In computer networks, a reverse DNS lookup or reverse DNS resolution (rDNS) is the querying technique 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 of an IP address uses PTR records. rDNS involves searching domain name registry and registrar tables. This may be used to try to identify the originator's domain name to track, for example, a spammer sending spam emails or the domain name of a computer trying to break into a firewall or someone trying to hack the system. It may also be used to determine the name of the internet service provider assigned to a particular IP address. The reverse DNS database of the Internet is rooted in the .arpa top-level domain.
Although the informational RFC 1912 (Section 2.1) recommends 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.
Inverse queries take the form of a single resource record (RR) in the answer section of the message, with an empty question section. The owner name of the query RR and its time to live (TTL) are not significant. The response carries questions in the question section which identify all names possessing the query RR which the name server knows. Since no name server knows about all of the domain name space, the response can never be assumed to be complete. Thus inverse queries are primarily useful for database management and debugging activities. Inverse queries are not an acceptable method of mapping host addresses to host names; use the
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), to 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 220.127.116.11 the PTR record for the domain name
18.104.22.168.in-addr.arpa would be looked up, and found to point to
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.
Reverse DNS lookups for IPv6 addresses use the special domain
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
While most rDNS entries only have one PTR record, DNS does not restrict the number. Multiple PTR records are used, for example, when a web server supports many virtual hosts-- i.e. multiple hostnames then resolve to a single address, and multiple hostnames will be returned for a PTR lookup for that shared address. However, DNS lookups typically occur over UDP, and since UDP has a limited message size, in extreme cases, multiple PTRs could cause a DNS response to exceed those UDP limits.
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.
The most common uses of the reverse DNS include:
A reverse DNS lookup is a DNS query for the domain name associated with a given IP address. This accomplishes the opposite of the more-commonly-used forward DNS lookup, in which the DNS system is queried to return an IP address.