Nameservers contain zone files (database files for domains). Zone files contain resource records. Resource records describe specific network resources that are available. The ability to use names for resources such as e-mail exchangers, nameservers and web servers are configured within zone files using resource records.
SPECIAL Resource Record Types
Start of Authority (SOA) and Nameserver (NS) records are special because they must be included in every database zone file. Most DNS systems do not make distinctions between forward and reverse zones, the resolution process works the same.
The only difference between forward and reverse resolution is the files the DNS nameserver uses to find the answers to queries and the records inside those zone files.
FORWARD Lookup Resource RECORDs
The following resource record types are used in zone files configured to provide forward resolution (name to IP address translation) of domains and host names.
Reverse Lookup Resource RECORDs
The reverse zone has only three types of records, PTR, NS and SOA. Reverse zones allow DNS nameservers to turn IP addresses into fully qualified domain names (FQDN). Why is reverse resolution important and why do we need to set it up?
With the growth of SPAMHAUSES (places that send out unsolicited bulk commercial e-mail) it is easier to translate the IP address back into a domain name and block the domain name than to block the individual IP addresses of the spammer's e-mail servers. Some mail servers automatically block mail received from hosts they cannot reverse resolve, assuming it is an attack or junk mail.
Another use is for the authorization to download software with encryption capabilities. If the IP address of the host contacting the dowload server can be reverse resolved to a host in a domain that is registered inside the continental United States, then the download can be permitted.
When performing reverse lookup, a specific resource record type is used: