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Difficulty: MediumDNS Lookup and Name Resolution Roles in Enterprise Networks

A network administrator is evaluating the operation of DNS protocol mechanisms and transport layer behavior within an enterprise network infrastructure. Which two statements accurately describe how DNS lookup processes and transport protocols function during name resolution? (Choose two.)

  1. Standard DNS queries for IP address resolution (A or AAAA records) typically utilize UDP port 53 to reduce latency and connection overhead.Answer
  2. B
    DNS queries use TCP port 53 by default for all standard client host lookups to guarantee packet sequencing and built-in retransmissions.
  3. When a DNS response payload exceeds 512 bytes without EDNS0 support, the server sets the TrunCation (TC) bit, prompting the client to re-query using TCP port 53.Answer
  4. D
    Internal recursive DNS servers require TCP port 53 exclusively when forwarding queries to external root name servers.
  5. E
    Reverse DNS lookup requests for PTR records must use TCP port 53 because UDP does not support pointer record data structures.

Answer

The two correct statements are that standard DNS queries for IP address resolution typically utilize UDP port 53 to reduce latency, and when a DNS response payload exceeds 512 bytes without EDNS0 support, the server sets the TrunCation (TC) bit to prompt the client to re-query using TCP port 53.
Standard DNS lookups operate over UDP port 53 because single-packet requests and responses do not warrant the overhead of a TCP connection. However, if a DNS response message is truncated due to exceeding 512 bytes (without extension mechanisms like EDNS0), the DNS server sets the TC (TrunCation) bit in the header. Upon receiving a truncated response, the client initiates a TCP connection on port 53 to reliably retrieve the complete set of resource records.

Step-by-Step Solution

1
Identify the primary transport protocol used for standard DNS queries.
DNS uses UDP port 53 for normal client-to-server and server-to-server queries to maintain low latency and minimize connection overhead.
Establishing a TCP connection for small request/response pairs adds unnecessary delay.
2
Determine the conditions under which DNS transitions from UDP to TCP transport.
DNS falls back to TCP port 53 when the response exceeds 512 bytes (indicated by the TC bit in the DNS header) or during DNS zone transfers (AXFR/IXFR).
TCP guarantees reliable delivery and reassembly for large payloads that cannot fit into standard UDP DNS packets.

Key Concept

DNS Transport Protocols and TrunCation (TC) Mechanism
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DNS Lookup and Name Resolution Roles in Enterprise Networks

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