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Difficulty: MediumTroubleshooting DNS and Name Resolution Services

A network engineer is troubleshooting a name resolution issue on a corporate client workstation. When the engineer runs `nslookup internal.example.com 10.20.4.15`, the hostname successfully resolves to `10.20.4.100`. However, standard web browsers and ping utilities on the workstation report an error stating that the host cannot be found. Which of the following potential root causes best explain why direct `nslookup` queries succeed while system applications fail to resolve the hostname? (Select TWO.)

  1. The local hosts file on the workstation contains an outdated or incorrect static mapping for the target hostname, which local system applications evaluate prior to querying DNS.Answer
  2. The workstation's active network adapter is configured with an incorrect primary DNS server IP address that differs from the server specified in the command line query.Answer
  3. C
    The authoritative DNS server lacks an AAAA record for the host, causing the local DNS client to fail fallback to standard IPv4 resolution during application lookups.
  4. D
    The corporate firewall is blocking TCP port 53 traffic, which forces nslookup to operate using ICMP while standard applications rely on TCP.

Answer

The resolution failure during standard application usage is caused by either an outdated local hosts file entry overriding DNS resolution or an incorrect primary DNS server address assigned to the network interface card.
The nslookup utility bypasses the operating system's standard name resolution sequence by directly sending queries over UDP/TCP port 53 to the server specified in the command line argument. Standard applications rely on the OS resolver API, which evaluates the local hosts file first and then sends requests to the DNS servers configured on the network adapter interface. An erroneous hosts entry or a misconfigured adapter DNS IP address will cause application lookups to fail despite direct DNS server reachability.

Step-by-Step Solution

1
Analyze the behavior of the diagnostic command versus OS application behavior.
Recognize that nslookup queries a specific DNS server directly (10.20.4.15) and bypasses the operating system's local resolver caching and static hosts configuration.
This isolation proves that the remote DNS server holds valid records, while local client-side configuration or resolution order is causing standard applications to fail.
2
Evaluate local system resolution mechanics.
The OS resolver checks the local hosts file prior to contacting DNS servers, and relies on network adapter TCP/IP settings for automatic server selection.
An invalid entry in the hosts file or a mismatched primary DNS IP on the network interface will break general application resolution while leaving explicit command-line queries unaffected.

Key Concept

Operating System Name Resolution Order vs Command-Line Query Tools
Estimated Time:1m 30s
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Topics

Troubleshooting DNS and Name Resolution Services

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