Add DNS Server

Normally we just add the IP Address, Subnet Mask and Gateway to the ifcfg-eth0 file, as that is all we need. But if it is required that we need to update the OS, or add a piece of software (htop for example) then we may need to add the DNS settings.
There are two ways this can be done.

Method 1

Use “PEERDNS=no”. This option will prevent /etc/resolv.conf from being modified by a DHCP server. So instead of using DHCP-provided DNS, you can specify any arbitrary DNS servers you want to use in /etc/resolv.conf.
The configuration file for your network interface (e.g., eth0) looks like the following.
sudo vi /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
PEERDNS=no
Then, add static DNS to /etc/resolv.conf
sudo vi /etc/resolv.conf
nameserver 8.8.8.8
nameserver 8.8.4.4

Method 2

Alternatively, you can specify DNS servers directly in the interface configuration file, instead of modifying /etc/resolv.conf yourself. That is:
sudo vi /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
DNS1=8.8.8.8
DNS2=8.8.4.4
The DNS servers specified with “DNS1/DNS2” directives will then automatically be added to /etc/resolv.conf when the interface is activated. So there is no need to modify /etc/resolv.conf yourself.