1. Overview
Domain Name System (DNS) is a vital service for the Intranet and Internet. It’s responsible for translating machine names into IP addresses. In this tutorial, we’ll explain multiple ways to configure DNS in the Linux operating system.
2. Domain Name System
DNS is a mechanism to make the internet human-friendly. Computers communicate with each other using their IP addresses. There are lots of IP addresses in the internet world, and it’s impossible to remember them all. To solve this issue and make it more human-friendly, DNS was invented. DNS servers map IP addresses to hostnames. When we enter a domain name like baeldung.com into our browser, the computer finds our nearest DNS server and asks what’s the correct IP address for baeldung.com.
Then, it returns the IP address to our system so that it can communicate with the baeldung.com server. For example, DNS translates the domain name baeldung.com to IP address 172.66.40.248. In Linux, there are DNS lookup tools like nslookup and dig, which are made to query DNS servers.** For getting the IP address of a domain name, we can use the nslookup command:
$ nslookup baeldung.com
Server: 8.8.8.8
Address: 8.8.8.8#53
Non-authoritative answer:
Name: baeldung.com
Address: 172.66.40.248
Name: baeldung.com
Address: 172.66.43.8
3. DNS Configuration
There are two approaches to configuring a Linux system’s DNS service: Resolver Configuration File and Hosts.
3.1. Resolver Configuration File
We’re able to set the DNS configuration in network interface config files. However, this is not the only way. We can configure a DNS service using the /etc/resolv.conf file. If we want to change our DNS configuration, we can use the nameserver keyword:
$ cat /etc/resolv.conf
nameserver 8.8.8.8
nameserver 4.2.2.4
In the above configuration, we set the DNS servers to public DNS servers like the Google server (8.8.8.8). Also, note that any changes made manually to the /etc/resolv.conf configuration file is bound to be overwritten upon changes in the network or upon system reboot.
3.2. Hosts
Another way to configure a Linux system’s DNS service is by manipulating the /etc/hosts file. The /etc/hosts file contains server names and their IP addresses statically saved:
$ cat /etc/hosts
127.0.0.1 localhost
This file can be changed by the root user and will map domain names to IP addresses. The /etc/hosts file has a higher priority than /etc/resolv.conf file.
4. DNS Priority
DNS priority tells the system about the priority of DNS lookup. Linux normally performs lookups in /etc/hosts before it uses DNS. We can modify this behavior by editing the /etc/nsswitch.conf file, and specifically, the hosts line. Let’s check the DNS lookup order in /etc/nsswitch.conf file:
$ cat /etc/nsswitch.conf | grep hosts
hosts: files mdns4_minimal [NOTFOUND=return] dns
The above configuration means that DNS lookup refers to files (/etc/hosts) first, and then DNS servers specified in DNS (/etc/resolv.conf). This means when the system wants to find the IP address of a domain name, it first reads the /etc/hosts file and then /etc/resolv.conf.
5. Conclusion
In this article, we explained the DNS service and various ways of configuring it in the Linux operating system. Additionally, we discussed how Linux determines the DNS lookup priority.