In computer networks, a proxy server is a server (a computer system or an application program) which services the requests of its clients by forwarding requests to other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a different server. The proxy server provides the resource by connecting to the specified server and requesting the service on behalf of the client. A proxy server may optionally alter the client's request or the server's response, and sometimes it may serve the request without contacting the specified server. In this case, it would 'cache' the first request to the remote server, so it could save the information for later, and make everything as fast as possible.

A proxy server that passes all requests and replies unmodified is usually called a gateway or sometimes tunneling proxy.

A proxy server can be placed in the user's local computer or at specific key points between the user and the destination servers or the Internet.

Types and functions

Proxy servers implement one or more of the following functions:

Caching proxy server

A caching proxy server is the original proxy and can accelerate service requests by retrieving content saved from a previous request, made by the same client or even other clients. Caching proxies keep local copies of frequently requested resources, allowing large organizations to significantly reduce their upstream bandwidth usage and cost, while significantly increasing performance. Most ISP's and large businesses have a caching proxy. These machines are built to deliver superb file system performance (often with RAID and journaling) and also contain hot-rodded versions of TCP.

The HTTP 1.0 and later protocols contain many types of headers for declaring static (cacheable) content and verifying content freshness with an original server, e.g. ETAG (validation tags), If-Modified-Since (date-based validation), Expiry (timeout-based invalidation), etc. Other protocols such as DNS support expiry only and contain no support for validation.

Some poorly-implemented caching proxies have had downsides (e.g., an inability to use user authentication). Some problems are described in RFC 3143 (Known HTTP Proxy/Caching Problems).

Web proxy

A proxy that focuses on WWW traffic is called a "web proxy". The most common use of a web proxy is to serve as a web cache. Most proxy programs (e.g. Squid) provide a means to deny access to certain URLs in a blacklist, thus providing content filtering. This is usually used in a corporate environment, though with the increasing use of Linux in small businesses and homes, this function is no longer confined to large corporations. Some web proxies reformat web pages for a specific purpose or audience (e.g., cell phones and PDAs).