Computer security is a hot topic, more and more Internet users are concerning about computer security. Linux is a operating system like Unix. It has all the features of Unix operating system. Linux system has a very strict structure like Unix in security. This paper discusses the network security of Linux system from the technical aspects of network security. The paper introduces methods of protecting network security which set the right to accessing the customer machine and use firewall technology. Describe detailed the Netfilter`s structure and Netfilter`s position in network.