DNS normally uses UDP port 53. In which situation does a DNS client or server fall back to TCP on port 53?
LPIC-1 Exam 102-500, objective 109. Networking fundamentals hard
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The options
Correct When the response is too large for a single UDP datagram and comes back with the truncation flag set, and for zone transfers between servers
Correct. A truncated (TC bit) response tells the resolver to retry over TCP, and zone transfers (AXFR/IXFR) always use TCP because they are bulk, reliable transfers.
Not correct Whenever the query asks for an MX or TXT record rather than an A record
Wrong. The record type does not choose the transport. Any record type is queried over UDP first; only the response size matters.
Not correct When the query is a reverse lookup in the in-addr.arpa zone
Wrong. Reverse lookups are ordinary queries and use UDP first exactly like forward lookups.
Not correct Only when the resolver is configured with the `options tcp` directive in /etc/resolv.conf
Wrong. There is no `tcp` option in resolv.conf. The glibc resolver does accept `options use-vc`, which forces TCP, but the fallback described above happens automatically without any configuration.
Why
DNS is a rare protocol that uses both transports on the same port number. Queries go out over UDP 53 for speed; if the answer exceeds what the negotiated UDP size allows, the server returns a short reply with the TC (truncated) bit set and the client repeats the query over TCP 53. Zone transfers use TCP unconditionally. A firewall that permits only UDP 53 therefore breaks large responses such as DNSSEC-signed answers.
Where this comes from
- Cited
- LPI exam objective 109.1
- What it says
- Understand the difference between TCP and UDP and which transport common services use.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 109 and space the ones you get wrong.