications by IP, the default gateway is either present or not relevant to this specific failure.