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Difficulty: HardDNS Lookup and Name Resolution Roles in Enterprise Networks

An enterprise network engineer observes that a Cisco IOS device attempting to resolve a hostname receives a DNS response from the DNS server with the Truncation (TC) bit set in the DNS header. Which action does the client device perform to obtain the complete set of resource records?

  1. It initiates a TCP connection to the DNS server on port 53 and re-issues the lookup request.Answer
  2. B
    It retransmits the identical UDP query to the secondary configured DNS server address on port 53.
  3. C
    It sends an ICMP Packet Too Big message back to the DNS server to request IP layer packet fragmentation.
  4. D
    It aborts the DNS lookup process and sends a local subnet broadcast query using Link-Local Multicast Name Resolution.

Answer

The client device initiates a TCP connection to the DNS server on port 53 and re-issues the lookup request.
Standard DNS transactions utilize UDP port 53 due to lower overhead. However, basic UDP DNS responses are restricted to a maximum size of 512 bytes. If the answer exceeds this size limit, the DNS server sets the Truncation (TC) bit in the DNS header. When a client receives a response with the TC bit set, it recognizes that data was omitted and re-issues the query over a TCP connection to port 53, allowing the full set of resource records to be transferred reliably.

Step-by-Step Solution

1
Analyze standard DNS transport protocol limits.
Standard DNS resolution operates over UDP port 53 with a maximum message size limit of 512 bytes to minimize network overhead.
UDP provides fast name resolution without requiring connection setup state.
2
Identify the purpose of the DNS Truncation (TC) bit.
When a DNS response payload exceeds 512 bytes, the responding DNS server sets the TC bit to 1 in the DNS response header.
This signals to the client that the returned data was truncated and incomplete.
3
Determine the RFC-compliant DNS client behavior upon receiving a truncated response.
The client opens a reliable TCP connection to the same DNS server on port 53 and re-sends the query.
TCP streaming handles large payloads by managing segmentation and reliable delivery without the 512-byte restriction.

Key Concept

DNS Transport Protocols and Truncation Mechanics (UDP vs TCP Port 53)
Estimated Time:1m 30s
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Topics

DNS Lookup and Name Resolution Roles in Enterprise Networks

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