What is DNS and where is it used?
What many don’t realize is that there is much more behind it than just name to IP translation (called DNS lookup) and the other way around (that is called reverse DNS lookup).
There are hidden services which are critical for the proper functionality of the Internet like mail, ftp, web – just to name the most well-known. All these services are used every day by billions of people, devices and online services around the world without even thinking at them. The only time when they are aware of their existence is, when they don’t function anymore. But before going into this, let’s briefly go through the most important of them: email and web.
Mail transfer agents use DNS to find out where to deliver e-mail for a particular address. The domain to mail exchanger mapping provided by MX records (Mail eXchange) is another example of how DNS works. MX represents the entity (mail server) that can receive email for a domain. The MX record is used by mail servers to exchange emails and it is configured as a subdomain like mx.domain.com. For example, if a user Bob@domain1.com wants to send an email to Alice@domain2.com, the two servers must communicate via their MX records (domain1.com connects to mx.domain2.com), negotiate and agree on certain parameters and then finally exchange the email message.
The first and most important thing that must happen is that the servers are able to contact each other. When trying to contact mx.domain2.com, the mail transfer agent running on domain1.com must be able to locate domain2.com (this is called A-Record). If the DNS resolution for a domain doesn’t work at all (the name to IP address translation doesn’t work) then it is impossible for that domain to receive any emails.
Ever wondered why do you have to almost always put a “www.” in front of a domain so that you can view its website? “www.“ is actually a subdomain for the main domain and it was historically chosen as an acronym for “World Wide Web” or simply said, the website of that domain. Same as for the email, if the main domain doesn’t get found, then you usually can’t see the website anymore.
Other uses of DNS
There are also other uses of the DNS which are even more hidden than the two mentioned above. Best example for such a service built on top of the DNS are white- and black-lists used to filter good and bad domains, respectively. A service makes a specially created query to a certain domain and get back an answer in form of an IP address (that’s what DNS does, right?). Many services use 127.0.0.1 for when the address is in the list and 127.0.0.2 when the address is not in the list.
Now you know exactly what DNS is used for and where. In our next part we will talk about what happens when DNS doesn’t work, so stay tuned!