The Ansible Hardening role from the OpenStack project is a great way to secure Linux boxes in a reliable, repeatable and customisable manner.

It was created by former colleague of mine Major Hayden and while it was spun out of OpenStack, it can be applied generally to a number of the major Linux distros (including Fedora, RHEL, CentOS, Debian, SUSE).

The role is based on the Secure Technical Implementation Guide (STIG) out of the Unites States for RHEL, which provides recommendations on how best to secure a host and the services it runs (category one for highly sensitive systems, two for medium and three for low). This is similar to the Information Security Manual (ISM) we have in Australia, although the STIG is more explicit.