What is DNS?

DNS lets users connect to websites using domain names instead of IP addresses. Learn how DNS works.

The Domain Name System (DNS) is the phonebook of the Internet. Humans access information online through domain names like nytimes.com or espn.com. Web browsers interact through Internet Protocol (IP) addresses. DNS translates domain names to IP addresses so browsers can load Internet resources.

Each device connected to the Internet has a unique IP address that other machines use to find the device. DNS servers eliminate the need for humans to memorize IP addresses such as 192.168.1.1 (in IPv4), or more complex newer alphanumeric IP addresses such as 2400:cb00:2048:1::c629:d7a2 (in IPv6).

How does DNS work?

DNS resolution involves converting a hostname (such as www.example.com) into a computer-friendly IP address (192.168.1.1). An IP address is given to each device on the Internet, and that address is necessary to find the appropriate Internet device – like a street address is used to find a particular home. When a user wants to load a webpage, they must translate what they type into their web browser (example.com) and the machine-friendly address necessary to locate the example.com webpage.

To understand the process behind the DNS resolution, it’s essential to learn about the different hardware components a DNS query must pass between. For the web browser, the DNS lookup occurs “behind the scenes” and requires no interaction from the user’s computer apart from the initial request.

There are 4 DNS servers involved in loading a webpage:

DNS recursor – The recursor can be thought of as a librarian who is asked to go find a particular book somewhere in a library. The DNS precursor is a server designed to receive queries from client machines through applications such as web browsers. Typically, the precursor makes additional requests to satisfy the client’s DNS query.
Root nameserver – The root server is the first step in translating (resolving) human-readable host names into IP addresses. It can be thought of like an index in a library that points to different racks of books – typically, it serves as a reference to other more specific locations.
TLD nameserver – The top-level domain server (TLD) can be considered a specific rack of books in a library. This nameserver is the next step in the search for a specific IP address, and it hosts the last portion of a hostname (In example.com, the TLD server is “com”).
Authoritative nameserver – This final nameserver can be considered a dictionary on a rack of books, in which a specific name can be translated into its definition. The authoritative nameserver is the last stop in the nameserver query. Suppose the authoritative name server has access to the requested record. In that case, it will return the IP address for the requested hostname to the DNS Recursor (the librarian) that made the initial request.

What’s the difference between an authoritative DNS server and a recursive DNS resolver?

Both concepts refer to servers (groups of servers) that are integral to the DNS infrastructure. Still, each performs a different role and lives in various locations inside the pipeline of a DNS query. One way to think about the difference is the recursive resolver is at the beginning of the DNS query, and the authoritative nameserver is at the end.

Recursive DNS resolver

The recursive resolver is the computer that responds to a recursive request from a client and takes the time to track down the DNS record. It makes a series of requests until it reaches the authoritative DNS nameserver for the requested record (or times out or returns an error if no record is found). Luckily, recursive DNS resolvers do not always need to make multiple requests to track down the records needed to respond to a client; caching is a data persistence process that helps short-circuit the necessary requests by serving the requested resource record earlier in the DNS lookup.

Authoritative DNS server

Put, an authoritative DNS server is a server that holds and is responsible for DNS resource records. This server at the bottom of the DNS lookup chain will respond with the queried resource record, ultimately allowing the web browser to request to reach the IP address needed to access a website or other web resources. An authoritative nameserver can satisfy queries from its data without needing to query another source, as it is the final source of truth for certain DNS records.

In instances where the query is for a subdomain such as foo.example.com or CNN.com, an additional nameserver will be added to the sequence after the authoritative nameserver, which is responsible for storing the subdomain’s CNAME record.

Different DNS recursive resolvers such as Google DNS, OpenDNS, and providers like Comcast maintain data center installations of DNS recursive resolvers. These resolvers allow for quick and easy queries through optimized clusters of DNS-optimized computer systems.

What are the steps in a DNS lookup?

DNS is concerned with a domain name being translated into the appropriate IP address for most situations. To learn how this process works, it helps to follow the path of a DNS lookup as it travels from a web browser through the DNS lookup process and back again. Let’s take a look at the steps.

Note: DNS lookup information is often cached locally inside the querying computer or remotely in the DNS infrastructure. There are typically eight steps in a DNS lookup. When DNS information is cached, steps are skipped from the DNS lookup process, making it quicker. The example below outlines all eight steps when nothing is cached.

The eight steps in a DNS lookup:

A user types ‘example.com’ into a web browser, and the query travels into the Internet and is received by a DNS recursive resolver.
The resolver then queries a DNS root nameserver.
The root server then responds to the resolver with the address of a Top Level Domain (TLD) DNS server (such as .com or .net), which stores the information for its domains. When searching for example.com, our request is pointed toward the .com TLD.
The resolver then requests the .com TLD.
The TLD server then responds with the IP address of the domain’s nameserver, example.com.
Lastly, the recursive resolver sends a query to the domain’s nameserver.
The IP address, for example ,.com, is then returned to the resolver from the nameserver.
The DNS resolver responds to the web browser with the domain’s IP address initially requested.
The browser makes an HTTP request to the IP address.
The server at that IP returns the webpage to be rendered in the browser (step 10).

What is a DNS resolver?

The DNS resolver is the first stop in the DNS lookup and is responsible for dealing with the client who made the initial request. The resolver starts the sequence of queries that ultimately leads to a URL being translated into the necessary IP address.

Note: A typical uncached DNS lookup involves recursive and iterative queries.

It’s important to differentiate between a recursive DNS query and a recursive DNS resolver. The query refers to the request made to a DNS resolver requiring the resolution of the query. A DNS recursive resolver is a computer that accepts a recursive query and processes the response by making the necessary requests.

What are the types of DNS queries?

In a typical DNS lookup, three types of queries occur. By using a combination of these queries, an optimized process for DNS resolution can result in a reduction of distance traveled. In an ideal situation, cached record data will be available, allowing a DNS server to return a non-recursive query.

Three types of DNS queries:

Recursive query – In a recursive query, a DNS client requires that a DNS server (typically a DNS recursive resolver) respond to the client with the requested resource record or an error message if the resolver can’t find the record.
Iterative query – in this situation, the DNS client will allow a DNS server to return its best answer. If the queried DNS server does not have a match for the query name, it will return a referral to a DNS server authoritative for a lower level of the domain namespace. The DNS client will then make a query to the referral address. This process continues with additional DNS servers down the query chain until an error or timeout occurs.
The non-recursive query typically occurs when a DNS resolver client queries a DNS server for a record that it can access either because it’s authoritative for the record or the form exists inside its cache. Typically, a DNS server will cache DNS records to prevent additional bandwidth consumption and load on upstream servers.

What is DNS caching? Where does DNS caching occur?

The purpose of caching is to temporarily store data in a location that improves performance and reliability for data requests. DNS caching involves storing data closer to the requesting client so that the DNS query can be resolved earlier and additional queries further down the DNS lookup chain can be avoided, thereby improving load times and reducing bandwidth/CPU consumption. DNS data can be cached in various locations, each storing DNS records for a set amount of time determined by a time-to-live (TTL).

Browser DNS caching

Modern web browsers are designed by default to cache DNS records for a set time. The purpose here is obvious; the closer the DNS caching occurs to the web browser, the fewer processing steps must be taken to check the cache and make valid requests to an IP address. When a request is made for a DNS record, the browser cache is the first location checked for the requested record.

In Chrome, you can see the status of your DNS cache by going to chrome://net-internals/#dns.

Operating system (OS) level DNS caching

The operating system-level DNS resolver is the second and last local stop before a DNS query leaves your machine. The process inside your operating system designed to handle this query is commonly called a “stub resolver” or DNS client. When a stub resolver gets a request from an application, it first checks its cache to see if it has the record. If it does not, it sends a DNS query (with a recursive flag set) outside the local network to a DNS recursive resolver inside the Internet service provider (ISP).

When the recursive resolver inside the ISP receives a DNS query, like all previous steps, it will also check if the requested host-to-IP-address translation is already stored inside its local persistence layer.

The recursive resolver also has additional functionality depending on the types of records it has in its cache:

Suppose the resolver does not have the A records but does have the NS records for the authoritative nameservers. In that case, it will query those name servers directly, bypassing several steps in the DNS query. This shortcut prevents lookups from the root and .com nameservers (in our search for example.com) and helps resolve the DNS query more quickly.
If the resolver does not have the NS records, it will query the TLD servers (.com in our case), skipping the root server.
If the resolver does not have records pointing to the TLD servers, it will query the root servers. This event typically occurs after a DNS cache has been purged.