In computer networks, a reverse DNS lookup or reverse DNS resolution (rDNS) is the querying of the Domain Name System (DNS) to determine the domain name associated with an IP address – the reverse of the usual "forward" DNS lookup of an IP address from a domain name. The process of reverse resolving an IP address uses PTR records. The reverse DNS database of the Internet is rooted in the.arpa top-level domain.
Contents
- IPv4 reverse resolution
- Classless reverse DNS method
- IPv6 reverse resolution
- Multiple pointer records
- Records other than PTR records
- Uses
- References
Although the informational RFC 1912 (Section 2.1) specifies that "Every Internet-reachable host should have a name" and that "For every IP address, there should be a matching PTR record...", it is not an Internet Standard requirement, and not all IP addresses have a reverse entry.
IPv4 reverse resolution
Reverse DNS lookups for IPv4 addresses use the special domain in-addr.arpa
. In this domain, an IPv4 address is represented as a concatenated sequence of four decimal numbers, separated by dots, to which is appended the second level domain suffix .in-addr.arpa
. The four decimal numbers are obtained by splitting the 32-bit IPv4 address into four octets and converting each octet into a decimal number. These decimal numbers are then concatenated in the order: least significant octet first (leftmost), most significant octet last (rightmost). It is important to note that this is the reverse order to the usual dotted-decimal convention for writing IPv4 addresses in textual form.
For example, to do a reverse lookup of the IP address 8.8.4.4
the PTR record for the domain name 4.4.8.8.in-addr.arpa
would be looked up, and found to point to google-public-dns-b.google.com
.
If the A record for google-public-dns-b.google.com
in turn pointed back to 8.8.4.4
then it would be said to be forward-confirmed.
Classless reverse DNS method
Historically, Internet registries and Internet service providers allocated IP addresses in blocks of 256 (for Class C) or larger octet-based blocks for classes B and A. By definition, each block fell upon an octet boundary. The structure of the reverse DNS domain was based on this definition. However, with the introduction of Classless Inter-Domain Routing, IP addresses were allocated in much smaller blocks, and hence the original design of pointer records was impractical, since autonomy of administration of smaller blocks could not be granted. RFC 2317 devised a methodology to address this problem by using CNAME records.
IPv6 reverse resolution
Reverse DNS lookups for IPv6 addresses use the special domain ip6.arpa
(previously ip6.int
). An IPv6 address appears as a name in this domain as a sequence of nibbles in reverse order, represented as hexadecimal digits as subdomains. For example, the pointer domain name corresponding to the IPv6 address 2001:db8::567:89ab
is b.a.9.8.7.6.5.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa
.
Multiple pointer records
While most rDNS entries only have one PTR record, DNS does not restrict the number. However, having multiple PTR records for the same IP address is generally not recommended, unless there is a specific need. For example, if a web server supports many virtual hosts, there may be one PTR record for each host and some versions of name server software will allocate this automatically. Multiple PTR records can cause problems, however, including triggering bugs in programs that only expect single PTR records. In the case of a large web server, having hundreds of PTR records can cause the DNS packets to be much larger than normal, which can cause the query to be requested over TCP when they exceed the DNS 512 byte UDP message limit.
Records other than PTR records
Record types other than PTR records may also appear in the reverse DNS tree. For example, encryption keys may be placed there for IPsec, SSH and IKE. DNS-Based Service Discovery uses specially-named records in the reverse DNS tree to provide hints to clients about subnet-specific service discovery domains. Less standardized usages include comments placed in TXT records and LOC records to identify the geophysical location of an IP address.
Uses
The most common uses of the reverse DNS include: