Hosts file in Linux

The Hosts file allows us to resolve DNS queries locally, which can be very useful for new projects if DNS changes have not yet propagated for a domain we own or if we do not yet have a domain and need to run tests with DNS queries.

Changes we make to the hosts file will only affect the local server we are configuring and will have no effect on other servers. Below, we explain how to enable editing of the hosts file so that changes persist when we restart the server.

Procedure for disabling overwriting of the hosts file

The first step is to prevent the Cloud Init service from managing the hosts file, because if we try to edit the hosts file directly and restart the server, we will find that the hosts file has been overwritten with the initial values.

To prevent Cloud init from controlling the hosts file, we must perform the following steps:

Ubuntu 18.04

Edit the following Cloud-Init configuration file:

# vi /etc/cloud/cloud.cfg

Find the "manage_etc_hosts" variable and set it to false. 

manage_etc_hosts: false 

Ubuntu 16.04, Debian 9 and Centos 7

Edit the following Cloud-Init configuration file:

# vi /etc/cloud/cloud.cfg

Add the following variable below "disable_root: false": 

manage_etc_hosts: false 

 In the section "cloud_init_modules", we must remove the following module:

- update_etc_hosts 

Centos 8 and Debian 10

Edit the following Cloud-Init configuration file:

# vi /etc/cloud/cloud.cfg.d/99_clouding.cfg 

Add the following variable below "disable_root: false": 

manage_etc_hosts: false 

In the section "cloud_init_modules", we must remove the following module: 

- update_etc_hosts 

 With these changes, we can now edit the hosts file, and the changes will persist when we restart the server.

Example of an entry in the Hosts file

If we wanted our server to perform the DNS query for "mail.example.com" locally, we would need to perform the following steps:

Edit the hosts file:

# vi /etc/hosts

Add the following line:

127.0.0.1 mail.example.com 

Before making the change, if we ping mail.example.com, the result would be as follows:

root@mail:~# ping mail.example.com
ping: mail.example.com: Name or service not known  

However, after adding "127.0.0.1 mail.example.com", we can see that the DNS query now returns a response with the server's own "loopback" IP:

root@mail:~# ping mail.example.com
PING mail.example.com (127.0.0.1) 56(84) bytes of data.
64 bytes from mail.example.com (127.0.0.1): icmp_seq=1 ttl=64 time=0.039 ms
64 bytes from mail.example.com (127.0.0.1): icmp_seq=2 ttl=64 time=0.040 ms  

If the DNS query we want to make refers to an external server, simply replace the IP 127.0.0.1 with the external server's public IP. 

With these simple steps, we can add entries to "/etc/hosts" so that the server resolves DNS queries locally.