Reverse DNS

Reverse DNS is the process of using DNS to translate IP addresses to hostnames.

Reverse DNS is the opposite of forward DNS, which is used to translate hostnames to IP addresses.

Internet names are the names which we use to refer to hosts on the Internet, such as www.tech-faq.com and www.freebsd.org.

IP addresses are the numbers which Internet routers use to move traffic across the Internet, such as 216.17.138.115 and 216.136.204.117.

Reverse DNS Lookups

One of the best ways to understand reverse DNS is to use the DNS testing tool, `nslookup` to do some a sample reverse DNS lookup.

Let's use `nslookup` to do a reverse DNS lookup on the IP address 216.136.204.117:

bash-2.05a$ nslookup 216.136.204.117
Server: localhost.net
Address: 127.0.0.1

Name: www.freebsd.org
Address: 216.136.204.117

Reverse DNS PTR Records

Reverse DNS is setup by configuring PTR records (Pointer Records) in your DNS server.

This is in contrast to Forward DNS, which uses A records (Address Records).

Reverse DNS Delegation

When you register a domain name with a domain registrar, you usually become responsible for that Forward DNS domain. In DNS terms, the domain is delegated to you.

However, you are not also responsible for your reverse records. Your Reverse DNS records are still most likely to be the responsibility of your hosting facility or ISP.

To make changed to your Reverse DNS PTR records, you must contact the company where you get your IP addresses from, usually a hosting facility or an ISP.

Alternatively, your ISP or hosting company may delegate a range of IP addresses to you, in which case you must configure Reverse DNS and PTR records in your DNS server.

Is Reverse DNS Necessary?

Some junior DNS administrators configure forward DNS and forget to configure reverse DNS.

When they do this, some things work fine. Internet web browsing, for example, works great. However, not everything works.

Reverse DNS is required by some Internet protocols and by extensions to some other Internet protocols. Without reverse DNS, you will experience trouble with r-commands, IRC, some SMTP servers, most enterprise management systems, and many network backup systems.

Troubleshooting problems caused by faulty or non-existant reverse DNS can take considerable time and effort. It is much better to ensure that reverse DNS is configured correctly from the beginning.