What Is Domain Name Server
Domain Name Server (DNS) is a database system that translates a computer's fully qualified domain name into an ipaddress.
Networked computers use IP addresses to locate and connect to each other, but IP addresses can be difficult for people to remember. For example, on the web, it's much easier to remember the domain name www.gmail.com than it is to remember its corresponding IP address (74.125.79.83). DNS allows you to connect to another networked computer or remote service by using its user-friendly domain name rather than its numerical IP address. Conversely, Reverse DNS (rDNS) translates an IP address into a domain name.
How Domain Name Servers Work
If you spend any time on the Internet sending email or browsing the Web, then you use domain name servers without even realizing it. Domain name servers, or DNS, are an incredibly important but completely hidden part of the internet , and they are fascinating. The DNS system forms one of the largest and most active distributed databases on the planet. Without DNS, the Internet would shut down very quickly.
When you use the Web or send an e-mail message, you use a domain name to do it. For example, the URL "http://www.gmail.com" contains the domain name gmail.com. So does the e-mail address "vndpundir2007@gmail.com."
Human-readable names like "gmail.com" are easy for people to remember, but they don't do machines any good. All of the machines use names called IP addresses to refer to one another. For example, the machine that humans refer to as "www.gmail.com" has the IP address 74.125.79.83. Every time you use a domain name, you use the Internet's domain name servers (DNS) to translate the human-readable domain name into the machine-readable IP address. During a day of browsing and e-mailing, you might access the domain name servers hundreds of times!
DNS Servers and IP Addresses
Domain name servers translate domain names to IP addresses. That sounds like a simple task, and it would be -- except for five things:
- There are billions of IP addresses currently in use, and most machines have a human-readable name as well.
- There are many billions of DNS requests made every day. A single person can easily make a hundred or more DNS requests a day, and there are hundreds of millions of people and machines using the Internet daily.
- Domain names and IP addresses change daily.
- New domain names get created daily.
- Millions of people do the work to change and add domain names and IP addresses every day.
The DNS system is a database, and no other database on the planet gets this many requests. No other database on the planet has millions of people changing it every day, either. That is what makes the DNS system so unique.
IP Addresses
To keep all of the machines on the Internet straight, each machine is assigned a unique address called an IP address. IP stands for Internet protocol, and these addresses are 32 bit number formally expressed as four "octets" in a "dotted decimal number." A typical IP address looks like this:
74.125.79.83
The four numbers in an IP address are called octets because they can have values between 0 and 255 (28 possibilities per octet).
Every machine on the Internet has its own IP address.A server has a static IP address that does not change very often. A home machine that is dialing up through a modem often has an IP address that is assigned by the ISP when you dial in. That IP address is unique for your session and may be different the next time you dial in. In this way, an ISP only needs one IP address for each modem it supports, rather than for every customer.
If you are working on a Windows machine, you can view your current IP address with the command WINIPCFG.EXE (IPCONFIG.EXE for Windows 2000/XP). On a UNIX machine, type nslookup along with a machine name (such as "nslookup www.gmail.com") to display the IP address of the machine (use the command hostname to learn the name of your machine).
As far as the Internet's machines are concerned, an IP address is all that you need to talk to a server. For example, you can type in your browser the URL http://74.125.79.83 and you will arrive at the machine that contains the Web server for gmail.com. Domain names are strictly a human convenience
The Distributed System
Name servers do two things all day long:
- They accept requests from programs to convert domain names into IP addresses.
- They accept requests from other name servers to convert domain names into IP addresses.
When a request comes in, the name server can do one of four things with it:
- It can answer the request with an IP address because it already knows the IP address for the domain.
- It can contact another name server and try to find the IP address for the name requested. It may have to do this multiple times.
- It can say, "I don't know the IP address for the domain you requested, but here's the IP address for a name server that knows more than I do."
- It can return an error message because the requested domain name is invalid or does not exist.
When you type a URL into your browser, the browser's first step is to convert the domain name and host name into an IP address so that the browser can go request a Web page from the machine at that IP address . To do this conversion, the browser has a conversation with a name server.
When you set up your machine on the Internet, you (or the software that you installed to connect to your ISP) had to tell your machine what name server it should use for converting domain names to IP addresses. On some systems, the DNS is dynamically fed to the machine when you connect to the ISP, and on other machines it is hard-wired. If you are working on a Windows 95/98/ME machine, you can view your current name server with the command WINIPCFG.EXE (IPCONFIG for Windows 2000/XP). On a UNIX machine, type nslookup along with your machine name. Any program on your machine that needs to talk to a name server to resolve a domain name knows what name server to talk to because it can get the IP address of your machine's name server from the Operating system.
The browser therefore contacts its name server and says, "I need for you to convert a domain name to an IP address for me." For example, if you type "www.gmail.com" into your browser, the browser needs to convert that URL into an IP address. The browser will hand "www.gmail.com" to its default name server and ask it to convert it.
The name server may already know the IP address for www.gmail.com. That would be the case if another request to resolve www.gmail.com came in recently (name servers cache IP addresses to speed things up). In that case, the name server can return the IP address immediately. Let's assume, however, that the name server has to start from scratch.
A name server would start its search for an IP address by contacting one of the root name servers. The root servers know the IP address for all of the name servers that handle the top-level domains. Your name server would ask the root for www.gmail.com, and the root would say (assuming no caching), "I don't know the IP address for www.gmail.com, but here's the IP address for the COM name server." Obviously, these root servers are vital to this whole process, so:
- There are many of them scattered all over the planet.
- Every name server has a list of all of the known root servers. It contacts the first root server in the list, and if that doesn't work it contacts the next one in the list, and so on.
The root server knows the IP addresses of the name servers handling the several hundred top-level domains. It returns to your name server the IP address for a name server for the COM domain. Your name server then sends a query to the COM name server asking it if it knows the IP address for www.gmail.com. The name server for the COM domain knows the IP addresses for the name servers handling the gmail.com domain, so it returns those. Your name server then contacts the name server for gmail.com and asks if it knows the IP address for www.gmail.com. It does, so it returns the IP address to your name server, which returns it to the browser, which can then contact the server for www.gmail.com to get a Web page.
One of the keys to making this work is redundancy. There are multiple name servers at every level, so if one fails, there are others to handle the requests. There are, for example, three different machines running name servers for gmail.com requests. All three would have to fail for there to be a problem.
The other key is caching. Once a name server resolves a request, It caches all of the IP addresses it receives. Once it has made a request to a root server for any COM domain, it knows the IP address for a name server handling the COM domain, so it doesn't have to bug the root servers again for that information. Name servers can do this for every request, and this caching helps to keep things from bogging down.
Name servers do not cache forever, though. The caching has a component, called the Time To Live (TTL), that controls how long a server will cache a piece of information. When the server receives an IP address, it receives the TTL with it. The name server will cache the IP address for that period of time (ranging from minutes to days) and then discard it. The TTL allows changes in name servers to propagate. Not all name servers respect the TTL they receive, however. When gmail moved its machines over to new servers, it took three weeks for the transition to propagate throughout the Web.
How Can I Create a New Domain Name
When someone wants to create a new domain, he or she has to do two things:
- Find a name server for the domain name to live on.
- Register the domain name.
Technically, there does not need to be a machine in the domain -- there just needs to be a name server that can handle the requests for the domain name.
There are two ways to get a name server for a domain:
- You can create and administer it yourself.
- You can pay an ISP or hosting company to handle it for you.
Most larger companies have their own domain name servers. Most smaller companies pay someone.
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