Security Guide for Linux by NSA

Guide for Linux is presented as Hardening Tips for the Red Hat Enterprise Linux 5 and Guide to the Secure Configuration of Red Hat Enterprise Linux 5. Of course most of recommendation suit other distributions. Here is the introduction quote from latter guide:

"The purpose of this guide is to provide security configuration recommendations for the Red Hat Enterprise Linux (RHEL) 5 operating system. The guidance provided here should be applicable to all variants (Desktop, Server, Advanced Platform) of the product. Recommended settings for the basic operating system are provided, as well as for many commonly-used services that the system can host in a network environment.
The guide is intended for system administrators. Readers are assumed to possess basic system administration skills for Unix-like systems, as well as some familiarity with Red Hat’s documentation and administration conventions. Some instructions within this guide are complex. All directions should be followed completely and with understanding of their effects in order to avoid serious adverse effects on the system and its security."

Above mentioned guide covers the following directions: system-wide configuration (for example, iptables and ip6tables setup, logging, selinux and etc.) and services configuring (SSH, Avahi server, MTA, LDAP and many others).