608 lines
15 KiB
HTML
608 lines
15 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Overview</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Linux Security HOWTO"
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HREF="index.html"><LINK
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REL="PREVIOUS"
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TITLE="Introduction"
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HREF="x21.html"><LINK
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REL="NEXT"
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TITLE="Physical Security"
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HREF="physical-security.html"></HEAD
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><BODY
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CLASS="sect1"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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><DIV
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SUMMARY="Header navigation table"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Linux Security HOWTO</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="x21.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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></TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="physical-security.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="AEN82"
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></A
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>2. Overview</H1
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><P
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> This document will attempt to explain some procedures and commonly-used
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software to help your Linux system be more secure. It is
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important to discuss some of the basic concepts first, and create a
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security foundation, before we get started.
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</P
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="AEN85"
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></A
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>2.1. Why Do We Need Security?</H2
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><P
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> In the ever-changing world of global data communications, inexpensive
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Internet connections, and fast-paced software development, security is
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becoming more and more of an issue. Security is now a basic
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requirement because global computing is inherently insecure. As your
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data goes from point A to point B on the Internet, for example, it may
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pass through several other points along the way, giving other users
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the opportunity to intercept, and even alter, it. Even other
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users on your system may maliciously transform your data into
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something you did not intend. Unauthorized access to your system may
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be obtained by intruders, also known as "crackers", who then use
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advanced knowledge to impersonate you, steal information from you, or
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even deny you access to your own resources. If you're wondering
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what the difference is between a "Hacker" and a "Cracker", see Eric
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Raymond's document, "How to Become A Hacker", available at <A
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HREF="http://www.catb.org/~esr/faqs/hacker-howto.html"
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TARGET="_top"
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>http://www.catb.org/~esr/faqs/hacker-howto.html</A
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>.
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</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="AEN89"
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></A
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>2.2. How Secure Is Secure?</H2
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><P
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> First, keep in mind that no computer system can ever be completely
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secure. All you can do is make it increasingly difficult for someone
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to compromise your system. For the average home Linux user, not much
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is required to keep the casual cracker at bay. However, for
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high-profile Linux users (banks, telecommunications companies, etc),
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much more work is required.
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</P
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><P
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> Another factor to take into account is that the more secure your
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system is, the more intrusive your security becomes. You need to
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decide where in this balancing act your system will still be usable,
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and yet secure for your purposes. For instance, you could require
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everyone dialing into your system to use a call-back modem to call
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them back at their home number. This is more secure, but if someone is
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not at home, it makes it difficult for them to login. You could also
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setup your Linux system with no network or connection to the Internet,
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but this limits its usefulness.
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</P
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><P
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> If you are a medium to large-sized site, you should establish a
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security policy stating how much security is required by your site
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and what auditing is in place to check it. You can find a well-known
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security policy example at <A
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HREF="http://www.faqs.org/rfcs/rfc2196.html"
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TARGET="_top"
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>http://www.faqs.org/rfcs/rfc2196.html</A
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>. It has been recently
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updated, and contains a great framework for establishing a security
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policy for your company.
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</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="AEN95"
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></A
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>2.3. What Are You Trying to Protect?</H2
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><P
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> Before you attempt to secure your system, you should determine what
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level of threat you have to protect against, what risks you should or
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should not take, and how vulnerable your system is as a result. You
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should analyze your system to know what you're protecting,
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why you're protecting it, what value it has, and who has
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responsibility for your data and other assets.
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</P
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><P
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>
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<P
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></P
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><UL
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><LI
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><P
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> <EM
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>Risk</EM
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> is the possibility that an intruder may be successful in
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attempting to access your computer. Can an intruder read or write
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files, or execute programs that could cause damage? Can they delete
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critical data? Can they prevent you or your company from getting important work
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done? Don't forget: someone gaining access to your account, or your
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system, can also impersonate you.
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</P
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><P
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> Additionally, having one insecure account on your system can result in
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your entire network being compromised. If you allow a single user
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to login using a <TT
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CLASS="literal"
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>.rhosts</TT
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> file, or to use an insecure
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service such as <TT
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CLASS="literal"
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>tftp</TT
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>, you risk an intruder getting 'his
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foot in the door'. Once the intruder has a user account on your
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system, or someone else's system, it can be used to gain access to
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another system, or another account.
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</P
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></LI
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><LI
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><P
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> <EM
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>Threat</EM
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> is typically from someone with motivation to gain unauthorized
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access to your network or computer. You must decide whom you trust to
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have access to your system, and what threat they could pose.
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</P
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><P
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> There are several types of intruders, and it is useful to keep their
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different characteristics in mind as you are securing your systems.
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</P
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><P
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></P
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><UL
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><LI
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><P
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> <EM
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>The Curious</EM
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> - This type of intruder is basically
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interested in finding out what type of system and data you have.
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</P
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></LI
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><LI
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><P
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> <EM
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>The Malicious</EM
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> - This type of intruder is out to either
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bring down your systems, or deface your web page, or otherwise force you
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to spend time and money recovering from the damage he has caused.
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</P
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></LI
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><LI
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><P
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> <EM
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>The High-Profile Intruder</EM
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> - This type of intruder is
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trying to use your system to gain popularity and infamy. He might use
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your high-profile system to advertise his abilities.
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</P
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></LI
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><LI
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><P
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> <EM
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>The Competition</EM
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> - This type of intruder is interested in
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what data you have on your system. It might be someone who thinks you
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have something that could benefit him, financially or otherwise.
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</P
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></LI
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><LI
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><P
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> <EM
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>The Borrowers</EM
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> - This type of intruder is interested in
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setting up shop on your system and using its resources for their own
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purposes. He typically will run chat or irc servers, porn archive
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sites, or even DNS servers.
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</P
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></LI
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><LI
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><P
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> <EM
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>The Leapfrogger</EM
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> - This type of intruder is only
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interested in your system to use it to get into other systems. If your
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system is well-connected or a gateway to a number of internal hosts,
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you may well see this type trying to compromise your system.
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</P
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></LI
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></UL
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></LI
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><LI
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><P
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> Vulnerability describes how well-protected your computer is from
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another network, and the potential for someone to gain unauthorized
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access.
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</P
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><P
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> What's at stake if someone breaks into your system? Of course the
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concerns of a dynamic PPP home user will be different from those of a
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company connecting their machine to the Internet, or another large
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network.
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</P
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><P
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> How much time would it take to retrieve/recreate any data that was
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lost? An initial time investment now can save ten times more time
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later if you have to recreate data that was lost. Have you checked
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your backup strategy, and verified your data lately?
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</P
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></LI
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></UL
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>
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</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="AEN133"
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></A
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>2.4. Developing A Security Policy</H2
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><P
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> Create a simple, generic policy for your system that your users can
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readily understand and follow. It should protect the data you're
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safeguarding as well as the privacy of the users. Some things to
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consider adding are: who has access to the system (Can my friend use my
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account?), who's allowed to install software on the system, who owns
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what data, disaster recovery, and appropriate use of the system.
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</P
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><P
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> A generally-accepted security policy starts with the phrase
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</P
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><P
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> <SPAN
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CLASS="QUOTE"
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>"<EM
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> That which is not permitted is prohibited</EM
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>"</SPAN
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>
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</P
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><P
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> This means that unless you grant access to a service for a user, that
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user shouldn't be using that service until you do grant access. Make
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sure the policies work on your regular user account. Saying, "Ah, I
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can't figure out this permissions problem, I'll just do it as root"
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can lead to security holes that are very obvious, and even ones that
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haven't been exploited yet.
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</P
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><P
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> <A
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HREF="ftp://www.faqs.org/rfcs/rfc1244.html"
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TARGET="_top"
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>rfc1244</A
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>
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is a document that describes how to create your own network security
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policy.
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</P
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><P
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> <A
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HREF="ftp://www.faqs.org/rfcs/rfc1281.html"
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TARGET="_top"
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>rfc1281</A
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>
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is a document that shows an example security policy with detailed
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descriptions of each step.
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</P
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><P
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> Finally, you might want to look at the COAST policy archive at <A
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HREF="ftp://coast.cs.purdue.edu/pub/doc/policy"
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TARGET="_top"
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>ftp://coast.cs.purdue.edu/pub/doc/policy</A
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> to see what some
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real-life security policies look like.
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</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="AEN147"
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></A
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>2.5. Means of Securing Your Site</H2
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><P
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> This document will discuss various means with which you can secure
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the assets you have worked hard for: your local machine,
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your data, your users, your network, even your reputation. What would
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happen to your reputation if an intruder deleted some of your users'
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data? Or defaced your web site? Or published your company's
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corporate project plan for next quarter? If you are planning a network
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installation, there are many factors you must take into account before
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adding a single machine to your network.
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</P
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><P
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> Even if you have a single dial up PPP account, or just a small site,
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this does not mean intruders won't be interested in your systems.
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Large, high-profile sites are not the only targets -- many intruders
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simply want to exploit as many sites as possible, regardless of their
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size. Additionally, they may use a security hole in your site to gain
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access to other sites you're connected to.
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</P
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><P
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> Intruders have a lot of time on their hands, and can avoid guessing
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how you've obscured your system just by trying all the
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possibilities. There are also a number of reasons an intruder may be
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interested in your systems, which we will discuss later.
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</P
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><DIV
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CLASS="sect3"
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><H3
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CLASS="sect3"
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><A
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NAME="AEN152"
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></A
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>2.5.1. Host Security</H3
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><P
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> Perhaps the area of security on which administrators concentrate most is
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host-based security. This typically involves making sure your own
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system is secure, and hoping everyone else on your network does the
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same. Choosing good passwords, securing your host's local network
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services, keeping good accounting records, and upgrading programs with
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known security exploits are among the things the local security
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administrator is responsible for doing. Although this is absolutely
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necessary, it can become a daunting task once your network becomes
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larger than a few machines.
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</P
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></DIV
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><DIV
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CLASS="sect3"
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><H3
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CLASS="sect3"
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><A
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NAME="AEN155"
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></A
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>2.5.2. Local Network Security</H3
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><P
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> Network security is as necessary as local host security. With
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hundreds, thousands, or more computers on the same network,
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you can't rely on each one of those systems being secure. Ensuring
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that only authorized users can use your network,
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building firewalls, using strong encryption, and ensuring
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there are no "rogue" (that is, unsecured) machines on your network are all
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part of the network security administrator's duties.
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</P
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><P
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> This document will discuss some of the techniques used to secure your
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site, and hopefully show you some of the ways to prevent an intruder
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from gaining access to what you are trying to protect.
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</P
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></DIV
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><DIV
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CLASS="sect3"
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><H3
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CLASS="sect3"
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><A
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NAME="AEN159"
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></A
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>2.5.3. Security Through Obscurity</H3
|
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><P
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|
> One type of security that must be discussed is "security through
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obscurity". This means, for example, moving a service that has known
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security vulnerabilities to a non-standard port in hopes that attackers
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won't notice it's there and thus won't exploit it. Rest assured that
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they can determine that it's there and will exploit it. Security
|
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through obscurity is no security at all. Simply because you may have a
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small site, or a relatively low profile, does not mean an intruder
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won't be interested in what you have. We'll discuss what you're
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protecting in the next sections.
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</P
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></DIV
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="AEN162"
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></A
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>2.6. Organization of This Document</H2
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><P
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> This document has been divided into a number of sections. They cover
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several broad security issues. The first,
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<A
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HREF="physical-security.html"
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>Section 3</A
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>,
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covers how you need to protect your physical machine from
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tampering. The second,
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<A
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HREF="local-security.html"
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>Section 4</A
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>, describes how to
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protect your system from tampering by local users. The third,
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<A
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HREF="file-security.html"
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>Section 5</A
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>,
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shows you how to setup your file systems and permissions on your
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files. The next, <A
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HREF="password-security.html"
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>Section 6</A
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>, discusses how to use encryption to better secure
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your machine and network.
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<A
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HREF="kernel-security.html"
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>Section 7</A
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> discusses what kernel
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options you should set or be aware of for a more secure system.
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<A
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HREF="network-security.html"
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>Section 8</A
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>, describes how to
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better secure your Linux system from network attacks.
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<A
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HREF="secure-prep.html"
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>Section 9</A
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>, discusses how to
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prepare your machine(s) before bringing them on-line. Next,
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<A
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HREF="after-breakin.html"
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>Section 10</A
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>,
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discusses what to do when you detect a system compromise in progress
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or detect one that has recently happened. In <A
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HREF="sources.html"
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>Section 11</A
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>, some primary security resources are enumerated.
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The Q and A section <A
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HREF="q-and-a.html"
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>Section 13</A
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>,
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answers some frequently-asked questions, and finally a conclusion in
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<A
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HREF="conclusion.html"
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>Section 14</A
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>
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</P
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><P
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> The two main points to realize when reading this document are:
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</P
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><P
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>
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<P
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></P
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><UL
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><LI
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><P
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> Be aware of your system. Check system logs such as
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<TT
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CLASS="literal"
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>/var/log/messages</TT
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> and keep an eye on your system, and
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</P
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></LI
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><LI
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><P
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> Keep your system up-to-date by making sure you have installed the
|
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current versions of software and have upgraded per security alerts.
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Just doing this will help make your system markedly more secure.
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</P
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></LI
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></UL
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>
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</P
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></DIV
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="x21.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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|
WIDTH="34%"
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|
ALIGN="center"
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|
VALIGN="top"
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|
><A
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|
HREF="index.html"
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|
ACCESSKEY="H"
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|
>Home</A
|
|
></TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
><A
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|
HREF="physical-security.html"
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|
ACCESSKEY="N"
|
|
>Next</A
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|
></TD
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
>Introduction</TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
> </TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
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|
VALIGN="top"
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|
>Physical Security</TD
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|
></TR
|
|
></TABLE
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></DIV
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></BODY
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|
></HTML
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> |