Linux Server Security, Second Edition

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

44

|

Chapter 3: Hardening Linux and Using iptables

This chapter lays the groundwork for much of what follows. Whereas most of the

rest of this book is about hardening specific applications, this chapter covers system-

hardening principles and specific techniques for hardening the core operating

system.

OS Hardening Principles

Operating-system hardening can be time consuming and even confusing. Like many

OSes designed for a wide range of roles and user levels, Linux has historically tended

to be “insecure by default”: most distributions’ default installations are designed to

present the user with as many preconfigured and active applications as possible.

Therefore, securing a Linux system not only requires you to understand the inner

workings of your system; you may also have to undo work others have done in the

interest of shielding you from those inner workings!

Having said that, the principles of Linux hardening and OS hardening in general can

be summed up by a single maxim: “That which is not explicitly permitted is forbid-

den.” As I mentioned in the previous chapter, this phrase was coined by Marcus

Ranum in the context of building firewall rules and access-control lists. However, it

scales very well to most other information security endeavors, including system hard-

ening.

Another ...