Reverse DNS lookup

Reverse DNS lookup ( rDNA ) refers to a DNS request, in which is to be determined to an IP address of the name. Alternative names are inverse queries, reverse lookup or inverse requests. The equivalent of the phone is the so-called reverse search, so the search for the name to a given phone number.

Background

In most cases the domain will Name System (DNS ) is used to determine a domain name of the associated IP address. There are, however, the reverse situation where the IP address given to one of the name is required. If this resolution is to allow a reverse domain will be created.

Technology

Since it would be extremely time-consuming to search for an inverse query the entire domain tree for the desired IPv4 address - it is not known to what road you will find the program you want to - a separate domain for inverse Hits was formed in -addr.arpa domain. Below this domain exist only three sub-domain levels, so that a maximum of three steps to resolve an IPv4 address are required.

The immediate subdomains of in-addr.arpa have as a label is a number between 0 and 255, representing the first component of an IPv4 address. (Examples: 10.in -addr.arpa or 192.in -addr.arpa ).

The next level in the tree represents the second component of an IPv4 address (example:. 16.172.in -addr.arpa contains the IPv4 addresses 172.16.xy ) and the lowest level, finally, the third component (example: 2.0.192.in - addr.arpa contains all known IPv4 addresses of the network 192.0.2.0/24 - so, for example, 192.0.2.69 ).

As can be seen from the examples, contains a reverse IP address name components in the reverse order. This structure enables a refining the reverse address space in several steps. For example, the network 198.51.0.0/16 initially by reverse domain 51.198.in - addr.arpa. are represented. All newly assigned IPv4 addresses from this segment are entered there. Later subdomains can be created (for example 100.51.198.in -addr.arpa ). Inverse queries to IP addresses that do not exist are subdomains, while continuing to be amortized over the global domain.

Reverse domains work just like normal. The same also applies for the appropriate zone files (see: Zone ). At the beginning of a reverse domain associated zone file an SOA resource record is followed by one or more NS resource records. As an additional RR types but only PTR resource records are allowed. For a PTR RR left is an IP address and a name right - in contrast to the A resource record, which left a name and an IP address is right.

For IPv6 addresses, this principle is used again. In addition to the previous domain ip6.arpa has been introduced. The hexadecimal digits of the IPv6 addresses are not listed individually reverse in blocks of four but.

Example:

1.113.0.203.in - addr.arpa. 1285 IN PTR server1.example.com. ba9.8.7.6.5.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.bd0.1.0.0.2.ip6.arpa. 1285 IN PTR server1.example.com. The corresponding entry in the example.com domain, then looks like this:

Server1.example.com. 1800 IN A 203.0.113.1 server1.example.com. 1800 IN AAAA 2001: db8 :: 567:89 from In order to determine the appropriate host name is analogous to the determination of an IP to a hostname using nslookup command. The query for the above example would look as follows.

Nslookup 203.0.113.1 nslookup 2001: db8 :: 567:89 from comments