What is 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.

Free White Papers on Networking

Books on DNS

DNS and BIND

DNS and BIND is about one of the Internet's fundamental building blocks: the distributed host information database that's responsible for translating names into addresses, routing mail to its proper destination, and many other services. As the authors say in the preface, if you're using the Internet, you're already using DNS--even if you don't know it.

This edition brings you up to date on the new 9.1.0 and 8.2.3 versions of BIND along with the older 4.9 version. There's also more extensive coverage of NOTIFY, IPv6 forward and reverse mapping, transaction signatures, and the new DNS Security Extensions; and a new section on accommodating Windows 2000 clients, servers and Domain Controllers.

Whether you're an administrator involved daily with DNS or a user who wants to be more informed about the Internet and how it works, you'll find this book essential reading.

Topics include:

What DNS does, how it works, and when you need to use it
How to find your own place in the Internet's name space
Setting up name servers
Using MX records to route mail
Configuring hosts to use DNS name servers
Subdividing domains (parenting)
Securing your name server: restricting who can query your server, preventing unauthorized zone transfers, avoiding bogus name servers, etc.
Mapping one name to several servers for load sharing
Troubleshooting: using nslookup, reading debugging output, common problems
DNS programming, using the resolver library and Perl's Net::DNS module

DNS and BIND Cookbook

The DNS and BIND Cookbook presents solutions to the many problems faced by network administrators responsible for a name server. Following O'Reilly's popular problem-and-solution cookbook format, this title is an indispensable companion to DNS and BIND, the definitive guide to the critical task of name server administration. The cookbook contains dozens of code recipes showing solutions to everyday problems, ranging from simple questions, like, "How do I get BIND?" to more advanced topics like providing name service for IPv6 addresses. This book is full of BIND configuration files that you can adapt to your sites requirements.

With the wide range of recipes in this book, you'll be able to:

Check whether a name is registered
Register your domain name and name servers
Create zone files for your domains
Delegate subdomains and check delegation
Set up back-up mail servers and virtual email addresses
Use incremental transfer
Secure zone transfers
Restrict which queries a server will answer
Protect your name server from abuse
Perform logging and troubleshooting
Upgrade to BIND 9 from earlier version
Use IPv6

DNS on Windows Server 2003

While computers and other devices identify each other on networks or the Internet by using unique addresses made up of numbers, humans rely on the DNS (Domain Name System), a distributed database that allows us to identify machines by name. DNS does the work of translating domain names into numerical IP addresses, routing email to its proper destination, and many other services, so that users require little or no knowledge of the system. If you are a network or system administrator, however, configuring, implementing, and maintaining DNS can be a formidable challenge. Now, with Windows Server 2003, an understanding of the workings of DNS is even more critical.

DNS on Windows Server 20003 is a special Windows-oriented edition of the classic DNS and BIND, newly updated to document the many changes to DNS, large and small, found in Windows Server 2003. Cricket Liu, Matt Larson, and Robbie Allen explain the whole DNS system in terms of the new Windows Server 2003, from starting and stopping the DNS service to establishing an organization's namespace in the global hierarchy.

Besides covering general issues like installing, setting up, and maintaining the server, DNS on Windows Server 2003 tackles issues specific to the Windows 2003 environment, including the use of the `dnscmd` program to manage the Microsoft DNS Server from the command line and development using the WMI DNS provider to manage the name server programmatically. DNS on Windows Server 2003 also documents new features of the Microsoft DNS Server in Windows Server 2003, including conditional forwarding and zone storage in AD (Active Directory) application partitions.

DNS on Windows Server 2003 provides DNS administrators with a thorough grounding in:

System tuning
Caching
Zone change notification
Security issues
Troubleshooting
Planning for growth

If you're a Windows administrator, this book is the operational manual you need for working with DNS every day. If you're a Windows user who simply wants to take the mystery out of the Internet, this book is a readable introduction to the architecture and inner workings of the Internet.