A program which forwards HTTP, HTTPS and FTP requests, often but not necessarily caching them. A caching proxy server serves requests from the Cache where possible, to reduce latency and bandwidth.
Proxies are also frequently used in corporate networks, where FireWalls often block access to the outside world, and only proxies are allowed to communicate with it. With a TransparentProxy, this can happen without client configuration -- a sometimes problematic scheme also employed by ISPs to reduce the amount of traffic crossing their external links.
Like WebServers, proxies often keep access logs. These logs are useful for debugging, optimisation and maintenance tasks but can also be used to show who is doing what, when and how much it's costing.
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