A proxy server is a computer network service which allows 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, possibly 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 computer network is a system for communication among two or more computers. ... In computer science, a cache (pronounced kăsh) is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data are expensive (usually in terms of access time) to fetch or compute relative to reading the cache. ...

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. Web caching is the caching of web resources (HTML pages, images, etc. ... Graphic representation of the world wide web around Wikipedia The World Wide Web (WWW, or simply Web) is an information space in which the items of interest, referred to as resources, are identified by global identifiers called Uniform Resource Identifiers (URI). ... A webpage or Web page is a page or file of the World Wide Web, usually in HTML/XHTML format (the file extensions are typically htm or html) and with hypertext links to enable navigation from one page or section to another. ... A file in a computer system is a stream (sequence) of bits stored as a single unit, typically in a file system on disk or magnetic tape. ... The term web server can mean one of two things: a computer responsible for serving web pages, mostly HTML documents, via the HTTP protocol to clients, mostly web browsers; a software program that is working as a daemon serving web documents. ... A local area network (LAN) is a computer network covering a local area, like a home, office or small group of buildings such as a college. ... In computing, a client is a system that accesses a (remote) service on another computer by some kind of network. ...

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 (LRU) and Least Frequently Used (LFU). LRU removes the least-recently used documents, and LFU removes the least-frequently used documents. A Uniform Resource Locator, URL (either pronounced as earl (IPA: [ɜː˞l]; SAMPA: [3:`l]) or spelled out), or Web address, is a standardized address for some resource (such as a document or image) on the Internet (or elsewhere). ... Flowcharts are often used to represent algorithms. ... Cache algorithms are optimizing instructions that a computer program can follow to manage a cache of information stored on the computer. ...

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. Censorware is a term used to describe content filtering software by its opponents. ... Application software is a subclass of computer software that employs the capabilities of a computer directly to a task that the user wishes to perform. ... Skweezer is a free Web portal for users of mobile devices such as personal digital assistants, Palm Pilots, Pocket PCs, Smartphones, and XHTML-enabled mobile phones. ... A stylised representation of a mobile phone A mobile phone is a device which behaves as a normal telephone whilst being able to move over a wide area (compare cordless phone which acts as a telephone only within a limited range). ... Palm IIIxe PDA Personal digital assistants (PDAs or palmtops) are handheld devices that were originally designed as personal organizers, but became much more versatile over the years. ... In computer security technology, a virus is a self-replicating program that spreads by inserting copies of itself into other executable code or documents (for a complete definition: see below). ...

Network address translation proxies

UDP datagram exchanges. NAT is also known as IP masquerading. The User Datagram Protocol (UDP) is a minimal message-oriented transport layer protocol that is currently documented in IETF RFC 768. ... In computer networking, network address translation (NAT, also known as network masquerading or IP-masquerading) is a technique in which the source and/or destination addresses of IP packets are rewritten as they pass through a router or firewall. ...

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 or NAT 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. A transparent proxy or transproxy combines a proxy server with NAT so that connections are routed into the proxy without client-side configuration. A corporation is a legal entity (distinct from a natural person) that often has similar rights in law to those of a Civil law systems may refer to corporations as moral persons; they may also go by the name AS (anonymous society) or something similar, depending on language (see below). ... A school is most commonly a place designated for learning. ... This article is about the domestic group. ... Censorware is a term used to describe content filtering software by its opponents. ...

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 end-to-end principle is one of the central design principles of the Internet Protocol (IP) that is the basis of the Internet. ... The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet runs. ...

The term proxy is also used in a different sense in SIP, the Session Initiation Protocol used in many modern voice over IP systems. A SIP Proxy, unlike a Web proxy, does not handle the content of client data. The three letter abbreviation SIP can refer to the following: In computing, the IETF standard Session Initiation Protocol. ... Voice over IP (also called VoIP, IP Telephony, and Internet telephony) is technology enabling routing of voice conversations over the Internet or any other IP network. ...

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. An IP address (Internet Protocol address) is a unique number, a kind of telephone number, used by machines (usually computers) to refer to each other when sending information through the Internet. ... An open proxy is an Internet proxy server which is accessible by unauthorized users, specifically those from elsewhere on the internet. ... A typical spam advertisement Spam by e-mail is a type of spam that involves sending identical or nearly identical messages to thousands (or millions) of recipients. ... Microsoft Windows is a range of closed source proprietary commercial operating environments for personal computers and servers. ... In computer security technology, a virus is a self-replicating program that spreads by inserting copies of itself into other executable code or documents (for a complete definition: see below). ... Internet Relay Chat (IRC) is a form of instant communication over the Internet. ...

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 [1] (http://www.blitzed.org/bopm/). Likewise, an email server may be configured to automatically test e-mail senders for open proxies, using software such as Michael Tokarev's proxycheck [2] (http://www.corpit.ru/mjt/proxycheck.html). 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 Blitzed OPM [3] (http://opm.blitzed.org/info) and CBL [4] (http://cbl.abuseat.org/). A DNS-based Blackhole List, or DNSBL, is a means by which an Internet site may publish a list of IP addresses, in a format which can be easily queried by computer programs on the Internet. ... An IP address (Internet Protocol address) is a unique number, a kind of telephone number, used by machines (usually computers) to refer to each other when sending information through the Internet. ...

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. [5] (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 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 destination 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 have 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

Secure Sockets Layer (SSL) and Transport Layer Security (TLS), its successor, are cryptographic protocols which provide secure communications on the Internet. ...

Popular proxy servers

• The Squid cache is the popular HTTP proxy server in UNIX/Linux world
• The Apache HTTP Server can be configured to act as a proxy server.
• Wingate is a small Internet Protocol proxy server that can be used to redirect any kind traffic on a Microsoft Windows host. A more free proxy server is AnalogX (http://www.analogx.com/)'s proxy (http://www.analogx.com/contents/download/network/proxy.htm)
• Windows 98se and other versions of Windows released later have Internet Connection Sharing (ICS) built-in. This gives NAT functionality to any Windows computer with an Internet connection and a LAN connection.
• The Linux kernel contains modules for NAT functionality as part of the ipchains and iptables packet filtering systems.
• OpenBSD's Packet Filter (http://www.openbsd.org/faq/pf/) is an extremely flexible interface for inspecting network traffic that can provide, among many other things, NAT functionality.
• The elhttp server (http://www.cr0.net:8040/code/network/) is a lightweight HTTP proxy for UNIX and Windows (GPL license)
• Microsoft's ISA Server (http://www.microsoft.com/isaserver) is a product that runs on Windows 2000/2003 servers and combines the functions of both a proxy server and a firewall.

See also

• Anonymity and Internet privacy
• SOCKS
• Web Proxy Autodiscovery Protocol WPAD
• Skweezer
• Firewall (networking)

Anonymity is the state of not being identifiable within a set, called the anonymity set. When referring to human beings, we say that a person is anonymous when the identity of that person is not known. ... The Internet is a series of connected networks and computers that communicate through the Transmission Control Protocol/Internet Protocol, TCP/IP, to provide information and transfer data. ... For the garment, see sock. ... Web Proxy Autodiscovery Protocol (WPAD) is a method of storing proxy autoconfiguration file in a guessable url, for example wpad. ... Skweezer is a free Web portal for users of mobile devices such as personal digital assistants, Palm Pilots, Pocket PCs, Smartphones, and XHTML-enabled mobile phones. ... In computing, a firewall is a piece of hardware or software which functions in a networked environment to prevent some communications forbidden by the security policy, analogous to the function of firewalls in building construction. ...

External links

• Proxy (http://www.windowsnetworking.com/articles_tutorials/proxy.html) - An article on how proxies work

Web-based proxy servers & Proxy list

• ProxyBlind Free Proxy servers (http://www.proxyblind.org/)
• Free Proxy Server (http://uncensor.info)
• Anonymous Browsing Quick-Start Page (http://www.space.net.au/~thomas/quickbrowse.html)
• Free proxies (http://www.digitalcybersoft.com/ProxyList/)
• http://proxify.com/
• http://www.anonymizer.com/
• http://www.the-cloak.com
• http://nonymouse.com/
• http://www.guardster.com/
• http://www.proxyone.com/
• http://www.proxyweb.net/
• http://webwarper.net/
• http://www.snoopblocker.com/
• http://pxy01.pink-banana.com/cgi-bin/redirect.cgi
• http://uncensor.net/
• http://www.megaproxy.com/