2.1. What do you want this system for?
Securing Debian is not very different from securing any other system; in order to do it properly, you must first decide what you intend to do with it. After this, you will have to consider that the following tasks need to be taken care of if you want a really secure system.
You will find that this manual is written from the bottom up, that is, you will read some information on tasks to do before, during and after you install your Debian system. The tasks can also be thought of as:
Decide which services you need and limit your system to those. This includes deactivating/uninstalling unneeded services, and adding firewall-like filters, or tcpwrappers.
Limit users and permissions in your system.
Harden offered services so that, in the event of a service compromise, the impact to your system is minimized.
Use appropriate tools to guarantee that unauthorized use is detected so that you can take appropriate measures.
