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Reverse DNS Delegations
Posted by admin on 11 September 2009 09:40 PM
The hardest part to understand about PTR records for inexperienced DNS administrators is delegation. For forward zones (i.e. domains), the domain is registered using a domain registrar, and the registrar is told to “point” the domain to a particular server. This is delegation.

In the case of reverse zones, the situation is similar, except that instead of a registrar delegating the zone, it’s usually the ISP that creates the delegation. Or doesn’t, in many cases, leaving the operator of the subnet unable to effectively create PTR records. In such cases, the operator of the subnet must rely on the controller of the reverse zone (usually an ISP) to manage the PTR records properly.

The key point is that having control over a domain and a subnet doesn’t give control over the reverse zone for that subnet. It’s up to whoever assigns the subnet to also assign control of the reverse zone, or decide not to do so. In the case of subnets smaller than a class C (a 24-bit subnet), RFC 2317 provides a method of delegating control of the PTR records.

Note that, for private subnets (e.g. 192.168.0/24 or, there is no delegation of the reverse zone. In such cases, the DNS server that the client machines (and internal servers such as mail servers) use for DNS lookups usually must either host the reverse zone or know where to find it (what other servers to query).
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