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

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).

Content Filtering Web Proxy

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.

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.

Hostile proxy

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).

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

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.

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

Split proxy server

A split proxy is a proxy implemented as two programs installed on two different 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, as well as 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 then unpacks the data and presents it to the browser in the standard fashion.

Some Web accelerators are proxy servers. Some reduce the quality of JPEG images to speed transmission. Some use a split proxy with special protocols and local and remote caching. (See Google Web Accelerator.)

Reverse proxy server

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 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.
  
• 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.
• 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.

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, chatrooms, messenger, offensive material, internet pornography, social networking, 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 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 are cracking down on the web sites and online services that are made available in their buildings. Since circumventors are used to bypass censors in computers, web sites like Orkut, MySpace, Bebo, Xanga, Silkroad Online, YouTube, Miniclip, Facebook, Gaiaonline and other non-work or school related social web sites have become targets of mass banning.

Proxy Web server creators have become more clever 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 proxying 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".

Managed 'clean-pipe' proxy servers

Used in an increasing number of work-places, especially those with multiple Internet breakout points. Currently an emerging technology to rival in-house, hardware solutions. Many consider this a branch of Software as a Service or Security as a Service. Providers include AT&T and ScanSafe.

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.

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.