Reverse DNS is the opposite of DNS lookup. Normally when a domain name or an address for a web site is entered, this is resolved to the respective IP address using the domain name service. The reverse DNS on the other hand enables resolving the IP address back to the name of the Domain address. Reverse DNS is normally not used for general purposes and is possibly useful only to those people who are tracing a specific IP address that they have otherwise obtained.

DNS reverse search or reverse DNS query is used if the IP address is known to you. From the IP address a reverse resolution is done which would help in identifying the ‘reverse-IN-ADDR’ entry for that IP address. From this ‘reverse-IN-ADDR’ entry, a PTR record or a pointer record is normally obtained. This pointer record is the storage area where the canonical name or the web name that is normally used is stored for the IP address and shows up when you look for the reverse DNS. When there is an error in the PTR record or in the ‘reverse-IN-ADDR’ then ‘DNS reverse lookup not working’ error or ‘451 bad reverse DNS’ error shows up. Such reverse DNS errors or reverse DNS lookup failures do not happen every day.

The reverse DNS lookup was used originally to trace back a route. These use to be a part of the network trouble shooting tools such as traceroute and ping. DNS reverse lookup helps in tracking the emails and forums to identify the path that an email has taken or the forum posting has taken. Spam emails are identified by appropriately identifying the rDNS or the reverse DNS associates of that email. This would also help in identifying the phishers, particularly if the person is using zombies to create domains. Reverse DNS would thus reduce both spamming and phishing.

Sometimes the DNS reverse entry in the PTR record might not be found. Under this condition, the reverse DNS lookup might fail. The whois utility which is used for identifying the owner of the DNS is employed in such cases. The whois reverse DNS lookup would help in these cases to identify the canonical name. The reverse DNS lookup whois is the tool for this purpose.

There are many utilities used for administering a web site. Some of these like the cPanel are also sometimes used for tracing the reverse route. The cPanel reverse DNS can be enabled so that it passes the IP address with every one of the mail that goes out. This is insisted by some of the Internet service providers.