
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
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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 ...