In light of another thread, I would like to discuss necessary security measures one should implement on any given Linux server. For the sake of argument, let's say we have several servers running a website, an sql database, a file server, and an email server. I'm in the process of setting up my very first file server on a VLAN. It happens to be an Ubuntu 12.04 server, but I will also be installing a Centos server in the future. I will eventually set up an email, web, and sql server as well. Utlimately they will face the internet. But I want to learn the basics before I unleash them upon the world. I'm interested in your personal opinions, links to articles, tutorials, horror stories. Pretty much anything because it's all brand new to me. Thanks!