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 computer network is an interconnection of a group of computers. ... In information technology, a server is an application or device that performs services for connected clients as part of a client-server architecture. ... In computing, a client is a system that accesses a (remote) service on another computer by some kind of network. ...

A proxy server that passes all requests and replies unmodified is usually called a gateway or sometimes tunneling proxy. It has been suggested that this article or section be merged into Gateway (telecommunications). ...

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) This article is about the computer term. ...

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, NetCache) 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). Web caching is the caching of web documents (e. ...

Content Filtering Web Proxy

Further information: 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. DansGuardian blocking whitehouse. ... This article or section does not cite its references or sources. ... An acceptable use policy (AUP; also sometimes acceptable usage policy) is a set of rules applied by many transit networks which restrict the ways in which the network may be used. ...

Common methods used for content filtering include: URL or 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 provider. Blacklisted redirects here. ... A DNS Blacklist, or DNSBL (definition below), is a means by which an Internet site may publish a list of IP addresses that some people may want to avoid and in a format which can be easily queried by computer programs on the Internet. ... A Uniform Resource Locator, URL (spelled out as an acronym, not pronounced as earl), or Web address, is a standardized address name layout for resources (such as documents or images) on the Internet (or elsewhere). ... For mime as an art form, see mime artist. ...

A content filtering proxy will often support user authentication, to control web access. It also usually produces logs, either to give detailed information about the URLs accessed by specific users, or to monitor bandwidth usage statistics. It may also communicate to daemon based and/or ICAP based antivirus software to provide security against virus and other malware by scanning incoming content in real time before it enters the network. For other uses of the terms authentication, authentic and authenticity, see authenticity. ... A server log is a file (or several files) automatically created and maintained by a server of activity performed by it. ... In Unix and other computer multitasking operating systems, a daemon is a computer program that runs in the background, rather than under the direct control of a user; they are usually instantiated as processes. ... See ICAP/4 for the computer program. ... Antivirus redirects here. ... A screenshot of a malicious website attempting to install spyware via an ActiveX Control in Internet Explorer 6 Malware is software designed to infiltrate or damage a computer system without the owners informed consent. ...

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 anonymizers. This page may meet Wikipedia’s criteria for speedy deletion. ...

Access control: Some proxy servers implement a logon requirement. In large organizations, authorized users must log on to gain access to the web. The organization can thereby track usage to individuals. The World Wide Web and WWW redirect here. ...

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. Eavesdropping is the intercepting and reading of messages and conversations by unintended recipients. ...

Intercepting proxy server

An intercepting proxy (also known as a "transparent proxy") combines a proxy server with a gateway. Connections made by client browsers through the gateway are redirected through the proxy without client-side configuration (or often knowledge). It has been suggested that this article or section be merged into Gateway (telecommunications). ...

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 article: 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 email server may be configured to automatically test e-mail senders for open proxies. An open proxy is a proxy server which is accessible by any Internet user. ... Internet Relay Chat (IRC) is a form of instant communication over the Internet. ... A mail transfer agent or MTA (also called a mail server, or a mail exchange server in the context of the Domain Name System) is a computer program or software agent which transfers electronic mail messages from one computer to another. ... Electronic mail, abbreviated e-mail or email, is a method of composing, sending, and receiving messages over electronic communication systems. ...

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. A DNS Blacklist, or DNSBL (definition below), is a means by which an Internet site may publish a list of IP addresses that some people may want to avoid and in a format which can be easily queried by computer programs on the Internet. ... An IP address (or Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ... The Abusive Hosts Blocking List is an abuse tracking and filtering system developed by The Summit Open Source Development Group, and based on the original Summit Blocking List (2000-2002). ... CBL is Composite Blocking List. ... NJABL is Not Just Another Bogus List. ... The Sorbs are a Slavic minority indigenous to the region known as Lusatia in the current German states of Saxony and Brandenburg (in former GDR territory). ...

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. [1] Others consider the client to have solicited the scan by connecting to a server whose terms of service include testing. A port scanner is a piece of software designed to search a network host for open ports. ...

Reverse proxy server

Main article: 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: A reverse proxy is a proxy server that is installed within the neighborhood of one or more 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: 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.

Secure Sockets Layer (SSL) and Transport Layer Security (TLS), its successor, are cryptographic protocols which provide secure communications on the Internet. ... In computer networking, load balancing is a technique (usually performed by load balancers) to spread work between many computers, processes, hard disks or other resources in order to get optimal resource utilization and decrease computing time. ...

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. This article is about the corporation. ... elgooG Logo elgooG is a mirror image (a flipped around image version) of the Google search engine. ...

Students are able to access blocked sites (games, chatrooms, 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. A chat room is an online forum where people can chat online (talk by broadcasting messages to people on the same forum in real time). ... Internet pornography is pornography that is distributed via the Internet, primarily via websites, peer-to-peer file sharing, or Usenet newsgroups. ...

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 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". For other uses, see PHP (disambiguation). ... The Common Gateway Interface (CGI) is a standard protocol for interfacing external application software with an information server, commonly a web server. ... An IP address (or Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ... Anonymous redirects here. ...

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, and has been demonstrated, for a malicious proxy server to record everything sent to the proxy: including unencrypted logins and passwords. However, if you use an HTTPS proxy such as LinkItTo your information will be encrypted thus making it harder for thieves to steal your info. HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ...

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 spammed or trolled the site. This article is about electronic spam. ... An Internet troll, or simply troll in Internet slang, is someone who posts controversial and usually irrelevant or off-topic messages in an online community, such as an online discussion forum, with the intention of baiting other users into an emotional response[1] or to generally disrupt normal on-topic...

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'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 URL-rewriting web proxy designed primarily for providing remote access to sites that authenticate users by IP address.
• JAP - A local proxy, web anonymizer software connecting to proxy server chains of different organisations
• Microsoft Internet Security and Acceleration Server is a product that runs on Windows 2000/2003 servers and combines the functions of both a proxy server and a firewall.
• 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.
• 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 cache is a popular HTTP proxy server in the UNIX/Linux world.
• Tor - A proxy-based anonymizing Internet communication system.
• Varnish is designed to be a high-performance caching reverse proxy.
• 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.

AlchemyPoint is a Firefox, and Flock extension and mashup platform that allows users to create web mashups, perform screen scraping, and engage in semantic web browsing. ... The Apache HTTP Server, commonly referred to simply as Apache, is a web server notable for playing a key role in the initial growth of the World Wide Web. ... Blue Coat Systems NASDAQ: BCSI is a maker of proxy server appliances used for WAN optimization, content filtering, remote access and web cache. ... Blue Coat Systems NASDAQ: BCSI is a maker of proxy server appliances used for WAN optimization, content filtering, remote access and web cache. ... Blue Coat Systems NASDAQ: BCSI is a maker of Proxy Server appliances used for WAN Optimization, Content Filtering, and Web cache. ... A rewrite engine is a piece of web server software used to modify URLs before fetching the requested item, for a variety of purposes. ... An IP address (or Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ... Java Anon Proxy, also known as Java Anonymous Proxy or JAP, is a proxy system designed to allow browsing the Web anonymously. ... Microsoft Internet Security and Acceleration Server (ISA Server) is a stateful packet and application layer inspection firewall, virtual private network and web cache (both forward caching and reverse caching) server which runs on Windows 2000 and Windows Server 2003. ... Windows 2000 (also referred to as Win2K) is a preemptive, interruptible, graphical and business-oriented operating system designed to work with either uniprocessor or symmetric multi-processor computers. ... Windows Server 2003 is a server operating system produced by Microsoft. ... This article is about the network security device. ... nginx (pronounced engine X) is a lightweight web server/reverse proxy and mail (IMAP/POP3) proxy. ... Post Office Protocol version 3 (POP3) is an application layer Internet standard protocol used to retrieve email from a remote server to a local client over a TCP/IP connection. ... Privoxy is a web proxy program, frequently used in combination with Tor and Squid. ... Open source refers to projects that are open to the public and which draw on other projects that are freely available to the general public. ... Proxomitron, the Universal Web filter was created by Scott R. Lemmon and is a configurable personal web proxy tool. ... SSH redirects here. ... Look up Socks in Wiktionary, the free dictionary. ... Sun Java System Web Proxy Server is a freely available HTTP 1. ... A reverse proxy is a proxy server that is installed within the neighborhood of one or more servers. ... Squid is a proxy server and web cache daemon. ... Tor (The Onion Router) is a free software implementation of second-generation onion routing – a system enabling its users to communicate anonymously on the Internet. ... Varnish is a high performance HTTP accelerator designed for content-heavy dynamic web sites. ... WinGate is an integrated Internet Gateway Management system. ... Windows redirects here. ... WWWOFFLE is a proxy server and web caching software, allowing dial-up or broadband users to cache data for offline use. ... yProxy is a non-transparent NNTP proxy server for the Windows operating system. ... Ziproxy is a forwarding, non-caching, http proxy targetted for traffic optimization. ...

References

See also

• Captive portal
• Internet privacy
• Proxy list
• Proxyjudge
• SOCKS
• Transparent SMTP proxy
• Web cache
• HTTP

The captive portal technique forces an HTTP client on a network to see a special web page (usually for authentication purposes) before surfing the Internet normally. ... Internet privacy consists of privacy over the media of the Internet: the ability to control what information one reveals about oneself over the Internet, and to control who can access that information. ... A proxy list is a list of websites classified as proxies all on one website. ... Look up Socks in Wiktionary, the free dictionary. ... This article or section does not cite its references or sources. ... Web caching is the caching of web documents (e. ... HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ...

External links

• Overview of how proxies and NTLM works, and a guide to configure NTLM-ASP for proxy bypassing
• Proxy software and scripts at the Open Directory Project
• Free web-based proxy services at the Open Directory Project
• Free http proxy servers at the Open Directory Project