Proxy server

The term "forced proxy" is ambiguous. It means both "intercepting proxy" (because it filters all traffic on the only available gateway to the Internet) and its exact opposite, "non-intercepting proxy" (because the user is forced to configure a proxy in order to access the Internet).
Because proxies might be used for abuse, system administrators have developed a number of ways to refuse service to open proxies. IRC networks such as the Blitzed network automatically test client systems for known types of open proxy [1]. Likewise, an email server may be configured to automatically test e-mail senders for open proxies, using software such as Michael Tokarev's proxycheck. Groups of IRC and electronic mail operators run DNSBLs publishing lists of the IP addresses of known open proxies, such as AHBL, CBL, NJABL, and SORBS.
The ethics of automatically testing clients for open proxies are controversial. Some experts, such as Vernon Schryver, consider such testing to be equivalent to an attacker portscanning the client host. [3] Others consider the client to have solicited the scan by connecting to a server whose terms of service include testing.
A split proxy is a proxy implemented as two programs installed on two different computers. Since they are effectively two parts of the same program, they can communicate with each other in a more efficient way than they can communicate with a more standard resource or tool such as a website or browser. This is ideal for compressing data over a slow link, such as a wireless or mobile data service and also for reducing the issues regarding high latency links (such as satellite internet) where establishing a TCP connection is time consuming. Taking the example of web browsing, the user's browser is pointed to a local proxy which then communicates with its other half at some remote location. This remote server fetches the requisite data, repackages it and sends it back to the user's local proxy, which unpacks the data and presents it to the browser in the standard fashion.
Some Web accelerators are proxy servers. Some reduce the quality of JPEG images to speed transmission. Some use a split proxy with special protocols and local and remote caching. (See Google Web Accelerator.)
A reverse proxy is a proxy server that is installed in the neighborhood of one or more web servers. All traffic coming from the Internet and with a destination of one of the web servers goes through the proxy server. There are several reasons for installing reverse proxy servers:
* Security: the proxy server is an additional layer of defense and therefore protects the web servers further up the chain
* Encryption / SSL acceleration: when secure web sites are created, the SSL encryption is often not done by the web server itself, but by a reverse proxy that is equipped with SSL acceleration hardware. See Secure Sockets Layer.
* Load balancing: the reverse proxy can distribute the load to several web servers, each web server serving its own application area. In such a case, the reverse proxy may need to rewrite the URLs in each web page (translation from externally known URLs to the internal locations)
* Serve/cache static content: A reverse proxy can offload the web servers by caching static content like pictures and other static graphical content
* Compression: the proxy server can optimize and compress the content to speed up the load time.
* Spoon feeding: reduces resource usage caused by slow clients on the web servers by caching the content the web server sent and slowly "spoon feeds" it to the client. This especially benefits dynamically generated pages.
* Extranet Publishing: a reverse proxy server facing the Internet can be used to communicate to a firewalled server internal to an organisation, providing extranet access to some functions while keeping the servers behind the firewalls.

See also: