what is Reverse lookup

Reverse lookup

In most DNS lookups, clients typically perform a forward lookup, which is a search based on the DNS name of another computer as stored in an address (A) resource record. This type of query expects an IP address as the resource data for the answered response.

DNS also provides a reverse lookup process, enabling clients to use a known IP address during a name query and look up a computer name based on its address. A reverse lookup takes the form of a question, such as "Can you tell me the DNS name of the computer that uses the IP address 192.168.1.20?"

DNS was not originally designed to support this type of query. One problem for supporting the reverse query process is the difference in how the DNS namespace organizes and indexes names and how IP addresses are assigned. If the only method to answer the previous question was to search in all domains in the DNS namespace, a reverse query would take too long and require too much processing to be useful.

To solve this problem, a special domain, the in-addr.arpa domain, was defined in the DNS standards and reserved in the Internet DNS namespace to provide a practical and reliable way to perform reverse queries. To create the reverse namespace, subdomains within the in-addr.arpa domain are formed using the reverse ordering of the numbers in the dotted-decimal notation of IP addresses.

This reversed ordering of the domains for each octet value is needed because, unlike DNS names, when IP addresses are read from left to right, they are interpreted in the opposite manner. When an IP address is read from left to right, it is viewed from its most generalized information (an IP network address) in the first part of the address to the more specific information (an IP host address) contained in the last octets.

For this reason, the order of IP address octets must be reversed when building the in-addr.arpa domain tree. The IP addresses of the DNS in-addr.arpa tree can be delegated to companies as they are assigned a specific or limited set of IP addresses within the Internet-defined address classes.

Finally, the in-addr.arpa domain tree, as built into DNS, requires that an additional resource record (RR) type — the pointer (PTR) RR — be defined. This RR is used to create a mapping in the reverse lookup zone that typically corresponds to a host (A) named RR for the DNS computer name of a host in its forward lookup zone.

Note

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The in-addr.arpa domain applies for use in all TCP/IP networks that are based on Internet Protocol version 4 (IPv4) addressing. The New Zone Wizard automatically assumes that you are using this domain when creating a new reverse lookup zone. If you are installing DNS and configuring reverse lookup zones for an Internet Protocol version 6 (IPv6) network, you can specify an exact name in the New Zone wizard. This will permit you to create reverse lookup zones in the DNS console that can be used to support IPv6 networks, which uses a different special domain name, the ip6.arpa domain. Additional information is available on IPv6 and DNS, including examples of how to create and use ip6.arpa domain names in Request for Comments (RFC) 3596, "DNS Extensions to support IP version 6." For more information, refer directly to this RFC, which can be obtained from the RFC Editor Web site.

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Example: Reverse query (for IPv4 networks)

The following figure shows an example of a reverse query initiated by a DNS client (host-b) to learn the name of another host (host-a) based on its IP address, 192.168.1.20.

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The reverse query process as shown in this figure occurs in the following steps:

1.	The client, "host-b", queries the DNS server for a pointer (PTR) RR that maps to the IP address of 192.168.1.20 for "host-a". Because the query is for PTR records, the resolver reverses the address and appends the in-addr.arpa domain to the end of the reverse address. This forms the fully qualified domain name ("20.1.168.192.in-addr.arpa.") for which to be searched in a reverse lookup zone.
2.	Once located, the authoritative DNS server for "20.1.168.192.in-addr.arpa" can respond with the PTR record information. This includes the DNS domain name for "host-a", completing the reverse lookup process.

Keep in mind that if the queried reverse name is not answerable from the DNS server, normal DNS resolution (either recursion or iteration) can be used to locate a DNS server that is authoritative for the reverse lookup zone and that contains the queried name. In this sense, the name resolution process used in a reverse lookup is identical to that of a forward lookup.