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A proxy server can also serve as a Firewall . 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 Server s, 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 documents that have been left the longest, while LFU removes the least popular 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 PDA s. Network operators can also deploy proxies to intercept Computer Virus es and other hostile content served from remote Web pages. A special case of web proxies are "CGI proxies." These are web sites which 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 . PROXY TRANSPARENCY Many organizations — including router to share an internet connection or a LAN. Such proxies may be difficult to configure for applications requiring a large port range out going and may only be able to route inward to a single server for a given UDP or TCP port (see Wingate 2.x versions). However such proxies may have more extensive logging or more customizable security than a simple NAT router box. A transparent proxy or '''transproxy''' (also known as a forced proxy) combines a proxy server with NAT so that connections are routed into the proxy without client-side configuration. However, RFC 3040 defines 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 Virus es 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 may be configured to automatically test E-mail senders for open proxies, using software such as Michael Tokarev's proxycheck {Link without Title} . Groups of IRC and electronic mail operators run DNSBL s publishing lists of the IP Address es of known open proxies, such as Blitzed OPM and CBL [http://cbl.abuseat.org/ . 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. {Link without Title} 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 a destination of 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 and also for reducing the issues regarding high latency links (such as satellite internet) where estabilishing 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 . Google's Web Accelerator ANONYMOUS PROXY RISKS 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 (see, for example, Sugarcane ) 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 requestor, 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. POPULAR PROXY SERVER SOFTWARE
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