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7.1 What is security?

For a computer to be secure it must be *physically secure* -- if we can get our hands on a host then we are never more than a screwdriver away from all of its assets--but assuming that hosts are physically secure, we then wish to deal with the issues of software security which is a much more difficult topic. Software security is about access control and software reliability. No single tool can make computer systems secure. Major blunders have been made out of the belief that a single product (e.g. a `firewall') would solve the security problem. For instance, a few years ago a cracker deleted all the user directories from a dialup login server belonging to a major Norwegian telecommunications company, from the comfort of his web browser. This was possible, even through a firewall, because the web server on the host concerned was incorrectly configured. The bottom line is that there is no such thing as a secure operating system, firewall or none. What is required is a persistent mixture of vigilence and adaptability.

For many, security is perceived as being synonymous with network privacy or network intrusion. Privacy is one aspect of security, but it is not the network which is our special enemy. Many breaches of security happen from within. There is little difference between the dangers of remote access from the network or direct access from a console: privacy is about access control, no matter where the potential intruder might be. If we focus exclusively on network connectivity we ignore a possible threat from internal employees (e.g. the janitor who is a computer expert and has an axe to grind, or the mischievous son of the director who was left waiting to play mom's office, or perhaps the unthinkable: a disgruntled employee who feels as though his/her talents go unappreciated). Software security is a vast subject, because modern computer systems are complex. It is only exacerbated by the connectivity of the internet which allows millions of people to have a go at breaking into networked systems. What this points to is the fact that a secure environment requires a tight control of access control on every host individually, not merely at specific points such as firewalls.

This article is not a comprehensive guide to security. Rather it is an attempt to illustrate how cfengine can be used to help you automate a level of host integrity on all the hosts of your network. Cfengine is a network configuration tool with two facets. It is a language used to build an `expert system'. An expert system describes the way you would like your hosts and network to look and behave. CFengine is also a software robot which compares the model you have described with what the world really looks like and then sets to work correcting any deviations from that picture. In many ways it is like an immune system, neutralizing and repairing damaged parts. Unlike many shell-script packages for sysadmin, cfengine is a C program which means light on system resources. Also it works on a principle of `convergence'. Convergence means that each time you run cfengine the system should get closer to the model which you have described until eventually when the system is the model, cfengine becomes quiescent, just like an immune system. In the words of one user your hosts `never get worse'. This assumes of course that the model you have is what you really want. Using cfengine, model building becomes synonymous with formulating and formalizing a system policy.

What makes cfengine a security tool is that security policy is a part of system policy: you cannot have one without the other. You will never have security unless you are in control of your network. Cfengine monitors and indeed repairs hosts with simple easily controllable actions. From an automation perspective, security is no different from the general day to day business of system maintenance, you just need to pay more attention to the details. We cannot speak of `have security' and `have not security'. There is always security, it is simply a matter of degree: weak or strong; effective or ineffective.


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