What is DNS?

Domain Name System (DNS) is an Internet Engineering Task Force (IETF) standard name service that allows your computer to register and resolve domain names. The DNS makes it possible to assign domain names to organizations independent of the routing of the numerical IP address. In other words, DNS is a system that translates domain names into IP addresses. This is necessary because computers only make use of IP addresses yet we use only human readable names since the names are easier to remember than IP addresses. Without this DNS resolution, the internet would be a very inconvenient place. DNS resolution is therefore a very important task. However, you may sometimes try to connect to a system by name and get a DNS error because the name did not resolve to the proper IP address. There can be several causes for this, such as:

The DNS server is down.
IP connectivity gets lost and thus the DNS cannot resolve
DNS cache poisoning.
Update and zone issues.
The DNS server does not have network connectivity to the root servers.

There are a number of ways to find out whether your system is resolving properly. You can use nslookup to verify name resolution. The nslookup command can be used to find various details relating to a particular DNS (Domain Name System) such as IP address, MX records etc.

Go to the command prompt and type in nslookup host_name server_IP_address. Replace the actual host name that you are trying to resolve for host_name and the IP address of the DNS server for server_IP_address then press enter.

This will allow you to verify if an error is on the server, if there is a widespread resolution error, or if the server is possibly down. Nslookup will also display the various types of DNS records, not just primary (A) records, or all records for a domain. You can then ping with the -a switch to also verify if DNS resolution is working fine.

Troubleshoot the dns client since most problems start with failed queries at the client. If a dns server provides incorrect data to queries that it successfully answers, then the most likely causes would be:

Resource records (RRs) were not dynamically updated in a zone.
An error was made when manually adding or modifying static resource records in the zone.
Stale resource records in the DNS server database, left from cached lookups or zone records not updated with current information or removed when they are no longer needed.

If the dns server does not resolve names for external networks, then the possible causes could be:

The recursive query times out before it can be completed.
A remote DNS server fails to respond.
A remote DNS server provides incorrect data.
DNS server recursion has been disabled.

Troubleshoot also the connectivity to the root servers. Verify that the DNS server used in a failed query can ping its root servers by IP address. If a ping attempt to one root server fails, it might indicate that an IP address for that root server has changed.

Books on DNS

DNS in Action: A Detailed and Practical Guide to DNS Implementation, Configuration, and Administration	The Domain Name System is one of the foundations of the internet. It is the system that allows the translation of human-readable domain names into machines-readable IP addresses and the reverse translation of IP addresses into domain names. This book describes the basic DNS protocol and its extensions; DNS delegation and registration, including for reverse domains; using DNS servers in networks that are not connected to the internet; and using DNS servers on firewall machines. Many detailed examples are used throughout the book to show perform various configuration and administration tasks.
	Read a sample chapter from DNS in Action

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 & BIND Cookbook	The DNS & 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 & BIND, 4th Edition, 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. It's 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 Protect your name server from abuse Set up back-up mail servers and virtual email addresses Delegate subdomains and check delegation Use incremental transfer Secure zone transfers Restrict which queries a server will answer Upgrade to BIND 9 from earlier version Perform logging and troubleshooting 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 Domain Name System (DNS), the distributed database that allows us to identify machines by name. DNS does the work of translating domain names into numerical IP addresses, routing mail to its proper destination, and many other services, so that users require little or no knowledge of the system. If you're a network or system administrator, however, configuring, implementing, and maintaining DNS zones can be a formidable challenge. And 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. Veteran O'Reilly authors, Cricket Liu, Matt Larson, and Robbie Allen explain the whole system in terms of the new Windows Server 2003, from starting and stopping a 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 the many issues specific to the new Windows 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. The book also documents new features of the Microsoft DNS Server in Windows Server 2003, including conditional forwarding and zone storage in Active Directory (AD) application partitions. DNS on Windows Server 2003 provides grounding in: Security issues System tuning Caching Zone change notification Troubleshooting Planning for growth If you're a Windows administrator, DNS on Windows Server 2003 is the operations 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 Internet's architecture and inner workings.