System Administation: Maximizing System Security, Part 1
One of the most hackneyed cliches in all of UNIX culture is that UNIX security is a contradiction in terms. While things aren't quite as hopeless as this cynical view, it is important to realize that a secure system is something you create, not something you get automatically when you install any current Linux distribution (or any other UNIX operating system for that matter).
This article provides an overview of UNIX security issues, and discusses the resources and tools available to Linux system administrators or anyone responsible for administering a Linux system—which are not necessarily synonymous. It considers what the most important issues are and what exists to defend the system. And since many of the most egregious “UNIX” security problems are actually vulnerabilities in TCP/IP networking and its component protocols, we naturally consider network security issues, as well as those relevant to an isolated computer system.
General discussions of computer security traditionally focus on the types of losses that can result from inadequate security measures:
Loss of equipment. The first or last threat to any computer system (depending on your point of view) is the loss of the computer itself. This can result from a variety of causes: theft, fire, water, earthquakes and other natural disasters, vandalism, and accidents (e.g., a user spilling coffee on it).
Loss of data. This type of loss can also occur in a variety of ways: data could be obtained by someone who should not have it (for example, a competitor), files could be accidentally or deliberately damaged or destroyed, or information that should have remained private could become publically accessible or broadcast.
Loss of use. A third type of loss can occur when neither the equipment nor its data is damaged, destroyed or removed, but the system is nevertheless unable to perform some or all of its normal functions. For example, an extended power outage could cause such a loss of use; the 1988 Internet worm incident is an example of software rendering a computer unusable.
Depending on your situation, some of these threats are obviously more potentially hazardous to you than others.
Effective thinking about security begins by considering potential losses rather than potential threats, because doing so allows you to place the threats in the context of your system and thereby make appropriate choices about how to prevent and address them. For example, every system has the potential of being broken into by an unauthorized person. However, the specific nature of that threat changes depending on the sort of loss that would be its most serious consequence—as do the corresponding measures to prevent the loss.
A successful intruder always has the potential to alter or destroy any file on the system, so every system needs to guard against and have a plan for recovering from that eventuality. In addition, for a system containing sensitive or proprietary data (customer credit card numbers, source codes for software products under development, and so on) one might need to consider ways of securing such data even from the root account. On the other hand, if loss of use is the primary loss against which a system needs to be protected, then devising ways of quickly identifying and neutralizing such an attack is much more important than providing extra security for any of the data on the system.
As these scenarios suggest, security involves more than just prevention against attacks. Equally important components of computer security are the recovery plans which specify what to do when something goes wrong. Computer security is not something you think about once in a while, but rather something that is an integral part of your thinking and actions in every administrative activity you perform. It includes the following concerns (not all of which will necessarily apply to any specific system):
Physical system access
Theft prevention—locks and so on
Prevention of physical and electronic vandalism
Ensuring continuous power via an uninterruptible power supply (UPS) unit
Fire control systems, surge suppressors, and other devices to prevent damage from the external environment
User authentication: passwords and other mechanisms
Modem access (dialin and dialout)
File ownership and protection
Encryption of very sensitive or private data
Network access policies and network software configuration
Procedures and policies related to building, testing, installing and using public domain software
Secure storage of backup media (including offsite copies)
Storage of original operating system media
Disaster recovery plans
User training for good security practices
A thorough discussion of all of these topics would consume several entire issues of Linux Journal, so we focus on operating system-level protections and solutions useful for Linux systems, in terms of both “standard” features and useful additional packages. Security facilities offered by the various Linux distributions vary considerably, but no current distribution includes everything that a prudent system administrator would want to have and use.
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