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.
Types
Web
Proxies that 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 that 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, called "Proxy Avoidance."
Intercepting
Many organizations — including corporations, schools, and families — use a proxy server to enforce acceptable network use policies (see censorware) or to provide security, anti-malware and/or caching services. A traditional web proxy is not transparent to the client application, which must be configured to use the proxy (manually or with a configuration script). In some cases, where alternative means of connection to the Internet are available (e.g. a SOCKS server or NAT connection), the user may be able to avoid policy control by simply resetting the client configuration and bypassing the proxy. Furthermore administration of browser configuration can be a burden for network administrators.
An intercepting proxy, often incorrectly called transparent proxy (also known as a forced proxy) combines a proxy server with NAT. Connections made by client browsers through the NAT are intercepted and redirected to 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.
Intercepting proxies are also commonly used by Internet Service Providers in many countries in order to reduce upstream link bandwidth requirements by providing a shared cache to their customers.
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.
Some poorly implemented intercepting proxies have historically had certain downsides, e.g. an inability to use user authentication if the proxy does not recognize that the browser was not intending to talk to a proxy. Some problems are described in RFC 3143. A well-implemented proxy however should not inhibit browser authentication at all.
The term transparent proxy, often incorrectly used instead of intercepting proxy to describe the same behavior, is defined in RFC 2616 (Hypertext Transfer Protocol -- HTTP/1.1) as:
"[A] proxy that does not modify the request or response beyond what is required for proxy authentication and identification."
Transparent SMTP proxy
A transparent SMTP proxy is similar in concept and function to a web proxy, only for e-mail.
Open
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' operating systems 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 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 [2].
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.
Reverse
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 websites 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: if a program is producing the web page on the web servers, the web servers can produce it, serve it to the reverse-proxy, which can spoon-feed it however slowly the clients need and then close the program rather than having to keep it open while the clients insist on being spoon fed.
Split
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 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 .
See Google's Web Accelerator
Circumventor
A circumventor is a web-based page that takes a site that is blocked and "circumvents" it through to an unblocked website, 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.
The most common use is in schools where many blocking programs block by site rather than by code; students are able to access blocked sites (games, chatrooms, messenger, weapons, racism, forbidden knowledge, etc.) through a circumventor. As fast as the filtering software blocks circumventors, others spring up. It should be noted, however, that 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 website.
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.
Anonymous
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.
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.