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Part 2: An introduction to DNS records

Brien Posey
When your Web browser checks the DNS server to find the IP address of the server hosting the Web site, it's actually looking for a record that lists the IP address of the Web site. DNS servers generally contain many different types of records. While it is beyond the scope of this tutorial to discuss all of them, here is an overview of some of the more important DNS records you should know about.

Start of Authority (SOA) records

There are often multiple DNS servers that service a domain. Multiple DNS servers might be used for load balancing, fault tolerance, or both. But, only one DNS server within a domain is considered authoritative.

The authoritative DNS server contains a read/write copy of the records database. Non-authoritative DNS servers are read-only. A Start of Authority (SOA) record points to the domain's authoritative DNS server. It also contains a sort of sequence number that is updated every time a change is made to the records contained within the zone. This helps non-authoritative DNS servers stay in sync with the authoritative DNS server.

Name Server (NS) records

A Name Server (NS) record is similar to an SOA record in that it points to the domain's DNS servers. The similarities end there though. Each DNS server in the domain gets its own NS record, regardless of whether it is authoritative or not.

Host records and Alias records

Probably the most common type of DNS record is a Host

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record (also called an A record). A Host record is simply used to associate a computer on your network with an IP address.

For example, I have a server on my network named RELEVANT. I therefore also have a Host record on my DNS server that links the name RELEVANT to the server's IP address.

It just so happens that the server RELEVANT is hosting a Web site on my domain. However, when people want to visit my Web site, they do not enter relevant.brienposey.com, even though that is the fully qualified domain name (FQDN) of the server hosting the Web site.

Instead, they enter www.brienposey.com. This is where the Alias record (also called a CNAME record) comes into play. An Alias record's job is to associate an alternate name with a computer for which there is already a Host record. For example, suppose that the host record for relevant looked like this:

Relevant Host (A) 147.100.100.97

When someone enters www.brienposey.com, I want them to be directed to the relevant.brienposey.com server. Since there is no server on the network named "www," I set up an Alias record that associates www with relevant.brienposey.com. The Alias record looks something like this:

www  Alias (CNAME) relevant.brienposey.com
MX Records

MX records

An e-mail message finds its way to its destination via an MX record.

An e-mail address is made up of a recipient name and a domain name (recipient@brienposey.com). I already explained how DNS queries are used to locate a DNS server for a specific domain. The process works exactly the same way when you send an e-mail message to a recipient in another domain.

The difference is that your mail client isn't interested in the IP address of your Web server -- it needs to know the IP address of your mail server. That's where the MX record comes in.

An MX record stores the IP address of your SMTP server, so e-mail clients can determine where a message should be sent. They simply perform a DNS query against a domain's MX record to get the IP address of the organization's SMTP server.


TUTORIAL: A PRIMER ON DNS AND MX RECORDS

 Home: Introduction
 Part 1: How DNS works
 Part 2: An introduction to DNS records
 Part 3: How to create an MX record
 Part 4: How to set up a Host record

ABOUT THE AUTHOR:   
Brien M. Posey, MCSE
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Exchange Server, and has previously received Microsoft's MVP award for Windows Server and Internet Information Server (IIS). Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal Web site at http://www.brienposey.com.

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