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Difficulty: MediumDNS Lookup and Name Resolution Roles in Enterprise Networks

An enterprise network engineer is analyzing DNS name resolution behavior across internal workstations and external edge firewalls. The infrastructure uses an internal Active Directory DNS server that forwards unknown domain requests to an external resolver. Which two statements correctly describe DNS transport protocol operation and name resolution roles in this environment?

  1. Standard DNS queries and responses primarily utilize UDP port 53, but DNS transitions to TCP port 53 when a response payload exceeds 512 bytes (without EDNS0 extensions).Answer
  2. B
    Standard host DNS queries sent to internal DNS servers default to TCP port 53 to guarantee reliable end-to-end transport for all name resolution requests.
  3. The internal Active Directory DNS server acts as a recursive resolver for enterprise client hosts while functioning as an authoritative server for the local internal domain namespace.Answer
  4. D
    An authoritative DNS server initiates recursive queries to root servers on behalf of enterprise client hosts seeking public internet domain addresses.
  5. E
    DNS zone transfers between internal primary and secondary DNS servers exclusively utilize UDP port 53 to maximize throughput during database replication.

Answer

The correct statements are that DNS queries primarily use UDP port 53 but switch to TCP port 53 when response payloads exceed 512 bytes without EDNS0, and that the internal Active Directory DNS server acts as a recursive resolver for internal clients while being authoritative for the local domain.
The correct options accurately identify that DNS relies primarily on UDP port 53 for standard client lookups, switching to TCP port 53 when responses exceed 512 bytes without EDNS0 extensions. Additionally, they correctly identify that an internal enterprise DNS server serves a dual role: authoritative for internal zone records and recursive resolver for endpoint clients requesting external or un-cached domain names.

Step-by-Step Solution

1
Analyze transport layer protocol usage for DNS queries and responses.
DNS uses UDP port 53 for standard low-overhead queries and responses under 512 bytes. If a response sets the Truncation (TC) bit because it exceeds 512 bytes, the client re-issues the query using TCP port 53.
UDP avoids connection establishment latency for small lookup packets, but TCP provides reliable stream delivery for oversized payloads.
2
Evaluate the distinction between authoritative DNS servers and recursive resolvers.
An internal Active Directory DNS server holds authoritative records for internal resources (such as domain controllers and local hostnames) and resolves non-local queries on behalf of client hosts recursively or via forwarders.
Clients do not query root and TLD servers directly; they rely on their configured recursive resolver to navigate the DNS hierarchy or forward requests.
3
Evaluate incorrect protocol and role assumptions.
Zone transfers require TCP port 53 due to database size and reliability requirements. Clients do not default to TCP port 53 for normal queries, nor do authoritative-only servers perform recursion for external clients.
Distinguishing between control plane operations (zone transfers), data plane lookups (client queries), and server roles (authoritative vs. recursive) clarifies correct DNS architecture.

Key Concept

DNS Transport Protocols and Server Roles in Enterprise Networks
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Topics

DNS Lookup and Name Resolution Roles in Enterprise Networks

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