What is DNS?

The Domain Name System (DNS) is a distributed Internet directory service. DNS is used to translate domain names into IP addresses, and to control Internet e-mail delivery. The DNS directory service consists of DNS data, DNS servers, and Internet protocols for fetching data from the servers. The billions of resource records in the DNS directory are split into millions of files called zones. Zones are kept on authoritative servers distributed all over the Internet, which answer queries according to the DNS network protocols. In contrast, caching servers simply query the authoritative servers and cache any replies. Most servers are authoritative for some zones and perform a caching function for all other DNS information. Most DNS servers are authoritative for just a few zones, but larger servers are authoritative for thousands of zones.

Since most dedicated server or shared server are Unix machines, the most used program is BIND (Berkeley Internet Name Domain). BIND 9.2.1 is the latest release of BIND. DNS software is made up of two components the name server and a resolver. The name server responds to browser requests by supplying name-to-address conversions. When it doesn’t know the address, the resolver will ask another name server for the information.

How does DNS work?

At the top of the DNS database tree are root name servers, which contain pointers to master name servers for each of the top-level domains (.com, .net, etc.). For example, to find out the numeric IP address of www.eukhost.com, a DNS server would ask the root name server for the address of the master name server for the .net domain. The master name servers for each of the top-level domains contain a record and name-server address of each domain name. So the DNS server asks the .net server for the name of the server that handles the simplythebest.net domain. The individual name server for each domain name contains detailed address information for the hosts in that domain. So the DNS server asks the eukhost.com server for the name of the server that handles the simplythebest.net domain. Finally this name server supplies the DNS server with the IP address of the machine called www.eukhost.com.

When you type in a URL, your browser sends a request to the closest name server. If that server has ever received a request for the same host name (within a time period set by the administrator to prevent passing old information), it will locate the information in its cache and reply. If the name server is unfamiliar with the domain name, the resolver will ask a server further up the tree. If that doesn’t help that server will ask yet another, until it finds a server that knows. A server that knows the address without asking another is an authoritative server. Once the address is located, it is sent back to your browser and you get to see the related web site.

Setting up a DNS server

There are two ways to configure DNS. You can use your ISP’s DNS server or you set up your own DNS server. In the first case you don’t have to register the name server (your ISP already did) with a domain registrar, in the latter case you will have to appoint a NS1 and NS2 name server, f.e. ns1.eukhost.com, and ns2.eukhost.com (a primary and a secondary name server).

In both cases, you will have to provide your domain registrar with at least two name server addresses to link to your domain. You will also have to add these DNS addresses to your TCP/IP stack on your server. If your ISP provides the DNS server you need to forward him any domain names which will reside on your server. Your ISP will then add the necessary MX records and A records to the DNS server.

MX records
If you want to receive e-mail from the Internet, you will need to have a Mail Exchange (MX) record for your domain in your ISPs or your own DNS database. An MX record has 3 parts: your domain name, the name of the machine that will accept mail for the domain, and a preference value. The preference value lets you build in a priority level, f.e. etech.net mail.etech.net 0. You can set multiple MX records, and the mail server will attempt the one with the lowest preference value first, and if it fails try the next, and so forth.

A records
A records associate IP addresses with computer names. For example, an A record links the name ‘yourdomain.com’ to the IP address of your Web server.

CNAME records
DNS allows machines to have a true (canonical name), as well as an unlimited number of aliases. The CNAME record takes care of aliases. CNAME records can be useful when you want a subdomain to point to a computer outside of your domain.
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