Centos 8

Ensure you have the epel-release repo
- yum install epel-release
Install the main nagios package
- yum install nagios
Install plugins if necessary (for us it is)
- yum install nagios-plugins nagios-plugins-* --skip-broken
- Note that several of the plugins covered by nagios-plugins-* require packages that do not seem to be easily available in Centos 8 yet. You could search for ways to install the dependencies manually, but we don't need those particular plugins so we can safely --skip-broken.
Note that this also installs httpd for you, as the main user interface for this service is a web-page.

Centos 7

Ensure you have the epel-release repo
- yum install epel-release
Install the main nagios package
- yum install nagios
Install plugins if necessary (for us it is)
- yum install nagios-plugins nagios-plugins-*
Note that this also installs httpd for you, as the main user interface for this service is a web-page.