RFC 2317 / START HERE
Classless reverse DNS, before the details
Classless IN-ADDR.ARPA delegation is a descriptive name for sharing control of reverse DNS. IN-ADDR.ARPA is the DNS tree used to look up names from IPv4 addresses.
It lets an organization manage reverse-DNS names for a small block such as 64 addresses, even though the ordinary reverse tree is divided at whole-byte boundaries.
Why it exists
An address prefix can end between bytes, while DNS delegation follows name labels. This publication connects the ordinary reverse names to separately administered names using existing DNS records and lookup rules.
A familiar example
For fictional 192.0.2.129, a parent CNAME leads to a delegated child name, which has a PTR record pointing to host1.b.example. The parent must cooperate; a reverse name is not proof of identity or a routing instruction.
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.
- Make room below an octet boundaryReverse DNS asks for a name associated with an IP address. A /26 IPv4 block contains 64 addresses, but the dotted reverse-name tree does not automatically create a boundary for those six host bits.→
- Delegate a real child zoneA delegation gives named authoritative servers responsibility for a child zone. The label 128-26 is an operator-chosen DNS label; a resolver does not interpret it as a subnet mask.→
- Connect the alias to the PTR ownerA CNAME record says that one DNS name is an alias of another. The parent holds the ordinary reverse alias; the child holds the PTR at the new target name.→
- Resolve through the alias and delegationA recursive resolver follows the CNAME target using ordinary DNS resolution. This chosen uncached sequence shows separate parent and child servers; caching or a parent that also serves the child can reduce the exchanges.→
- Plan the names and operational dependenciesThe aliases keep reverse lookup working even when PTR data lives elsewhere. That flexibility still depends on the delegating parent and on a consistent naming plan.→
This introduction explains RFC 2317 (1998). Read the original RFC ↗