I have download a series of good articles. I will summaried them as a sys configuration manual for our lab.And post it here also. BTW, lenx's article in this board is a good start point.
RTFM at first, and read Linux firewall howto as well. Don't ask question before you check all these articles - it's wasting both your time and ours. Also, there are many good papers at www.linuxsecurity.com.