Understanding Computer Attacks
There are many ways in which computer attacks can be divided, but perhaps the easiest is internal, which are computer attacks done by someone with access to a computer on the local network, and external, which are attacks by someone with access to a computer through the Internet. This might sound like a trivial separation to make, but it is actually important: Unless you routinely hire talented computer hackers or allow visitors to plug computers into your network, the worst internal attack you are likely encounter is from a disgruntled employee.
Hacker Versus Cracker
In earlier days, there was a distinction made between the words hacker and cracker. A hacker was someone who used technology to innovate in new or unusual ways, whereas a cracker was someone who used technology to attack another's computers and cause harm.
This distinction was lost on the general public, so the term hacker has now come to mean the same as cracker.
Although you should never ignore the internal threat, you should arguably be more concerned with the outside world. The big bad Internet is a security vortex. Machines connected directly to the outside world can be attacked by people across the world, and invariably are, even only a few minutes after having been connected.
This situation is not a result of malicious users lying in wait for your IP address to do something interesting. Instead, canny virus writers have created worms that exploit a vulnerability, take control of a machine, and then spread it to other machines around them. As a result, most attacks today are the result of these autohacking tools; there are only a handful of true hackers around, and, to be frank, if one of these ever actually targets you seriously, it will take a mammoth effort to repel him regardless of which operating system you run.
Autohacking scripts also come in another flavor: prewritten code that exploits a vulnerability and gives its users special privileges on the hacked machine. These scripts are rarely used by their creators; instead, they are posted online and downloaded by wannabe hackers, who then use them to attack vulnerable machines.
So, the external category is itself made up of worms, serious day job hackers, and wannabe hackers (usually called script kiddies). Combined they will assault your Internetfacing servers, and it is your job to make sure your boxes stay up, happily ignoring the firefight around them.
On the internal front, things are somewhat more difficult. Users who sit inside your firewall are already past your primary source of defense and, worse, they might even have physical access to your machines.
Regardless of the source of the attack, there is a five-step checklist you can follow to secure your box:
1. Assess your vulnerability. Decide which machines can be attacked, which services they are running, and who has access to them.
2. Configure the server for maximum security. Only install what you need, only run what you must, and configure a local firewall.
3. Secure physical access to the server.
4. Create worst-case-scenario policies.
5. Keep up-to-date with security news.