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A proxy server is a computer that offers a computer network service to allow clients to make indirect network connections to other network services. A client connects to the proxy server, then requests a connection, file, or other resource available on a different server.
The proxy provides the resource either by connecting to the specified server or by serving it from a cache. In some cases, the proxy may alter the client's request or the server's response for various purposes.
A proxy server can also serve as a firewall.
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1 Web proxies

A common proxy application is a caching Web proxy.
This provides a nearby cache of Web pages and files available on remote Web servers,
allowing local network clients to access them more quickly or reliably.

When it receives a request for a Web resource (specified by a URL), a caching proxy looks for the resulting URL in its local cache. If found, it returns the document immediately. Otherwise it fetches it from the remote server, returns it to the requester and saves a copy in the cache. The cache usually uses an expiry algorithm to remove documents from the cache, according to their age, size, and access history. Two simple cache algorithms are Least Recently Used ( and Least Frequently Used ( LRU removes the least-recently used documents, and LFU removes the least-frequently used documents.

Web proxies can also filter the content of Web pages served. Some censorware applications which attempt to block offensive Web content are implemented as Web proxies. Other web proxies reformat web pages for a specific purpose or audience; for example, Skweezer reformats web pages for cell phones and PDAs. Network operators can also deploy proxies to intercept computer viruses and other hostile content served from remote Web pages.

A special case of web proxies are "CGI proxies." These are web sites which allow a user to access a site through them.
They generally use PHP or CGI to implement the proxying functionality.
CGI proxies are frequently used to gain access to web sites blocked by corporate or school proxies.
Since they also hide the user's own IP address from the web sites they access through the proxy,
they are sometimes also used to gain a degree of anonymity.

2 Proxy transparency

Many organizations including corporations, schools, and families use proxy servers to enforce network use policies (see censorware) or provide security and caching services.

Usually, the web proxy is not transparent to the client application: it must be configured to use the proxy, manually or with a configuration script. Thus, the user can evade the proxy by simply resetting the client configuration, except in the case where the proxy is used instead of a NAT router to share an internet conection or a LAN. Such proxies may be difficult to configure for applications requiring a large port range out going and may only be able to route inward to a single server for a given UDP or TCP port (see Wingate 2.x versions). However such proxies may have more extensive logging or more customizable security than a simple NAT router box.

A transparent proxy or transproxy (also known as a forced proxy) combines a proxy server with NAT so that connections are routed into the proxy without client-side configuration. However, RFC 3040 defines this type as intercepting proxy.

Both NAT and transproxies are somewhat controversial in the Internet technical community, since both violate the end-to-end principle upon which TCP/IP was designed.

The term proxy is also used in a different sense in the Session Initiation Protocol (SIP) used in many modern voice over IP systems. A SIP Proxy, unlike a Web proxy, does not handle the content of client data


3 Open proxies, abuse, and detection

An open proxy is a proxy server which will accept client connections from any IP address and make connections to any Internet resource. Abuse of open proxies is currently implicated in a significant portion of e-mail spam delivery. Spammers frequently install open proxies on unwitting end users' Microsoft Windows computers by means of computer viruses designed for this purpose. Internet Relay Chat (IRC) abusers also frequently use open proxies to cloak their identities.

Because proxies could be implicated in 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 [4]. Likewise, an email server may be configured to automatically test e-mail senders for open proxies, using software such as Michael Tokarev's proxycheck [5].

Groups of IRC and electronic mail operators run DNSBLs publishing lists of the IP addresses of known open proxies, such as Blitzed OPM [6] and CBL [7].

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. [8] Others consider the client to have solicited the scan by connecting to a server whose terms of service include testing.


4 Reverse proxies

A reverse proxy is a proxy server that is installed in the neighborhood of one or more webservers. All traffic coming from the Internet and with a destination of one of the webservers is going 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 webservers further up the chain
  • Encryption / SSL acceleration: when secure websites are created, the SSL encryption is often not done by the webserver itself, but by a reverse proxy that is equipped with SSL acceleration hardware. See Secure Sockets Layer.
  • Load distribution: the reverse proxy can distribute the load to several webservers, each webserver serving its own application area. In such a case, the reverse proxy may need to rewrite the URLs in each webpage (translation from externally known URLs to the internal locations)
  • Serve/cache static content: A reverse proxy can offload the webservers by caching static content like pictures and other static graphical content (See Squid cache)
  • Compression: the proxy server can optimize and compress the content to speed up the load time.

5 Split proxies

A split proxy is essentially a pair of proxies installed across two 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 estabilishing 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.

Google's Web Accelerator is an example of a split proxy.


6 Anonymous proxy risks

In using a proxy server (for example, anonymizing HTTP proxy), all data sent to the service being used (for example, HTTP server in a website) must pass through the proxy server before being sent to the service, mostly in unencrypted form. It is therefore possible, and has been demonstrated (see, for example, Sugarcane) for a malicious proxy server to record everything sent to the proxy: including unencrypted logins and passwords.

By chaining proxies which do not reveal data about the original requestor, it is possible to obfuscate activities from the eyes of the user's destination. However, more traces will be left on the intermediate hops, which could be used or offered up to trace the user's activities. If the policies and administrators of these other proxies are unknown, the user may fall victim to a false sense of security just because those details are out of sight and mind.

The bottom line of this is to be wary when using proxy servers, and only use proxy servers of known integrity (e.g., the owner is known and trusted, has a clear privacy policy, etc.), and never use proxy servers of unknown integrity. If there is no choice but to use unknown proxy servers, do not pass any private information (unless it is properly encrypted) through the proxy.

29.04.2006 All rights reserved. Dedicated to my girlifriend- Monica.
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