5.6. Port Address Translation (PAT) from Userspace

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本文介绍了Port Address Translation (PAT)的概念及其工作原理。PAT是一种特定工具，用于将针对特定IP及端口组合的请求转发到另一个IP端口组合。通常被用于公共连接主机上，以便使内部服务对更大的网络可用。文章还探讨了当存在替代路由时PAT可能出现的问题，并讨论了PAT相对于NAT的一个主要优势：即在有限的公网IP地址资源下如何更有效地运行多个标准端口服务。

5.6. Port Address Translation (PAT) from Userspace

Port address translation (hereafter PAT) provides a similar functionality to NAT, but is a more specific tool. PAT forwards requests for a particular IP and port pair to another IP port pair. This feature is commonly used on publicly connected hosts to make an internal service available to a larger network.

PAT will break in strange and wonderful ways if there is an alternate route between the two hosts connected by the port address translation.

PAT has one important benefit over NAT (with the iproute2 tools). Let's assume that you have only five public IP addresses for which you have paid dearly. Additionally, let's assume that you want to run services on standard ports. You had hoped to connect four SMTP servers, two SSH servers and five HTTP servers. If you had wanted to accomplish this with NAT, you'd need more IP space.