How Does DNS Lookup Work?

Domain name system (DNS) lookups are how end users obtain the websites they search for. It is the way DNS services resolve end-user queries and acquire information related to domains.  

DNS Lookup

A DNS lookup is initiated when an end user enters a domain name and the resolver translates it into the corresponding identifier—the IP address. To understand this process, it is best to start with the basics of DNS—what it is, how it works, and what a query journey looks like. For a detailed explanation, visit our What is DNS resource page. 

A query journey includes all of the steps taken to translate the entered domain name to an IP address. When a person enters a web address into their browser, the search is initiated. The query first stops at the recursive server which contacts a series of authoritative servers to gain all of the information that it needs to translate it into language that a machine can read. Then, it returns the IP address related to the domain that was initially searched. There are instances in which the path may change or the domain is unreachable, but as a whole, that is the road most taken in query journeys.

For all of this to take place, the proper path needs to be established. In comes DNS records. DNS records set the rules and lay down the paths for the query to travel. They store all of the relevant information servers need to properly translate email addresses and domain names into meaningful numerical addresses to complete the DNS process.   

There are two different types of DNS lookups: forward DNS and reverse DNS lookups.

Forward DNS Lookups

Forward DNS (also known as a forward DNS lookup) is a request that is used to obtain an IP address by searching the domain. This follows the standard DNS query journey when the user types in a web page or sends an email and is provided with the related IP address.

This process allows an end client to translate a domain name or email address into the address of the device that would handle the server-side communication.  

Reverse DNS Lookup

Reverse DNS is the exact opposite of forward DNS. It is a lookup request that is used to obtain the domain name related to an IP address. Reverse lookups are typically used by email servers to ensure that the servers they are receiving messages from are valid. 

To complete this process, the mail server must have a pointer (PTR) record established. This type of record informs other mail servers that its IP address is authoritative for sending and receiving mail for its related domain. 

The IP owner (typically the ISP or hosting provider for the particular email server) delegates a zone for the server that ends in “in-addr.arpa” with some preceding numbers. The numbers at the beginning of the zone are the server’s IP block with the octets reversed. 

Example: The reverse DNS for the 192.168.1 class C would be “1.168.192.in-addr.arpa”. In this example, this reverse DNS zone would handle the reverse DNS for IPs 192.168.1.0 to 192.168.1.255. If the IP block is smaller than a class C, the zone might be “27/1.168.192.in-addr.arpa” or “0-25.1.168.192.in-addr.arpa”. The difference is the syntax.

For more information on reverse DNS, see our set-up tutorial. 

DNS Lookup Commands 

DNS information related to a domain can be found by using DNS lookup commands. They can provide details such as nameservers, mail servers, and configured records.   

Nslookup

A nameserver lookup, also known as an nslookup, allows you to locate the nameserver associated with a domain, along with any configured records. This information can be resolved using an IP address or a domain name as the search option. The command for an nslookup differs slightly across PC, Mac, and Linux. Using Windows 10, this is done through the command prompt and a Mac device uses the terminal. Linux users utilize dig, which is also a command line utility that allows users to locate domain information.

For a detailed look at how to run an nslookup, see our ….. resource.

DNS Lookup Online

While there are a few online tools that assist in retrieving DNS information, Constellix’s DNS Lookup Tool allows you to perform a search from any device, including cell phones and tablets. The lookup results can be shared and this has proven to be very useful for teams in troubleshooting domain configuration changes.

DNS Lookups Resolved 

DNS lookups utilize DNS records to translate IP addresses and domain names or email addresses. There are two types of lookups: forward DNS and reverse DNS. Both resolve information related to the domain, depending on the search method. If you have a domain name or email address and need the IP address resolved, forward DNS is used to return the requested information. It is a functional part of all IP-based networks, including the internet. If you have an IP address and need to locate an email address or domain name, reverse DNS resolves this request. 

Here are some related DNS resources:

DNS Zone Files

Primary vs Secondary DNS Servers

What is DNSSEC?