Keeping the System Up to Date

Many compromised systems owe their inglorious compromised status to lack of appropriate maintenance. A few minutes spent checking for and installing software updates on a regular basis can save uncountable hours of work later, because updated software frequently includes fixes for security bugs . If you update buggy software quickly enough, would-be intruders will not be able to exploit security vulnerabilities.

The Importance of Server Updates

Software bugs can take many forms and have many different types of effects. Bugs can corrupt data, crash the affected program, or make the program behave in some odd way. Some bugs are security- related . They may allow a person to write arbitrary files in arbitrary locations ( potentially overwriting critical configuration files), or give the abuser the ability to run programs under some other username. In sum, such bugs can compromise the system, giving a normal user superuser privileges.

Servers, like any other program, can be buggy. Buggy servers are particularly important because they're potentially more accessible than are buggy local programs. If a non-network program (say, man ) contains a security-related bug, only local users can exploit the bug. Assuming your users are trustworthy, and assuming a cracker hasn't gained local access to your system, such a bug won't cause harm. (Of course, those assumptions aren't always valid, so fixing such bugs is important.) Many servers, by contrast, are accessible to the world at large. If a flaw in a Web server allows any user to take control of the computer, then that Web server is vulnerable to attack from just about anybody. Thus, security bugs in servers are particularly critical, and it's vital you protect yourself against them.

The problem is exacerbated by the fact that many servers run as root . If a program (server or nonserver) that runs as an ordinary user is compromised, chances are little damage can be done with it. For instance, such a program can't ordinarily rewrite your /etc/passwd file. If a program that runs as root is compromised, though, the attacker has much greater power; if such a program can be made to write arbitrary files, changing /etc/passwd is very possible. Many servers need root privileges to function correctly. For instance, root access is needed to provide login services, or even to listen to the first 1024 ports, on which most servers run. (A super server runs as root , but can spawn a server that runs as another user, even when it serves a sub-1024 port.)

For all of these reasons, it's critical that you keep your servers up to date. You don't necessarily need to perform every server update, because many server updates exist to add features or fix nonsecurity bugs that might not affect you. You should upgrade whenever an update emerges that fixes a security bug, though.

How to Monitor for Updated Software

There are several ways to look for updated software packages:

  • Software package Web sites and mailing lists ” Most software packages, including most servers, have official Web sites, mailing lists, and occasionally newsgroups or other communication forums. You can monitor these resources on a regular basis to locate software updates. This approach can be tedious , though; a Linux system may have a dozen or more servers installed, and monitoring all the relevant forums can be difficult at best. This approach is best reserved for unusual packages ”those that aren't part of your normal distribution's software mix ”and perhaps for very popular servers you might be running.

  • Your distribution's Web site ” All distributions have Web pages that include information on software updates. Distribution maintainers do the work of monitoring various security resources, including the Web pages for the individual server packages included in the distribution. This provides you with a one-stop location for security and other update information. The drawback is that it may take some time for a security fix to filter down from its original source to your distribution's Web page. In a best-case scenario, the delay might be just a few minutes, but it's more likely to be a few hours or even days.

  • Generic security information sources ” The upcoming section, "Keeping Abreast of Security Developments," describes resources for information on security-related developments. These can be extremely useful and important. They usually include information on workarounds to problems, if they exist, so you may be able to take steps to minimize the risk before an official fix is available. You'll have to go back to the program maintainer or your distribution's updates page to obtain fixed software, though.

In most cases, some combination of the last two approaches is a good way to keep an eye on security developments. Reading your servers' Web sites can also be important, particularly if you're using unusual servers that aren't officially supported by your distribution. A quick check of two or three Web pages or newsgroups once a day can save untold hours of work recovering from a break-in. Even a once-a-week check is better than nothing, and a periodic comparison of installed packages against the latest versions available can help catch updates that might have slipped through the cracks, as it were.

Automatic Software Update Procedures

Unfortunately, manually checking for software updates can be tedious at best. For this reason, there are several tools available to help automate the process. These include the following:

  • apt-get ” This program is a standard part of the Debian distribution and its derivatives. It's used for installing software, and it can also check for updates to already installed packages. Specifically, typing apt-get update followed by apt-get dist-upgrade will retrieve updated package information and then upgrade any packages that have newer versions. Replace the second command with apt-get -s -u upgrade to receive a report on new packages without actually installing them. Using apt-get in this way will only work, however, if you list at least one Debian package distribution site in the /etc/apt/sources.list file. There are also ports of apt-get (part of the larger apt package) for RPM-based systems, such as the one created by Connectiva ( and apt4rpm (http://apt4rpm. sourceforge .net).

  • Red Hat's Update Agent ” Red Hat uses a package it calls the Update Agent to help keep systems up to date. This package requires you to register with Red Hat, and the program sends information on your computer's hardware and software to Red Hat. It can then keep your system updated. Configuration and use of the program is moderately complex, so you should consult its documentation at for more information.

Automatic security updates are desirable in many ways, because they can help protect you against security breaches. They aren't without their drawbacks, though. By giving an automatic process control of your computer, you're entrusting it with a huge responsibility. Automatic updates can and do fail in various ways. For instance, an updated package might include a new bug or an incompatibility with another important package ( especially if you've mixed packages from your distribution with others you build yourself or install from tarballs). It's also conceivable that a cracker could break into the automatic update site or a DNS server in order to deliver modified packages. Because Debian packages sometimes include installation scripts that require human interaction, you shouldn't run apt-get in a cron job or other automated procedure; you should run it manually, even if you plan to do so on a regular basis. (Using apt-get -s -u upgrade in a cron job should be safe, though.) These tools don't always differentiate between security updates and others that are less critical, but which might cause problems for your system.

On the whole, automated software updates can be quick and convenient , but I recommend using them only in a strictly supervised manner. Ideally, you should be able to authorize individual upgrades so as to head off problems due to an overzealous update agent. This is an area of active development, so it's likely that these tools will become more sophisticated and helpful in the future.

Advanced Linux Networking
Advanced Linux Networking
ISBN: 0201774232
EAN: 2147483647
Year: 2002
Pages: 203

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