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Difficulty: HardDNS Infrastructure and Record Types

A network engineer is investigating why enterprise client endpoints fail to locate a local software update repository server. According to system documentation, client agents perform a DNS query to discover service location details, including the designated port number, priority, and weight for load balancing.

The engineer executes the following command during troubleshooting:

text
$ dig +noall +answer _update._tcp.corp.example.com
_update._tcp.corp.example.com. 3600 IN CNAME repo1.corp.example.com.

Based on the diagnostic output, which of the following actions must the administrator take on the authoritative DNS server to resolve the discovery issue?

  1. Replace the CNAME record with an SRV record that specifies the target hostname, port number, priority, and weight for the service.Answer
  2. B
    Replace the CNAME record with a PTR record that maps the service name directly to the IPv4 address of the update repository server.
  3. C
    Reconfigure the authoritative DNS server to listen exclusively on TCP port 53 to allow larger service response payloads.
  4. D
    Modify the query name from TCP port 53 to UDP port 80 in order to match the HTTP transport port of the web repository.

Answer

Replace the CNAME record with an SRV record that specifies the target hostname, port number, priority, and weight for the service.
The correct answer is to replace the CNAME record with an SRV record. SRV records use the standard format `_service._proto.name` (e.g., `_update._tcp.corp.example.com`) and return priority, weight, port number, and target hostname. CNAME records only provide a domain alias and lack port and load-balancing parameters.

Step-by-Step Solution

1
Analyze the client requirement and command output.
The client queries for `_update._tcp.corp.example.com` expecting service discovery parameters (port, weight, priority, target host). The `dig` output shows a CNAME record mapping to `repo1.corp.example.com`.
A CNAME record only provides an alias pointing to another domain name; it cannot supply port numbers, priorities, or weights.
2
Identify the proper DNS record type required for service location discovery.
An SRV (Service) record follows the standard naming convention `_service._proto.name` and contains symbolic fields for Priority, Weight, Port, and Target hostname.
Applications relying on automatic service discovery require SRV records to determine which port and server to connect to.
3
Select the correct administrative remediation step.
Creating an SRV record with the proper parameters allows clients querying `_update._tcp.corp.example.com` to resolve both the target server and port.
Replacing the invalid CNAME configuration with an SRV record directly fulfills the technical requirement.

Key Concept

DNS SRV (Service Location) Record Configuration
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Topics

DNS Infrastructure and Record Types

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