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DNS (Domain Name System) is the heart of the Internet service, and is perhaps the most misunderstood part of it. You could not find your favorite Web page without it. There are actually 2 types of DNS service, IPV4 and IPV6. Since IPV6 is not widely used yet, we will restrict our discussions to IPV4.
When you enter your favorite URL or recover it from your shortcuts (eg. http://www.yellowhead.com/), your computer has no idea where to look for that page. The Internet works with numbers, not text, and it must be translated (http://[18.104.22.168]/) before it can be used. This is the function of the DNS service.
DNS service is usually provided by your ISP, although I have shown the OpenDNS servers below. IF you are using Windows, you can go to the command prompt and enter the command:
C:\>ipconfig /all Ethernet adapter Local Area Connection: Connection-specific DNS Suffix . : Description . . . . . . . . . . . : Intel(R) 82562V-2 10/100 Network Connection Physical Address. . . . . . . . . : 00-1D-09-7E-44-A2 DHCP Enabled. . . . . . . . . . . : No Autoconfiguration Enabled . . . . : Yes IPv4 Address. . . . . . . . . . . : 192.168.1.4(Preferred) Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : 192.168.1.1 Primary DNS Server. . . . . . . . : 22.214.171.124 Secondary DNS Server. . . . . . . : 126.96.36.199 NetBIOS over Tcpip. . . . . . . . : EnabledDNS servers are supposed to accept both UDP and TCP requests, but TCP requests are not universally accepted because some routers and firewalls do not pass along the TCP requests on port 53. UDP packets are limited in size (512 bytes), and delivery is not assured because there is no connection established. Therefore, the requesting computer will have to wait for the request to time out. When it times out on one server, it switches to the other. Why they use the names Primary and Secondary is not fully understood, as your computer will use whichever one it last had success with. For that reason it is not advisable to configure more than 2 DNS servers, as it will have to time out on all configured servers on a bad lookup, and that could result in a long wait.
The DNS protocol is very compact and highly cryptic, so for this discussion I will use screen shots from a program that I wrote called Client DNS. When this program is loaded, it finds the default DNS servers:
In this case, there is only one DNS server listed because I am using a NAT (Network Address Translation) router as DNS. It simply relays the requests to one of the DNS servers that it has in its own configuration. When you enter the domain name to translate, click on a server name, and click the "Send Query" button:
There is a lot of information in the header. First of all, it tells us that this is not an authoratative answer (Authoritative Answer = False). That is because this server does not host this particular domain, and recursion was requested and utilized to find the answer. This will be explained in more detail later. It also tells us that there were no errors and 1 Question (QDCount=1), 1 Answer (ANCount=1), 2 Nameserver (NSCount=2), and 1 Additional (ARCount=1) records received. Our ClientDNS program displays each of those record types on separate tabs. Clicking on the Question TAB:
Clicking on the Answer TAB:
And the Authority TAB:
And the Additional TAB:
But that is only part of the answer. Where does your listed DNS server get the answer from? To answer that question, we will follow the recursive procedure that your DNS server uses. Every domain name must be registered at one of the many Domain Registrars. The part of the domain behind the last dot indicates the high level domain, and the root servers will tell us where to find the servers that serve that particular high level domain (eg. com). Clicking on the "Root Servers" button loads the high level servers which are always in cache:
By selecting one of those servers and sending a query, we can examine the header:
Notice that these servers do not use recursion and do not provide the answer for us, but instead point us to 13 other Nameservers:
with their location:
Selecting one of those and querying again:
Notice that these servers also do not provide recursion, and again do not provide the answer, but instead point us to the actual authoratative Nameservers:
finally provides us with an authoratative answer:
MAN, WHAT A LOT OF WORK TO ANSWER A SIMPLE QUESTION. But you can see that when you register a domain, you are simply registering which DNS servers are being utilized to host that domain. Once your own DNS server has aquired the answer, it usually adds it to cache. How long does it remain in cache? Once again, the answer is not straight forward. The original query provided a TTL (Time To Live). The DNS server will generally use this number as the refresh timeout, but it is not under any obligation to do so. It can set its own minimum or maximum timeout. When the timeout has been decremented to zero without being called upon again (refreshed), it is deleted from cache.
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