Assuming a firewall (whether in hardware or in software via IPTables / IPChains / or another software firewall), then the bulk of your nefarious traffic is (hopefully) already being taken care of. However, what slips through, on legitimate ports, can sometimes be denial of service attacks. A truly distrubuted denial of service attack is something for where there exists no known solution (at least at this time). However, a single user dos (or a small number of users working together) can...