The Design And Implementation Of Content Filtering Firewall

With the fast development of the Internet, the network contents "garbage" has already invaded our life.In order to prevent and control illegal and harmful information to spread on Internet,it has become significant problem in network security field to filter the information spreading on internet.The core technology of content filtering is string matching algorithm.The classic string matching algorithm-BM algorithm adoptes heuristic skip,but computing right shifts decided by Goodsuffix is very complicated,which affects matching efficiency.BMH is a simplification of BM algorithm.Only using Badchar shift function computers the right shifts,which makes string algorithm simpler and faster.BMHS algorithm is enhancement of BMH algorithm, the shifts is determined by the occurence of rightmost+1 text character in the pattern string, which enhances the largest right shift from m to m+1 and improves time performace.But in some cases,its efficiency is not better than BMH algorithm.This dissertation introduced the present condition of the contents safety, the research contents and the development direction.On a basis of researching BM algorithm and some important enhancement, A new improved algorithm- BMG is proposed.This algorithm combines the merits of BMH and BMHS.At the same time, the uniqueness of next character is taken into account. This algorithm greatly enhances the probability of occurrence of the largest right shift m+1 ,thus improves the matching speed effectively.Based on anlysis of Netfilter firewall frame,content filtering module is designed and content function is implemented.In order to test BM algorithm and BMG algorithm time performance under the same network environment,these two algorithms are respectively embedded in content filtering module.Test shows BMG algorithm is dominant on string matching efficiency and distinctly improves the speed of firewall detecting illegal words.