HTTP代理服务器的作用 Web安全，应用集成，性能优化
1. 代理
In computer networks, a proxy server is a server (a computer system or an application) that acts as an intermediary for requests from clients seeking resources from other servers
A proxy server may reside on the user's local computer, or at various points between the user's computer and destination servers on the Internet.
- A proxy server that passes requests and responses unmodified is usually called a gateway or sometimes a tunneling proxy.
- A forward proxy is an Internet-facing proxy used to retrieve from a wide range of sources (in most cases anywhere on the Internet).
- A reverse proxy is usually an internal-facing proxy used as a front-end to control and protect access to a server on a private network. A reverse proxy commonly also performs tasks such as load-balancing, authentication, decryption or caching.
2. 缓存（Web cache /proxy cache）
保存远端资源的副本
3. 网关(gateway)
提供协议转换功能
4. 隧道
对数据进行忙转发，用于SSL
5. Agent代理
以上内容仅用于学习记录之用
参考资料：
[1] https://en.wikipedia.org/wiki/Proxy_server (proxy_server)
[2] https://en.wikipedia.org/wiki/Gateway_(telecommunications) (gateway)
[3] HTTP权威指南 David Gourley
[4] https://www.w3.org/Protocols/