Domain Name System (DNS)

 

One service that is used throughout networks and the Internet is the Domain Name System (DNS). Most users on the network connect to resources by using a friendly name such as www.gleneclarke.com—this style of name, known as a fully qualified domain name (FQDN), must be converted to an IP address before communication can occur. DNS is used as our solution to convert FQDNs to IP addresses.

    Let’s walk through an example of a name being resolved. You are sitting in your office on your network, and you try to connect to http://www.gleneclarke.com, which means that your client computer will send a query to the DNS server “Do you have an IP address for www.gleneclarke.com?” Your DNS server does not know who www.gleneclarke.com is, so it will then go out and query the DNS root servers and ask them if they have an IP address for

www.gleneclarke.com. There are only about 13 DNS root servers on the Internet; they don’t actually hold records for individual hosts, but they do forward the request to the name servers at the next level down, which are the .com name servers. DNS will find the IP address of the name requested by reading the name from right to left—in this case, .com is the far-right name part, so the request is forwarded to the .com name servers. The .com name servers look at the request and say, “No, we don’t have a clue who www.gleneclarke.com is, but why don’t you check out the gleneclarke.com name servers—they might know.”

       Your DNS server then queries the name servers for gleneclarke.com and asks them, “Do you have an IP address for www.gleneclarke.com?” In this case, the DNS servers for gleneclarke.com do have a record for www and they return the IP address to the DNS on your network. Now, your DNS server is smart—it will cache that data before sending an answer to your client computer that started the whole process, so that if anyone else is interested in the address, your DNS server will have the answer without going out on the web!

Top-Level Domains

The root servers are responsible for ensuring that any requests for an Internet resource are forwarded to the correct top-level domain. To help organize Internet resources in the DNS distributed database, there are specific top-level domain names created, and a company or organization has to register under a top-level domain. For example, if you are trying to connect to microsoft.com, the root DNS server forwards you to the .com top-level domain namespace.

The following is a description of the most popular DNS top-level domains found on the Internet:

• .com This is the commercial organizations group and is by far the largest.
Almost everyone wants to be found in this domain, because it is where most customers will try to find you.
• .org This is for nonprofit organizations.
• .net This is for networking organizations such as island.net and nfs.net, as well as for Internet service providers such as netzero.net.
• .mil This is for military organizations such as army.mil and navy.mil.
• .gov This is for U.S. government offices only.
• edu This is for educational organizations.

Country Domain Name

With only six top-level domain names and the requirement that all organizations register under one of those top levels if they wanted an Internet name, this soon became a problem because there were so many names to be registered and so few choices on the top levels. Eventually, top-level names were created for country domains; therefore, if you resided in a certain country, you could register under that country domain so that your customers could find you easily. Here are a few of the most common country domain names:

• .ca Canada
• .ie Ireland
• .uk United Kingdom
• .us United States

DNS Files

Most DNS servers maintain their DNS data in a number of files that exist on the hard disk of the server. In the old days, you managed the records by updating these text files, but today most DNS server environments support a graphic tool to create the records for your DNS server. When you create the records graphically, the DNS files are updated. Windows servers store their DNS files in %systemroot%\system32\DNS.

For Windows servers running Active Directory, the DNS database can be integrated with Active Directory, which allows you to have the DNS data replicated with Active Directory and also allows the zone data to be modified in multiple locations, given that Active Directory is a multimaster environment. If the DNS database is integrated with Active Directory, it will not be located in the %systemroot%\system32\DNS directory but be stored within the Active Directory database.

Hosts File

Before DNS became a popular solution to hostname resolution, there was a more manual method of creating and modifying a file on the local hard disk of every system. This file would need to be updated on every system if a change was made to a server’s IP address and you wanted the client applications to be aware of the change. This file, known as the hosts file, was located on each client system.

       Before DNS servers became a standard, network administrators used to create a text file known as the hosts file, which was used to resolve the FQDN to matching IP addresses. This text file was stored locally on each system; in the Windows world, it is stored in the %systemroot%\system32\drivers\etc folder and contains two columns—one for the IP address and the other for the FQDN.