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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta name="generator" content="HTML Tidy, see www.w3.org" /><title>Introduction</title><meta name="GENERATOR" content="Modular DocBook HTML Stylesheet Version 1.7" /><link rel="HOME" title="FreeBSD Handbook" href="index.html" /><link rel="UP" title="Security" href="security.html" /><link rel="PREVIOUS" title="Security" href="security.html" /><link rel="NEXT" title="Securing FreeBSD" href="securing-freebsd.html" /><link rel="STYLESHEET" type="text/css" href="docbook.css" /></head><body class="SECT1" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#840084"alink="#0000FF"><div class="NAVHEADER"><table summary="Header navigation table" width="100%" border="0" cellpadding="0"cellspacing="0"><tr><th colspan="3" align="center">FreeBSD Handbook</th></tr><tr><td width="10%" align="left" valign="bottom"><a href="security.html"accesskey="P">Prev</a></td><td width="80%" align="center" valign="bottom">Chapter 14 Security</td><td width="10%" align="right" valign="bottom"><a href="securing-freebsd.html"accesskey="N">Next</a></td></tr></table><hr align="LEFT" width="100%" /></div><div class="SECT1"><h1 class="SECT1"><a id="SECURITY-INTRO" name="SECURITY-INTRO">14.2 Introduction</a></h1><p>Security is a function that begins and ends with the system administrator. While allBSD <span class="TRADEMARK">UNIX</span>® multi-user systems have some inherentsecurity, the job of building and maintaining additional security mechanisms to keepthose users ``honest'' is probably one of the single largest undertakings of thesysadmin. Machines are only as secure as you make them, and security concerns are evercompeting with the human necessity for convenience. <span class="TRADEMARK">UNIX</span>systems, in general, are capable of running a huge number of simultaneous processes andmany of these processes operate as servers -- meaning that external entities can connectand talk to them. As yesterday's mini-computers and mainframes become today's desktops,and as computers become networked and internetwork, security becomes an even biggerissue.</p><p>Security is best implemented through a layered ``onion'' approach. In a nutshell, whatyou want to do is to create as many layers of security as are convenient and thencarefully monitor the system for intrusions. You do not want to overbuild your securityor you will interfere with the detection side, and detection is one of the single mostimportant aspects of any security mechanism. For example, it makes little sense to setthe <var class="LITERAL">schg</var> flags (see <ahref="http://www.FreeBSD.org/cgi/man.cgi?query=chflags&sektion=1"><spanclass="CITEREFENTRY"><span class="REFENTRYTITLE">chflags</span>(1)</span></a>) on everysystem binary because while this may temporarily protect the binaries, it prevents anattacker who has broken in from making an easily detectable change that may result inyour security mechanisms not detecting the attacker at all.</p><p>System security also pertains to dealing with various forms of attack, includingattacks that attempt to crash, or otherwise make a system unusable, but do not attempt tocompromise the <tt class="USERNAME">root</tt> account (``break root''). Security concernscan be split up into several categories:</p><ol type="1"><li><p>Denial of service attacks.</p></li><li><p>User account compromises.</p></li><li><p>Root compromise through accessible servers.</p></li><li><p>Root compromise via user accounts.</p></li><li><p>Backdoor creation.</p></li></ol><p>A denial of service attack is an action that deprives the machine of needed resources.Typically, DoS attacks are brute-force mechanisms that attempt to crash or otherwise makea machine unusable by overwhelming its servers or network stack. Some DoS attacks try totake advantage of bugs in the networking stack to crash a machine with a single packet.The latter can only be fixed by applying a bug fix to the kernel. Attacks on servers canoften be fixed by properly specifying options to limit the load the servers incur on thesystem under adverse conditions. Brute-force network attacks are harder to deal with. Aspoofed-packet attack, for example, is nearly impossible to stop, short of cutting yoursystem off from the Internet. It may not be able to take your machine down, but it cansaturate your Internet connection.</p><p>A user account compromise is even more common than a DoS attack. Many sysadmins stillrun standard <b class="APPLICATION">telnetd</b>, <b class="APPLICATION">rlogind</b>, <bclass="APPLICATION">rshd</b>, and <b class="APPLICATION">ftpd</b> servers on theirmachines. These servers, by default, do not operate over encrypted connections. Theresult is that if you have any moderate-sized user base, one or more of your userslogging into your system from a remote location (which is the most common and convenientway to login to a system) will have his or her password sniffed. The attentive systemadmin will analyze his remote access logs looking for suspicious source addresses evenfor successful logins.</p><p>One must always assume that once an attacker has access to a user account, theattacker can break <tt class="USERNAME">root</tt>. However, the reality is that in a wellsecured and maintained system, access to a user account does not necessarily give theattacker access to <tt class="USERNAME">root</tt>. The distinction is important becausewithout access to <tt class="USERNAME">root</tt> the attacker cannot generally hide histracks and may, at best, be able to do nothing more than mess with the user's files, orcrash the machine. User account compromises are very common because users tend not totake the precautions that sysadmins take.</p><p>System administrators must keep in mind that there are potentially many ways to break<tt class="USERNAME">root</tt> on a machine. The attacker may know the <ttclass="USERNAME">root</tt> password, the attacker may find a bug in a root-run server andbe able to break <tt class="USERNAME">root</tt> over a network connection to that server,or the attacker may know of a bug in a suid-root program that allows the attacker tobreak <tt class="USERNAME">root</tt> once he has broken into a user's account. If anattacker has found a way to break <tt class="USERNAME">root</tt> on a machine, theattacker may not have a need to install a backdoor. Many of the <ttclass="USERNAME">root</tt> holes found and closed to date involve a considerable amountof work by the attacker to cleanup after himself, so most attackers install backdoors. Abackdoor provides the attacker with a way to easily regain <tt class="USERNAME">root</tt>access to the system, but it also gives the smart system administrator a convenient wayto detect the intrusion. Making it impossible for an attacker to install a backdoor mayactually be detrimental to your security, because it will not close off the hole theattacker found to break in the first place.</p><p>Security remedies should always be implemented with a multi-layered ``onion peel''approach and can be categorized as follows:</p><ol type="1"><li><p>Securing <tt class="USERNAME">root</tt> and staff accounts.</p></li><li><p>Securing <tt class="USERNAME">root</tt>-run servers and suid/sgid binaries.</p></li><li><p>Securing user accounts.</p></li><li><p>Securing the password file.</p></li><li><p>Securing the kernel core, raw devices, and file systems.</p></li><li><p>Quick detection of inappropriate changes made to the system.</p></li><li><p>Paranoia.</p></li></ol><p>The next section of this chapter will cover the above bullet items in greaterdepth.</p></div><div class="NAVFOOTER"><hr align="LEFT" width="100%" /><table summary="Footer navigation table" width="100%" border="0" cellpadding="0"cellspacing="0"><tr><td width="33%" align="left" valign="top"><a href="security.html"accesskey="P">Prev</a></td><td width="34%" align="center" valign="top"><a href="index.html"accesskey="H">Home</a></td><td width="33%" align="right" valign="top"><a href="securing-freebsd.html"accesskey="N">Next</a></td></tr><tr><td width="33%" align="left" valign="top">Security</td><td width="34%" align="center" valign="top"><a href="security.html"accesskey="U">Up</a></td><td width="33%" align="right" valign="top">Securing FreeBSD</td></tr></table></div><p align="center"><small>This, and other documents, can be downloaded from <ahref="ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/">ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/</a>.</small></p><p align="center"><small>For questions about FreeBSD, read the <ahref="http://www.FreeBSD.org/docs.html">documentation</a> before contacting <<ahref="mailto:questions@FreeBSD.org">questions@FreeBSD.org</a>>.<br />For questions about this documentation, e-mail <<ahref="mailto:doc@FreeBSD.org">doc@FreeBSD.org</a>>.</small></p></body></html>⌨️ 快捷键说明
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