Netfilter-based Spam Filtering Gateway Design And Implementation

With the boom of Internet, Email is put into use widely for the great convenience of our work and life. At the same time, more and more Spam(or trash mail) became a big trouble for the Email users. How to filter Spams about all the types effectively is now a hotspot problem for many researchers.There are two ways for filtering Spam mostly, Client Filtering and Server Filtering. Present Spam-filtering systems using Server Filtering technique have their flaws. The first is that too much server resources are consumed by the Spam-filtering, which influence the common Email service. Secondly, they are complicated to configure and use in a smaller network, for example, a company Intranet.In the first part of this article, some traditional Spam-filtering techniques, including Real-time Black List, Reverse DNS Requirement, Bayes Filtering, Filtering based on rules, etc. are introduced. Through analyzing their features and deficiency, a Spam-filtering model based on Netfilter frameworks is proposed. It has features as separating from Email server, transparent for Email users and easy to configure and use. It consists of seven modules as following: IP packet re-direction, protocol analyse, attack protection, mail head analyse, Bayes filtering, filtering based on rules, virus check and administrator requirement. The first two of these are also called Email capturing engine.Consequent content of this article is about how to design and implement a high-speed Email capturing engine which is also the main course of my work. It includes constructing a transparent networks gateway, capturing IP packets, restoring SMTP Emails, and the implementation of a re-entered mail queue with two buffers. Afterwards, a relatively brief introduction to Spam Judge Subsystem is given, including mail head analyse, Bayes filtering and filtering based on rules.At the end, approaches to test this Spam-filtering system are illuminated and some successful tests are done in our lab's Intranet. An advice to improve it is also mentioned.