In computer networks, a proxy server is a server (a computer system or an application program) that acts as an intermediary for requests from clients seeking resources from 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 evaluates the request according to its filtering rules. For example, it may filter traffic by IP address or protocol. If the request is validated by the filter, the proxy provides the resource by connecting to the relevant 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 'caches' responses from the remote server, and returns subsequent requests for the same content directly.
A proxy server has many potential purposes, including:
- To keep machines behind it anonymous (mainly for security).[1]
- To speed up access to resources (using caching). Web proxies are commonly used to cache web pages from a web server.[2]
- To apply access policy to network services or content, e.g. to block undesired sites.
- To log / audit usage, i.e. to provide company employee Internet usage reporting.
- To scan transmitted content before delivery for malware.
- To scan outbound content, e.g. for data leak protection.
A proxy server that passes 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 various points between the user and the destination servers on the Internet.
A reverse proxy is a (usually) Internet-facing proxy used as a front-end to control and protect access to a server on a private network, commonly also performing tasks such as load-balancing, authentication, decryption or caching.
Web proxy
A proxy that focuses on World Wide Web 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 often used in a corporate, educational or library environment, and anywhere else where content filtering is desired. Some web proxies reformat web pages for a specific purpose or audience (e.g., cell phones and PDAs).
AOL dialup customers used to have their requests routed through an extensible proxy that 'thinned' or reduced the detail in JPEG pictures. This sped up performance but caused problems, either when more resolution was needed or when the thinning program produced incorrect results. This is why in the early days of the web many web pages would contain a link saying "AOL Users Click Here" to bypass the web proxy and to avoid the bugs in the thinning software.
Ivaylo,
ReplyDeleteThis was a great post. I never thought about proxy servers so this great. I never would have thought of it as an option for performance improvement but there appear to be many opportunities to use it as such.
Ivaylo,
ReplyDeleteI didn't realize that AOL used a proxy to compress pictures flowing through their service. I do remember many users having issues with the "AOL Users Click Here" link. I think I may be dating myself.