What is a DNS-server? How does it work? Types of DNS servers

Let's start, of course, with a definition.

DNS-server (Domain Name System) is a specialized "server" program responsible for converting domain names to their corresponding IP addresses and vice versa. The program can run on a dedicated server or a group of servers. It possible to say that DNS-servers are a kind of Internet directories that store IP-addresses of hosts (for example, servers with sites) and link them to the corresponding names. There are many of them on the Internet, and they all belong to different providers and serve their clients.

However, no single DNS-server will not store absolutely all names and addresses, but it will know where to pass the request. For this purpose, the hierarchical structure of DNS is used, which we will discuss further. 

DNS-servers play a key role in the network infrastructure of the Internet, providing a link between domain names that are understandable to the average user (for example, example.com), and numeric IP addresses of computers (for example, 192.0.2.1). Thus, this system makes it easier for people to navigate and work on the Internet without having to memorize long numerical addresses.

On a kind of address book functions of DNS-servers do not end. They can also cache requests and contribute to the security of the system, which we will discuss next.

How DNS technology works

The principles of DNS technology can be described by analogy with the use of the contact book in the phone in a two steps:

• In the beginning, the user chooses a contact name that is understandable to him (in our case it is the name of the site that is displayed in the browser bar, for example: www.example.com).
• After that, just as a smartphone needs a number to make a call, a computer needs its IP address to connect to a website. Therefore, the user's computer further applies to the DNS-server for the IP-address, corresponding to the letter query of the user in the address bar of the browser.

IP-address is a definitely formed series of digits format XXX.XXX.XXX.XXX.XXX.XXX, where each octet of the address occupies exactly one byte, and the numbers range from 0 to 255.

• And then the computer returns the answer to the user in a language he/she understands.

Types of DNS-servers. And why we need them

The main tasks of DNS-servers can be described by four functions: storing information about domain names, providing them at the request of users, caching DNS-records and security part.

Such functionality implemented with the help of several types of DNS-servers. Here are the main ones:

• Root DNS servers are at the top of the DNS hierarchy. They are responsible for specifying the location of authoritative DNS servers for each top-level domain (TLD), such as .com, .org, .net, etc. The interesting thing is that there are exactly 13 of them for the entire Internet (and a huge number of replicas - after all, the whole Internet is based on them), below will be a little more detailed.
• Authoritative DNS servers contain complete and accurate information about a domain or subdomain. They are directly responsible for resolving requests for IP addresses for specific domain names.
• Recursive DNS servers accept requests from client devices (such as a computer or router) and perform a full traversal of the DNS hierarchy, starting at the root servers, to find the requested IP address. They can cache the retrieved data (or store the necessary data, usually temporarily) to improve performance. Therefor the user open the requested site faster, reducing the load on the network.
• Forwarding DNS-servers receive requests from clients and forward them to other DNS-servers for resolution. They help to speed up the process of resolving DNS-queries and protect against attacks. This is most likely the DNS you are using on your home router.
• Local DNS servers are located within an organization's or ISP's local network and serve queries only to devices on that network. Using them it is possible to configure to cache DNS records and provide quick access to frequently used resources.
• Open (public) DNS servers are available for use by all Internet users and provide DNS query resolution services. Examples are Google Public DNS and OpenDNS.

What are DNS zones? Working from inside the site

Using a single domain with only one IP address is the easiest first step. In reality, things are usually more complicated. Firstly, many websites are now multi-assembly systems with a wide range of functionality. So to the initial base, the website, are added a mail server, a payment system, etc. For greater security and efficiency of all this construction, each of these resources goes with its own separate IP-address. Secondly, to solve different tasks (landing page for a separate project, event or advertising) websites often have subdomains, which also have separate IP-addresses.

DNS-zone, in turn, is a special file located on the DNS-server, which contains the full information described above: about the relationship of the site with subdomains, payment systems, mail system, etc. In the classic version of the file has the following types of records (here are the most important ones):

• A - specifies the IP address of the host bound to a specific domain name.
• MX - specifies the address of the mail server.
• CNAME - used to connect a subdomain.
• NS - specifies the address of the DNS server responsible for the zone.
• TXT - contains any textual information about the domain name.
• SPF - contains data about servers trusted to send mail on behalf of the specified domain.
• SOA - contains server information and template data about the domain name, showing the starting point of the zone.
• PTR - pointer record, used for reverse queries - converting an IP address to a name.
• SRV - contains information about a particular service (server, port, priority). It is necessary for a number of services like Active Directory. 

How a new domain comes to life

Activation of a new domain goes through several stages:

1. The domain name is successfully registered.
2. To inform DNS servers about the new domain name it is necessary to set up resource records through the control panel of the domain name registrar or hosting provider.
3. These DNS records are updated within a day on the local DNS server and propagated further to the rest of the DNS servers. At the end of this process, the new domain comes to life and can be used, for example, the finished site begins to open normally in the browser.

But the story about DNS-servers would be incomplete without a brief excursion into the global infrastructure.

Locations of the world's main DNS servers: how it all works

A map of the thirteen logical name servers, including anycasted instances, at the end of 2006

As we described above, at the heart of the top level are the root servers. Various global operators manage them. Historically, the first root server was located in the United States at the University of California, Berkeley (University of California, Berkeley) in 1984.

Currently, there are 13 major root DNS-servers in the world. They are distributed around the world and play a central role in the structure and operation of the Domain Name System (DNS). Each of these root servers serves one of the 13 top-level zones (TLDs) in the DNS hierarchy.

To improve Internet reliability in case of possible failures, the total number of root servers has been expanded to 123 through backups and replicas of these servers.

The current distribution of the main DNS centers reflects the high technological development of the region and the needs of the network infrastructure. It is possible to describe this map in the following way:

• North America is about 40-45% of the major DNS centers. The United States and Canada are home to many key DNS infrastructures, including DNS root servers.
• Europe - about 25-30% of the main DNS centers. Most of them are in countries with developed information technology infrastructure, such as Germany, France, and the UK and others.
• Asia - about 20-25% of the main DNS-centers. Here the leading countries are Japan, China, South Korea and India.
• Latin America - about 5-10% of the main DNS centers. Brazil, Mexico and Argentina also have their own DNS-infrastructures, providing the work of the Internet in this region.
• Oceania is about 1-5% of the major DNS centers. Australia and New Zealand also contribute to the global DNS network.
• And the UAE and Iceland are not lacking DNS servers, which correlates with the intensive use of the Internet in these regions.