Proxy server

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.

Contents
1 Web proxies
2 Network address translation proxies
3 Proxy transparency
4 Open proxies, abuse, and detection
5 Reverse Proxies
6 Split Proxies
7 Popular proxy servers
8 See also
9 External links

9.1 Web-based proxy servers & Proxy list

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.

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.

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.

Network address translation proxies

UDP datagram exchanges. NAT is also known as IP masquerading.

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. However RFC 3040 define this type as intercepting proxy.

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 term proxy is also used in a different sense in the Session Initiation Protocol (SIP) used in many modern voice over IP systems. A SIP Proxy, unlike a Web proxy, does not handle the content of client data.

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.

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]. 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 Blitzed OPM [3] and CBL [4].

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] 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:

Split Proxies

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. 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.

Google's Web Accelerator is an example of a split proxy.

Popular proxy servers

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

See also

External links

Web-based proxy servers & Proxy list

See also: Proxy server, Algorithm, Anonymity, Apache HTTP Server, Cache, Cache algorithms, Censorware, Client (computing), Computer file