RFC 8482 / START HERE
Minimal DNS ANY answers, before the details
DNS means Domain Name System. ANY names query type 255: a request for some or all available records.
RFC 8482 lets a DNS responder choose a smaller answer to an ANY query. It can return selected complete record sets, or a small Host Information (HINFO) record in the allowed context. A record set groups records with the same name, type and class.
The idea in one picture
Ask QTYPE ANY
Select a smaller answer
Request needed types
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
Large ANY answers can cost work and amplify spoofed User Datagram Protocol (UDP) traffic. A smaller, cacheable answer can reduce that cost without inventing a new error code. The behavior is optional and updates part of the original DNS specifications.
A familiar example
A fictional diagnostic query for lab.example receives only its two IPv4 address records. IPv6 and text records may still exist. A tool that needs IPv6 information can ask explicitly for AAAA instead of treating the ANY answer as a complete inventory.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Read ANY as some or all, not a complete inventoryA DNS question has a query type, QTYPE. ANY is the common name for type 255; it is a meta-query, not a promise to list every kind of record.→
- Select whole RRsets without inventing truncationA responder can send one available RRset or a larger subset. Choosing fewer types differs from cutting records out of a selected set or running out of packet space.→
- Build a synthesized HINFO only in its allowed contextHost Information (HINFO) is a DNS record with CPU (processor) and OS (operating system) character-string fields. RFC 8482 can use a small synthesized HINFO answer when the queried owner has no CNAME alias.→
- Cache normal records and choose TTL with careAn initiator can cache the answer normally. A time to live, TTL, limits how long the cached record remains usable; it is not a permanent ban on asking a name.→
- Keep transport policy, heuristics and security separateA responder can guess likely useful types, or use different ANY policies for User Datagram Protocol (UDP) and Transmission Control Protocol (TCP). These are optional choices with distinct tradeoffs, not an automatic sequence.→
This introduction explains RFC 8482 (2019). Read the original RFC ↗