在現今網際網路日益發達的時代，如何有效管理並且組織有效率的網路是非常重要的議題，VPN技術因此而誕生。透過VPN，可以有效的組織管理分散各地的區域網路，但是其所使用的穿隧技術，卻有著隱藏的安全性問題，若搭配更改Port Number等方式就有隱藏的危機出現。
本篇論文主要針對一些主要的VPN技術做基本的概念分析，並且介紹如何修改使它具有穿越防火牆的能力，造成網管人員或防火牆不易控管。另外，本篇論文並在後面提出解決方法，使其有效率的解決VPN安全性的問題。
最主要造成安全性的問題在於盜用其他通訊協定的Port Number，本篇論文之解決方法在於徹底解決不當更改Port Number的問題，使駭客無法利用如一般防火牆未阻擋的HTTP Port 80的方式進行不當連線。
我們的解決方法是製作一個基於網路裝置驅動程式的封包過濾器，利用IETF定義好的RFC規範建立封包檢查機制，將不合法的封包阻擋起來，以確保網路的安全。

In this age, the internet has becoming more and more popular recently. How to manage and organize the network effectively is a very important issue.Therefore, the technology of VPN was born. Through the VPN, we can manage and organize the local netork which spread everywhere effectively.But the tunneling technology which VPN used has a security problem. If we also change the VPN’s port number, it will have a big dangerous security problem.
In this paper, we will analyze some basic technology of VPN, and introduce how to modify the VPN. Let VPN have ability to pass through the firewall. This problem will make the people who managed whole network or firewall hard to control and manage it. Another, this paper will bring up the solution which can solve the security problem effectively.
The key of network security problem is to use another protocol’s port number. The solution in this paper will through solve this problem, so hacks can’t modify the TCP port number such as HTTP Port 80 at will.
Our solution is to implement a packet filter which is based on ethernet device driver.We use the RFC document which are defined by IETF to make the packet check rule. This packet filter can reject the illegal packet and make sure the network is safe.

第1章 導論
1.1 研究動機
1.2 解決方案
1.3 論文架構
第2章 相關研究
2.1 VPN
2.2 L2TP
2.3 IPSEC
2.4 PPTP
2.4.1 何謂PPTP
2.4.2 PPTP的安裝
2.4.2.1 PPTP Server
2.4.2.2 PPTP Client
2.5 ETHERNET DEVICE DRIVER
2.5.1 Ethernet Device Driver on Linux
2.5.2 Ethernet Device Driver on FreeBSD
第3章 想法與實作
3.1 MODIFY THE PPTP
3.1.1 Modify TCP Port Number
3.1.2 Modify IP Protocol Number
3.2 PPTP WITH NAT
3.3 PACKET FILTER ON LINUX
3.3.1 PPTP Packet Filter
3.3.2 FTP Packet Filter
3.3.3 HTTP Packet Filter
3.3.4 Telnet Packet Filter
3.3.5 User Configuration
3.4 PACKET FILTER ON FREEBSD
3.4.1 Sniff Packets in Device Driver
3.4.2 User Configuration
3.5 遇到的瓶頸與解決方法
3.6 PACKER FILTER的配置
第4章 效能評估
4.1 實驗環境
4.2 測試項目
4.3 測試結果
4.3.1 FTP Proformance
4.3.2 HTTP Performance
4.3.3 CPU Loading Evaluation
第5章 結論與未來研究方向
5.1 結論
5.2 下一代的VPN－MPLS VPN
5.3 未來研究方向
APPENDIX A PPTP CLIENT SETUP ON WINDOWS XP
APPENDIX B FTP PERFORMANCE EVALUATION DATA
REFERENCE

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