What is a DNS? Things you need to know about DNS

Here you will find the DNS definition explained in a simple way and the answers to the questions: What is a DNS server and what is the role of the DNS server? We will also see the link between DNS and domain.

In summary :

1. Definition of DNS
2. An example of using a DNS query
3. DNS at home
4. Domains and subdomains
5. Preferred DNS, Secondary DNS

Definition of DNS

First of all: DNS means "Domain Name System" or domain name system. I will explain first the functioning, then we will address the notion of domain name (or subdomain).

To explain, the role and operation of a DNS server, I will take again the example of the phone.As you might have read in the article "What is an IP address?", Computers exchange between them through their IP addresses. But how do they know which IP address to address?

• Want to call someone you do not know the phone number?
• What are you doing ? ....
• .... You use a directory!

Well, a DNS server is a computer directory. When you want to access a computer in the network, your computer will query the DNS server to retrieve the address of the computer you want to join. Once your computer has retrieved the address of the recipient, it will be able to join it directly with its IP address.

The DNS server will allow to make the relation between computer name and IP address. (Returned to the telephone, one obtains: name of the subscriber and no.).

You will tell me :

• "But I never try to join a computer in the network! 

To which, I will reply:

•   " of course yes ! "
•   Currently you are reading these lines that are not stored on your computer, so your computer has connected to a server to retrieve the pages that are displayed on your screen. "

Example

Let's go through the example and a little diagram, to see how it happens when you surf the Internet and when the DNS server comes in

In the example above, we see that the query "what is the address of www.google.com" answered 74.125.230.248. This query is called a domain name resolution.

If we continue, we can see that the DNS server is used only on the part 1-Question and 2-Response. After the computer retrieves the address of the server to be joined, it no longer needs the DNS server.

This IP address is not a random address. It corresponds to one of the servers of Google. (There are, of course, a large number).

So, if in your in the address bar of your browser, you type: http://74.125.230.248, the Google search page should be displayed.

Warning, this works at the time of writing, but it may not be the case when you read them. (The 74.125.230.248 server may be turned off or assigned to new tasks?)

To do the test click on: http://74.125.230.248

NB of 05/01/2016: As I said above, the address may have changed, and that is the case: Try now with http://74.125.206.94

And DNS at home?

Most people ignore this notion is understandable. So how can this work at home since I never informed these servers?

DNS servers work in cascade. To explain this one might say that when a DNS server does not know the answer, it will ask its "parent".

At home, the DNS server is your box (unless you have specified something else), and if your box does not know how to answer, it will ask its parent: the DNS server (s) of your ISP. This can go up to the root DNS servers. (We will say for the sake of simplicity that the root DNS servers are the basic servers of the system: one can not go back higher.)

The DNS configuration of your computer (or tablet, and any other equipment connected to you) is provided by the DHCP server.

A file

Perhaps you will hear about it if you are working in computers (or if you're tinkering a bit). A file on the machine can replace DNS queries.

This file is the "host" file. It will contain the correspondence between the requested addresses and the IP addresses. On each line of this file we will find at the beginning the IP address followed by a space or a tabulation and then the name (s) corresponding to this address.

Example: 74.125.230.248 www.google.com

Security

Let's just talk a little bit about security.

To understand what I am talking about, I will use the example of the telephone:

Imagine that you have a fake directory (pirated) or the phone numbers are not the right ones, and it sends you back to the pirates.

You need to call your bank: You call the phone number and you will necessarily come across hackers (but you do not know).

At the end of the line, you can ask for confidential information, and you will give them with confidence!

It must be understood that this can be the same with DNS servers and that there are viruses that modifies your network settings or your hosts file to point DNS queries to pirate servers.

You will see that browsers can display messages when this happens. I detail this part in the https part.

One of the jokes of network administrators is to modify, within the company, one of the DNS entries from one site to another site. (For example: make Facebook point to the site of the Ninja Turtles!: So every time someone wants to go on Facebook, they will find themselves on the site of the Ninja Turtles)

Additional information:

Preferred and auxiliary DNS servers

In the comments, Alao asks me the following question:

On the other hand, I have a concern:

In Windows XP or 7, at the level of the properties of connection to the local network, there are the addresses Preferred and auxiliary DNS server, I want to know where are its servers and are they physical or software?

Thank you in advance for your reply.

Here is my answer here, because I think it can interest the most:

There are two possibilities :

• if you are a particular DNS are often those of your ISP.
• If your computer is connected to a corporate network, DNS is usually servers located within the enterprise. These servers can be physical servers or virtual servers. (See here the definition of virtualization: What is virtualization?)
• And finally, what are the zones: Main DNS and Secondary DNS. It is strongly recommended to have 2 DNS servers in a network: the main DNS is the one that answers the requests in normal time, the secondary DNS taking over if Main does not respond.
• This information is often populated automatically by DHCP.

 As usual, all comments are welcome.