Proxy server 78768 225602469 2008-07-14T14:37:13Z BodhisattvaBot 6228552 robot Modifying: [[fr:Proxy ouvert]] {{Unreferenced|date=December 2006}} {{Expert-subject|Technology}} [[Image:Schematic Proxy Server.png|thumb|350px|right|[[Schematic]] representation of a proxy server, where the computer in the middle acts as the proxy server between the other two.]] In [[computer network]]s, a '''proxy server''' is a [[server (computing)|server]] (a computer system or an application program) which services the requests of its [[client (computing)|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 (computer networking)|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 proxy server can service requests without contacting the specified server, by retrieving content saved from a previous request, made by the same client or even other clients. This is called [[caching]]. 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. There are well-defined rules for caching. 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 (software)|Squid]], [[NetCache]]) provide a means to deny access to certain URLs in a [[blacklist (computing)|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). === Content Filtering Web Proxy === {{see|Content-control software}} A [[content filtering]] web proxy server provides administrative control over the content that may be relayed through the proxy. It is commonly used in commercial and non-commercial organizations (especially schools) to ensure that Internet usage conforms to [[acceptable use policy]]. Common methods used for content filtering include: [[Blacklist#Computing|URL]] or [[DNSBL|DNS blacklists]], [[URL]] regex filtering, [[MIME]] filtering, or content keyword filtering. Some products have been known to employ content analysis techniques to look for traits commonly used by certain types of content providers. A content filtering proxy will often support [[Authentication#Computer security|user authentication]], to control web access. It also usually produces [[Server log|logs]], either to give detailed information about the URLs accessed by specific users, or to monitor [[Bandwidth (computers)|bandwidth]] usage statistics. It may also communicate to [[Daemon (computer software)|daemon]] based and/or [[Internet Content Adaptation Protocol|ICAP]] based [[antivirus software]] to provide security against virus and other [[malware]] by scanning incoming content in real time before it enters the network. ===Anonymizing proxy server=== An anonymous proxy server (sometimes called a web proxy) generally attempts to anonymize web surfing. These can easily be overridden by site administrators, and thus rendered useless in some cases. There are different varieties of [[anonymizer]]s. '''Access control''': Some proxy servers implement a logon requirement. In large organizations, authorized users must log on to gain access to the [[World Wide Web|web]]. The organization can thereby track usage to individuals. ===Hostile proxy=== Proxies can also be installed by online criminals, in order to [[eavesdrop]] upon the dataflow between the client machine and the web. All accessed pages, as well as all forms submitted, can be captured and analyzed by the proxy operator. For this reason, passwords to online services (such as webmail and banking) should be changed if an unauthorized proxy is detected. ===Intercepting proxy server=== An '''intercepting proxy''' (also known as a "transparent proxy") combines a proxy server with a [[Gateway (computer networking)|gateway]]. Connections made by client browsers through the gateway are redirected through the proxy without client-side configuration (or often knowledge). Intercepting proxies are commonly used in businesses to prevent avoidance of acceptable use policy, and to ease administrative burden, since no client browser configuration is required. It is often possible to detect the use of an intercepting proxy server by comparing the external IP address to the address seen by an external web server, or by examining the HTTP headers on the server side. ===Transparent and non-transparent proxy server=== The term "transparent proxy" is most often used incorrectly to mean "intercepting proxy" (because the client does not need to configure a proxy and cannot directly detect that its requests are being proxied). However, RFC 2616 (Hypertext Transfer Protocol -- HTTP/1.1) offers different definitions: : "A 'transparent proxy' is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification". : "A 'non-transparent proxy' is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering". ===Forced proxy=== 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). Forced proxy operation is sometimes necessary due to issues with the interception of TCP connections and HTTP. For instance interception of HTTP requests can affect the usability of a proxy cache, and can greatly affect certain authentication mechanisms. This is primarily because the client thinks it is talking to a server, and so request headers required by a proxy are unable to be distinguished from headers that may be required by an upstream server (esp authorization headers). Also the HTTP specification prohibits caching of responses where the request contained an authorization header. ===Open proxy server=== {{main|open proxy}} Because proxies might be used for abuse, system administrators have developed a number of ways to refuse service to open proxies. many [[IRC]] networks automatically test client systems for known types of open proxy. Likewise, an [[mail transport agent|email server]] may be configured to automatically test [[electronic mail|e-mail]] senders for open proxies. Groups of IRC and electronic mail operators run [[DNSBL]]s publishing lists of the [[IP address]]es of known open proxies, such as [[AHBL]], [[Composite Blocking List|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. [http://groups.google.com/groups?selm=brd5b4%242kv3%241%40calcite.rhyolite.com&output=gplain] Others consider the client to have solicited the scan by connecting to a server whose terms of service include testing. ===Reverse proxy server=== {{main|reverse proxy}} 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: * 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 (computing)|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. * Security: the proxy server is an additional layer of defense and can protect against some OS and WebServer specific attacks. However, it does not provide any protection to attacks against the web application or service itself, which is generally considered the larger threat. * Extranet Publishing: a reverse proxy server facing the Internet can be used to communicate to a firewalled server internal to an organization, providing extranet access to some functions while keeping the servers behind the firewalls. If used in this way, security measures should be considered to protect the rest of your infrastructure in case this server is compromised, as it's web application is exposed to attack from the Internet. ===Circumventor=== A '''circumventor''' is a method of defeating blocking policies implemented using proxy servers. Ironically, most circumventors are also proxy servers, of varying degrees of sophistication, which effectively implement "bypass policies". A circumventor is a web-based page that takes a site that is blocked and "circumvents" it through to an unblocked web site, allowing the user to view blocked pages. A famous example is ''''''elgooG'''''', which allowed users in [[China]] to use [[Google]] after it had been blocked there. [[elgooG]] differs from most circumventors in that it circumvents only one block. Students are able to access blocked sites (games, [[chatroom]]s, messenger, offensive material, [[internet pornography]], social networking, etc.) through a circumventor. As fast as the filtering software blocks circumventors, others spring up. However, in some cases the filter may still intercept traffic to the circumventor, thus the person who manages the filter can still see the sites that are being visited. Circumventors are also used by people who have been blocked from a web site. Another use of a circumventor is to allow access to country-specific services, so that Internet users from other countries may also make use of them. An example is country-restricted reproduction of media and webcasting. The use of circumventors is usually safe with the exception that circumventor sites run by an untrusted third party can be run with hidden intentions, such as collecting personal information, and as a result users are typically advised against running personal data such as credit card numbers or passwords through a circumventor. ===At schools and offices=== Many work places and schools restrict the web sites and online services that are made available in their buildings. Since circumventors are used to bypass censors in computers, social networking and other sites deemed a waste of time or resources have become targets of mass banning. Proxy Web server creators have become more sophisticated, allowing users to encrypt links and any data going to and from other web servers. This allows users to access websites that would otherwise have been blocked. A special case of web proxies are "CGI proxies". These are web sites that allow a user to access a site through them. They generally use [[PHP]] or [[Common Gateway Interface|CGI]] to implement the proxy functionality. These types of 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]], called "Proxy Avoidance". ==Risks of using anonymous proxy servers== 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, as has been demonstrated, 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 requester, 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. In what is more of an inconvenience than a risk, proxy users may find themselves being blocked from certain Web sites, as numerous forums and Web sites block IP addresses from proxies known to have [[Spam (electronic)|spammed]] or [[Troll (Internet)|trolled]] the site. ==Proxy software== * [[AlchemyPoint]] is a user-programmable mashup proxy server that can be used to re-write web pages, emails, instant messenger messages, and other network transmissions on the fly. * The [[Apache HTTP Server]] can be configured to act as a proxy server. * [[Blue Coat Systems|Blue Coat]]'s (formerly [[Cacheflow]]'s) purpose-built [[SGOS]] proxies 15 protocols including HTTPS/SSL, has an extensive policy engine and runs on a range of appliances from branch-office to enterprise. * [[EZproxy]] is a [[rewrite engine|URL-rewriting]] web proxy designed primarily for providing remote access to sites that authenticate users by [[IP address]]. * [[Java Anon Proxy|JAP]] - A local proxy, web anonymizer software connecting to proxy server chains of different organisations * [[Novell BorderManager]] web proxy server, reverse proxy, [[Firewall]] and [[VPN]] end point. * [[Nginx]] Web and Reverse proxy server, that can act as [[POP3]] proxy server. * [[Privoxy]] is a free, [[open source]] web proxy with privacy and ad-blocking features. * [[Proxomitron]] - User-configurable web proxy used to re-write web pages on the fly. Most noted for blocking ads, but has many other useful features. * [[SafeSquid]] Linux based, complete content filtering HTTP1.1 proxy, allows distribution of 'profiled' internet access. * [[Secure Shell|SSH]] Secure Shell can be configured to proxify a connection, by setting up a [[SOCKS]] proxy on the client, and tunneling the traffic through the SSH connection. * [[Sun Java System Web Proxy Server]] is a caching proxy server running on Solaris, Linux and Windows servers that supports HTTP/S, NSAPI I/O filters, dynamic reconfiguration, SOCKSv5 and [[reverse proxy]]. * [[Squid (software)|Squid]] is a popular HTTP proxy server in the UNIX/Linux world. * [[tinyproxy]] is a fast light-weight HTTP proxy ideal for embedded use on POSIX operating systems. * [[Tor (anonymity network)|Tor]] - A proxy-based anonymizing Internet communication system. * [[Varnish cache|Varnish]] is designed to be a high-performance caching reverse proxy. * [[WinGate (computing)|WinGate]] is a multi-protocol proxy server and NAT solution that can be used to redirect any kind of traffic on a [[Microsoft Windows]] host. * [[WWWOFFLE]] has been around since the mid-1990s, and was developed for storing online data for offline use. * [[yProxy]] is an NNTP proxy server that converts yEnc encoded message attachments to UUEncoding, complete with SSL client support. * [[Ziproxy]] is a non-caching proxy for acceleration purposes. It recompresses pictures and optimizes HTML code. ==See also== *[[Captive portal]] *[[Internet privacy]] *[[Proxy list]] *[[SOCKS]] *[[Transparent SMTP proxy]] *[[Web cache]] *[[HTTP]] ==External links== <!--===========================({{NoMoreLinks}})=============================== | DO NOT ADD MORE LINKS TO THIS ARTICLE. WIKIPEDIA IS NOT A COLLECTION OF | | LINKS. 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