Domain Name System (DNS)

A fundamental part of the Internet, essentially a specialised distributed database, the DNS translates one type of thing into another. The usual type of translation, an “A” (for address) lookup, translates a domain name (the human-readable hostname, e.g. into an IP address. The address can then be used for sending IP packets to that system.Lookup types:

Lookup type Usage
A Address normal lookup
MX Mail Exchanger system accepting mail for the domain
CNAME Canonical name
PTR Pointer reverse lookup
TXT uninterpreted text available for any use
SOA Start Of Authority
HINFO Host Information
NS Name Server
AAAA IPv6 lookup
A6 Alternate IPv6 lookup

Reverse lookups (rDNS)

The special-purpose domain can to used to perform reverse lookups, from IP address to domain name. To do this, reverse the groups of the address, append and do a PTR lookup. E.g. to discover a name for do a PTR lookup on


On unix systems, “dig” and “nslookup”.

Resolver software

The most popular on Unix systems is “bind”.

DNSBL (DNS-accessible blacklists)

These use the capability of the DNS to publish lookup capability into lists of IP addresses. Typically the query is done as a “A” lookup of the reversed IP prepended to the list name. Some also offer TXT records.