This document will attempt to explain some procedures and commonly-used software to help your Linux system be more secure. It is important to discuss some of the basic concepts first, and create a security foundation, before we get started.
In the ever-changing world of global data communications, inexpensive Internet connections, and fast-paced software development, security is becoming more and more of an issue. Security is now a basic requirement because global computing is inherently insecure. As your data goes from point A to point B on the Internet, for example, it may pass through several other points along the way, giving other users the opportunity to intercept, and even alter, it. Even other users on your system may maliciously transform your data into something you did not intend. Unauthorized access to your system may be obtained by intruders, also known as "crackers", who then use advanced knowledge to impersonate you, steal information from you, or even deny you access to your own resources. If you're wondering what the difference is between a "Hacker" and a "Cracker", see Eric Raymond's document, "How to Become A Hacker", available at http://www.netaxs.com/~esr/faqs/hacker-howto.html.
First, keep in mind that no computer system can ever be completely secure. All you can do is make it increasingly difficult for someone to compromise your system. For the average home Linux user, not much is required to keep the casual cracker at bay. However, for high-profile Linux users (banks, telecommunications companies, etc), much more work is required.
Another factor to take into account is that the more secure your system is, the more intrusive your security becomes. You need to decide where in this balancing act your system will still usable, and yet secure for your purposes. For instance, you could require everyone dialing into your system to use a call-back modem to call them back at their home number. This is more secure, but if someone is not at home, it makes it difficult for them to login. You could also setup your Linux system with no network or connection to the Internet, but this limits its usefulness.
If you are a medium to large-sized site, you should establish a security policy stating how much security is required by your site and what auditing is in place to check it. You can find a well-known security policy example at http://www.faqs.org/rfcs/rfc2196.html. It has been recently updated, and contains a great framework for establishing a security policy for your company.
Before you attempt to secure your system, you should determine what level of threat you have to protect against, what risks you should or should not take, and how vulnerable your system is as a result. You should analyze your system to know what you're protecting, why you're protecting it, what value it has, and who has responsibility for your data and other assets.
Additionally, having one insecure account on your system can result in your entire network being compromised. If you allow a single user to login using a .rhosts file, or to use an insecure service such as tftp, you risk an intruder getting 'his foot in the door'. Once the intruder has a user account on your system, or someone else's system, it can be used to gain access to another system, or another account.
There are several types of intruders, and it is useful to keep their different characteristics in mind as you are securing your systems.
What's at stake if someone breaks into your system? Of course the concerns of a dynamic PPP home user will be different from those of a company connecting their machine to the Internet, or another large network.
How much time would it take to retrieve/recreate any data that was lost? An initial time investment now can save ten times more time later if you have to recreate data that was lost. Have you checked your backup strategy, and verified your data lately?
Create a simple, generic policy for your system that your users can readily understand and follow. It should protect the data you're safeguarding as well as the privacy of the users. Some things to consider adding are: who has access to the system (Can my friend use my account?), who's allowed to install software on the system, who owns what data, disaster recovery, and appropriate use of the system.
A generally-accepted security policy starts with the phrase
That which is not permitted is prohibited
This means that unless you grant access to a service for a user, that user shouldn't be using that service until you do grant access. Make sure the policies work on your regular user account. Saying, "Ah, I can't figure out this permissions problem, I'll just do it as root" can lead to security holes that are very obvious, and even ones that haven't been exploited yet.
rfc1244 is a document that describes how to create your own network security policy.
rfc1281 is a document that shows an example security policy with detailed descriptions of each step.
Finally, you might want to look at the COAST policy archive at ftp://coast.cs.purdue.edu/pub/doc/policy to see what some real-life security policies look like.
This document will discuss various means with which you can secure the assets you have worked hard for: your local machine, your data, your users, your network, even your reputation. What would happen to your reputation if an intruder deleted some of your users' data? Or defaced your web site? Or published your company's corporate project plan for next quarter? If you are planning a network installation, there are many factors you must take into account before adding a single machine to your network.
Even if you have a single dialup PPP account, or just a small site, this does not mean intruders won't be interested in your systems. Large, high-profile sites are not the only targets -- many intruders simply want to exploit as many sites as possible, regardless of their size. Additionally, they may use a security hole in your site to gain access to other sites you're connected to.
Intruders have a lot of time on their hands, and can avoid guessing how you've obscured your system just by trying all the possibilities. There are also a number of reasons an intruder may be interested in your systems, which we will discuss later.
Perhaps the area of security on which administrators concentrate most is host-based security. This typically involves making sure your own system is secure, and hoping everyone else on your network does the same. Choosing good passwords, securing your host's local network services, keeping good accounting records, and upgrading programs with known security exploits are among the things the local security administrator is responsible for doing. Although this is absolutely necessary, it can become a daunting task once your network becomes larger than a few machines.
Network security is as necessary as local host security. With hundreds, thousands, or more computers on the same network, you can't rely on each one of those systems being secure. Ensuring that only authorized users can use your network, building firewalls, using strong encryption, and ensuring there are no "rogue" (that is, unsecured) machines on your network are all part of the network security administrator's duties.
This document will discuss some of the techniques used to secure your site, and hopefully show you some of the ways to prevent an intruder from gaining access to what you are trying to protect.
One type of security that must be discussed is "security through obscurity". This means, for example, moving a service that has known security vunerabilities to a non-standard port in hopes that attackers won't notice it's there and thus won't exploit it. Rest assured that they can determine that it's there and will exploit it. Security through obscurity is no security at all. Simply because you may have a small site, or a relatively low profile, does not mean an intruder won't be interested in what you have. We'll discuss what you're protecting in the next sections.
This document has been divided into a number of sections. They cover several broad security issues. The first, Physical Security, covers how you need to protect your physical machine from tampering. The second, Local Security, describes how to protect your system from tampering by local users. The third, Files and Filesystem Security, shows you how to setup your filesystems and permissions on your files. The next, Password Security and Encryption, discusses how to use encryption to better secure your machine and network. Kernel Security discusses what kernel options you should set or be aware of for a more secure system. Network Security, describes how to better secure your Linux system from network attacks. Security Preparation, discusses how to prepare your machine(s) before bringing them on-line. Next, What To Do During and After a Break-in, discusses what to do when you detect a system compromise in progress or detect one that has recently happened. In Security Resources, some primary security resources are enumerated. The Q and A section Frequently Asked Questions, answers some frequently-asked questions, and finally a conclusion in Conclusion.
The two main points to realize when reading this document are:
/var/log/messages and keep an eye on your system, and