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<!ENTITY ubuntu '<phrase>Ubuntu</phrase>'>
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<chapter id="faq" status="needshelp">
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<title>Frequently Asked Questions</title>
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<para>Security-related questions are asked on support forums on a daily basis. Answers
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to these questions are varied. There is more than one right answer to most of them, and
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most of them require and deserve more thorough coverage than they generally get on these
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forums. We will address some of the most frequently asked general security questions here.
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<para><emphasis role="bold">Do I need a firewall?</emphasis></para>
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<para>It can't hurt. A properly configured firewall can protect your machine and your
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network from the hostile world of the Internet, where worms, viruses, and nefarious
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individuals are on constant missions to take control of your systems. Don't let this
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lull you into a false sense of security, though; running a firewall does not excuse
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you from security best-practices, such as keeping your software up to date, shutting
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down unnecessary services, etc. While your firewall can in fact provide significant
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protection on its own, its job is to be the first line of defense in a multi-layered
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security strategy.</para>
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<para><emphasis role="bold">Do I need anti-virus software?</emphasis></para>
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<para>Security is about trade-offs. Nearly everything you do to secure your system
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will involve some kind of sacrifice. You may be sacrificing performance, usability,
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maintainability, your precious time, stability, or in a few pleasant cases, nothing
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at all. Whether a security measure is a good idea is determined by asking yourself
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whether the cost is worth the mitigated risk. A firewall is almost always a good
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idea, because it provides a significant amount of protection from a wide range of
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threats without, if properly configured, interfering too much in your activities or
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requiring a great deal of maintenance. Once it's up and running, you can almost
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forget it's there.</para>
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<para>There are rumors that Linux is impervious to viruses. While it is true that
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Linux systems are much more resistant to viruses than Windows systems, and much
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more difficult to infect, this is not the case. No virus (if we distinguish viruses
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from worms) has ever been found infecting Linux machines in the wild, but it's not
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inconceivable that one could appear in the future. If it does, it is not likely to
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be very successful. Most Linux users - responsible Linux users - do not perform
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their daily activities, such as surfing the Internet, checking email, etc., with
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administrative (root) privileges. For this and other reasons, a virus would have
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a difficult time infecting system files or spreading itself further.</para>
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<para>To summarize the virus risk, it is unlikely but possible that you will ever
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encounter a virus threat to your Linux system. Anti-virus products are popular on
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Windows systems because the threat to them is much greater. If security is of
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paramount importance to you, you should consider installing an anti-virus system,
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but you should be aware of the cost. For anti-virus software to protect effectively,
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it must scan executable files whenever they are downloaded or executed. This can
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have an enormous impact on the performance of your system. There is an administrative
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cost, too. Virus definitions must be updated regularly -- preferably daily. While
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this can usually be set up once and forgotten, the antivirus engines must also be
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updated. As with most Linux distributions, &ubuntu; usually backports security
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updates and bugfixes to software in its repositories for stable releases. This means
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that your anti-virus engine will need to be updated from outside the official
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repositories to maintain its ability to cope with new virus threats. Finally,
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since anti-virus software is just as prone to bugs as any other software, and since
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it must scan all your executable files, libraries, documents, etc., it poses a risk
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to the stability of your system as well. In short, an anti-virus system may be worth
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the cost to some users, but most choose to omit it from their security arsenals. The
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decision is yours to make.</para>
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<para><emphasis role="bold">I don't have any sensitive information on my machine.
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Why should I care about security?</emphasis></para>
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<para>An unfortunately common attitude on the Internet is that security is only
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important to those with sensitive information stored on their computers. It is
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critical to debunk this myth. First, you probably have sensitive information on your
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machine whether you're aware of it or not. You might instruct your browser to save
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passwords to secure websites, like your banking site. Your email client may store
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authentication information for your mail server, where sensitive information may
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also be stored, and where it is available to anyone who gains access to your username
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<para>Even if you truly have no personal motivation to protect the information on
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your system, though, it is irresponsible and destructive for you to ignore security
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concerns. Most of the security threats on the Internet do not have the goal of
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uncovering secrets stored on your computer. They seek to gain access to your system
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in order to use it as a platform for launching attacks on other machines. If one of
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the unscrupulous citizens of the Internet takes advantage of an unsecured machine, he
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may or may not look for sensitive information. He is much more likely to turn your
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machine into a <quote>zombie</quote>, without your knowledge, and have it wait to do
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his bidding. At the attacker's command, your machine will be used to participate in
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distributed attacks on other networks, to send spam as fast as your computer can send
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it, or to launch a targeted attack on another network, masking the attacker's identity
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by making the attack appear to originate from your system.</para>
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<para>Security is important, then, not only to protect yourself, but to help make the
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Internet a safer place by ensuring that your computer and its resources are not
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subverted for the purpose of attacking others.</para>
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<para><emphasis role="bold">How secure is Linux?</emphasis></para>
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<para>Security is a guiding principle behind the development of Linux and most Linux
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software. It also benefits, like all open source software, from the fact that its
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code can be audited for security problems by anyone with the expertise to do so.
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&ubuntu; as a distribution is also very security-conscious and adheres to a list of
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rules that help to keep it as secure as possible out of the box while maintaining
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usability. Additionally, &ubuntu; benefits from the efforts of Debian's famed
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security team. But in the end, a Linux machine -- like any other machine -- is only
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as secure as its administrator makes it. A freshly installed &ubuntu; machine has
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no ports open to the outside world, which helps to make it very secure against
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external threats, but users inevitably install software that (out of necessity)
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exposes itself to the Internet, making it less secure. If these applications are
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properly configured and kept up to date, they should not represent a significant
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threat to security, but it is your responsibility as the administrator to do these
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<para id="secure-passwords" xreflabel="secure password">
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<emphasis role="bold">How long is a secure password?</emphasis></para>
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<para>Obviously there is no cutoff point where one could call a password of a certain
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length <quote>secure</quote> and all shorter passwords <quote>insecure</quote>. Also,
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there are other factors than length that contribute to the effectiveness of a password.
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The more <emphasis role="italics">character classes</emphasis> in your password, the
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more secure it is. Character classes include uppercase letters, lowercase letters,
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numbers, and special characters or punctuation. Any password that can be found in a
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dictionary is vulnerable to a <emphasis role="italics">dictionary attack</emphasis>
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and is therefore insecure. Appending digits to the end of a dictionary password does
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not significantly enhance its security. A password like <quote>forest179</quote>
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would succumb to a dictionary attack in a few seconds. A password like
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<quote>9gIn$nf2Y_!</quote> is reasonably secure against all forms of brute force
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attacks, because it is not in any dictionary, it is 11 characters long, and it uses
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all character classes (except non-printable characters, which are unsupported in some
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<para><emphasis role="bold">Should I write down my password?</emphasis></para>
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<para>The consensus on this question has shifted in recent years from an emphatic
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<quote>No</quote> to a cautious <quote>Yes</quote>. The problem is that as computers
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get faster, it becomes harder to choose a password that a user can remember, but
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which is still secure. The concept of password authentication is meant to solve the
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problem of proving who you are to the authenticating system. A password is,
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theoretically, something that only you know, so providing it to the system is supposed
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to prove who you are. If someone else can obtain your password, the system breaks.
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If you write down your password, it may be easier for an attacker to obtain that piece
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of paper than to get into your head, so that practice has historically been considered
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insecure. But as computers become more powerful, it becomes easier to obtain unwritten
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passwords without getting into your head, because you are likely to choose a password
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that can be broken, because the things that make it easy to remember (it's short, it's
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a word or phrase that can be pronounced, etc.) are the same things that make it easy to
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break. Choosing a more secure password that's difficult to remember, and writing that
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password down, may help to solve that problem (if you keep your password in a safe
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place - don't tape it to your monitor!). The authentication system is now based on
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<quote>something you have</quote> rather than <quote>something you know</quote>.
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Many, but certainly not all, security experts now agree that choosing a difficult
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password and writing it down is more likely to provide securite than trying to choose a
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good password that you can remember.</para>
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<para><emphasis role="bold">How can I make my system more secure?</emphasis></para>
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There are many steps you can take to secure your system. Some of these steps are
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more effective than others, but the most important consideration is that your system
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is secure <emphasis role="italics">in depth</emphasis>. If your first line of
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defense fails or is defeated, another countermeasure should be right behind it to
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back it up. For example, if you're too confident in your firewall and trust it as
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your only security measure, matters will be much worse if and when it fails than if
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you had secured your system in depth.</para>
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<para>Specific steps you can take to secure your system will be covered in detail in
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this document. If you're looking for the most useful information with the least
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possible text, the following list should get you started:</para>
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<listitem><para>Don't run--or even install--unnecessary software; the more software
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you have running, the more possible points of failure you have.</para></listitem>
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<listitem><para>For software that you do run, especially servers, ensure that it is not
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available (does not <quote>listen</quote>) on network interfaces or addresses
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that are not necessary.</para></listitem>
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<listitem><para>All software, especially server software, especially server software
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that listens on an Internet-facing device, should be configured as securely as
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possible while still providing necessary functionality. Remember the <xref
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linkend="least-privilege" />.</para></listitem>
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<listitem><para>A good firewall can protect you against many attacks to which you might
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otherwise be vulnerable.</para></listitem>
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<listitem><para>A <xref linkend="secure-passwords" /> is an absolute requirement for a
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secure system. (Note that secure non-password authentication counts as a secure
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password.)</para></listitem>
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<para id="least-privilege" xreflabel="principle of least privilege"><emphasis
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role="bold">What is the Principle of Least Privilege?</emphasis></para>
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<para>The principle of least privilege states that a system operates with the most
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restrictive set of permissions possible that still allows it to perform its required
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functions. A system can be a user, a piece of software, a computer, a network, or
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even a person. The more privileges a system has, the more damage it can do to
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other systems if its security is compromised. So, if each system operates under the
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principle of least privilege, all systems are safer because damage is limited in the
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event one of the systems is compromised.</para>
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