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. example.com) 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 in-addr.arpa can to used to perform reverse lookups, from IP address to domain name. To do this, reverse the groups of the address, append .in-addr.arpa and do a PTR lookup. E.g. to discover a name for 217.146.107.7 do a PTR lookup on 7.107.146.217.in-addr.arpa.
Tools
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.
Domain Name System (DNS)
Reverse lookups (rDNS)
The special-purpose domain in-addr.arpa can to used to perform reverse lookups, from IP address to domain name. To do this, reverse the groups of the address, append .in-addr.arpa and do a PTR lookup. E.g. to discover a name for 217.146.107.7 do a PTR lookup on 7.107.146.217.in-addr.arpa.
Tools
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.