It is a common mistake for people to assume that switching on a firewall makes them safe. This is not the case and, in fact, has never been the case. Each system has distinct security needs, and taking the time to customize its security layout will give you maximum security and the best performance.
The following list summarizes the most common mistakes:
Installing every package—Do you plan to use the machine as a DNS server? If not, why have BIND installed? Go through and ensure that you have only the software you need.
Enabling unused services—Do you want to administer the machine remotely? Do you want people to ...