摘要
随着全球经济的发展，无论是大企业还是中小企业，都在各地拥有自己的分支机构。 VPN技术以其管理简单,费用低廉的优点成为企业构建内部广域网络的首选。VPN服务市场在2005年取得了总销售额230亿美元的骄人业绩，预计到2009年还将增长22%，前景非常广阔。
VPN(虚拟专用网)不是真的专用网络，但它却能够通过公用网络实现专用网络的功能，能为企业的商业运作提供一个可靠、安全的数据传输网络。实现VPN技术的方式很多，本文以在Linux系统下常用的三种软件VPN：PPTP VPN ，SSL VPN 和IPSEC VPN为例，对各种VPN的工作原理做了详细的阐述，并对实现这些VPN的功能，需要在其服务器端和客户端如何配置做出了详细的总结。
在最后阶段对几种VPN进行比较，得出结论：IPSec VPN适合拥有较多的分支机构，维护成本较高，IT建设、管理和维护方面拥有一定经验的员工的大中型企业。而SSL VPN适用于那些需要很强灵活性的企业，员工需要在不同地点都可以轻易的访问公司内部资源，并可能通过各种移动终端或设备随时进行访问，企业的IT维护较低。相对而言，PPTP配置简单方便，适合那些规模较小，对安全要求较低的小企业。
Study of the VPN Technology
Abstract
Along with the development of global economy, regardless of the big enterprises or mid- and small-scale enterprises, all have their own branches in each place. The VPN technology, for its merits of simple management and the low expense, becomes the first choice of the enterprise to construct the internal wide area network (WAN). In 2005, the VPN service market has obtained excellent achievements with the total sales volume of 23 billion US dollars, which is surprising, it is estimated that it will increase by 22% in 2009, the prospect is extremely broad.
The VPN (Virtual Private Network) technology is not the real private network, but it can realizes the function of private network through the public network to offer a kind of reliable and safe data transmission network for enterprise's commercial operation. Taking three VPN software often used under Linux system such as PPTP VPN, SSL VPN, and IPSEC VPN as example, this article makes elaboration to each kind of VPNs’ principle of work, and summarizes in details its function how to configure in its server end and client end
In the end of the article, I carry on the comparison to several kinds of VPN, and then reach the conclusion, that is, IPSec VPN is suitable for large and middle-scale with more branches and higher maintenance cost, the IT construction, the management and the enterprises. Because their staff have the certain experience in the aspect of maintenance. But SSL VPN is suitable for the enterprises which need very strong flexibility and cost little on IT, there the staff need in different places to be allowed to easily visit interior resources of the company, and are able to carry on the visiting as necessary through each kind of mobile termination or the equipment. Relatively, as PPTP VPN can be configured simply and conveniently, it suits these small businesses with low safety requisition.
