Proxy Server

From Wikipedia, the free encyclopedia Proxy server Proxy server Types and functions Proxy servers implement one or more of the following functions: Caching proxy server A caching proxy server accelerates service Schematic representation of a proxy server, requests by retrieving content saved from a where the computer in the middle acts as the previous request made by the same client or proxy server between the other two. even other clients. Caching proxies keep local copies of frequently requested resources, In computer networks, a proxy server is a allowing large organizations to significantly server (a computer system or an application reduce their upstream bandwidth usage and program) that acts as a go-between for re- cost, while significantly increasing perform- quests from clients seeking resources from ance. Most ISPs and large businesses have a other servers. A client connects to the proxy caching proxy. These machines are built to server, requesting some service, such as a deliver superb file system performance (often file, connection, web page, or other resource, with RAID and journaling) and also contain available from a different server. The proxy hot-rodded versions of TCP. Caching proxies server evaluates the request according to its were the first kind of proxy server. filtering rules. For example, it may filter The HTTP 1.0 and later protocols contain traffic by IP address or protocol. If the re- many types of headers for declaring static quest is validated by the filter, the proxy (cacheable) content and verifying content provides the resource by connecting to the freshness with an original server, e.g. ETAG relevant server and requesting the service on (validation tags), If-Modified-Since (date- behalf of the client. A proxy server may op- based validation), Expiry (timeout-based in- tionally alter the client’s request or the serv- validation), etc. Other protocols such as DNS er’s response, and sometimes it may serve support expiry only and contain no support the request without contacting the specified for validation. server. In this case, it ’caches’ responses Some poorly-implemented caching proxies from the remote server, and returns sub- have had downsides (e.g., an inability to use sequent requests for the same content user authentication). Some problems are de- directly. scribed in RFC 3143 (Known HTTP Proxy/ A proxy server has two purposes: Caching Problems). • To keep machines behind it anonymous Another important use of the proxy server (mainly for security).[1] is to reduce the hardware cost. An organiza- • To speed up access to a resource (via tion may have many systems on the same net- caching). It is commonly used to cache work or under control of a single server, pro- web pages from a web server.[2] hibiting the possibility of an individual con- A proxy server that passes requests and nection to the Internet for each system. In replies unmodified is usually called a gate- such a case, the individual systems can be way or sometimes tunneling proxy. connected to one proxy server, and the proxy A proxy server can be placed in the user’s server connected to the main server. local computer or at various points between the user and the destination servers or the Web proxy Internet. A reverse proxy is a proxy used as a A proxy that focuses on WWW traffic is called front-end to accelerate and cache in-demand a "web proxy". The most common use of a resources. (such as a web page) web proxy is to serve as a web cache. Most proxy programs (e.g. Squid) provide a means 1 From Wikipedia, the free encyclopedia Proxy server to deny access to certain URLs in a blacklist, Because they are typically difficult to track, thus providing content filtering. This is often open proxies are especially useful to those used in a corporate, educational or library seeking online anonymity, from political dis- environment, and anywhere else where con- sidents to computer criminals. Apart from tent filtering is desired. Some web proxies re- these minorities, the majority of anonymity format web pages for a specific purpose or aficionados will be disappointed by the spam- audience (e.g., cell phones and PDAs). ming they’ll receive from "free" proxy AOL dialup customers used to have their services. requests routed through an extensible proxy Access control: Some proxy servers im- that ’thinned’ or reduced the detail in JPEG plement a logon requirement. In large organ- pictures. This sped up performance but izations, authorized users must log on to gain caused problems, either when more resolu- access to the web. The organization can tion was needed or when the thinning pro- thereby track usage to individuals. gram produced incorrect results. This is why Some anonymizing proxy servers may for- in the early days of the web many web pages ward data packets with header lines such as would contain a link saying "AOL Users Click HTTP_VIA, HTTP_X_FORWARDED_FOR, or Here" to bypass the web proxy and to avoid HTTP_FORWARDED, which may reveal the the bugs in the thinning software. IP address of the client. Other anonymizing proxy servers, known as elite or high an- Content-filtering web proxy onymity proxies, only include the Further information: Content-control REMOTE_ADDR header with the IP address software of the proxy server, making it appear that the A content-filtering web proxy server provides proxy server is the client. A website could administrative control over the content that still suspect a proxy is being used if the client may be relayed through the proxy. It is com- sends packets which include a cookie from a monly used in commercial and non-commer- previous visit that did not use the high an- cial organizations (especially schools) to en- onymity proxy server. Clearing cookies, and sure that Internet usage conforms to accept- possibly the cache, would solve this problem. able use policy. However often times mem- bers who disagree with the policy will revolt Hostile proxy by dowloading and using their own proxy. Proxies can also be installed in order to Some common methods used for content eavesdrop upon the dataflow between client filtering include: URL or DNS blacklists, URL machines and the web. All accessed pages, as regex filtering, MIME filtering, or content well as all forms submitted, can be captured keyword filtering. Some products have been and analyzed by the proxy operator. For this known to employ content analysis techniques reason, passwords to online services (such as to look for traits commonly used by certain webmail and banking) should always be ex- types of content providers. changed over a cryptographically secured A content filtering proxy will often support connection, such as SSL. user authentication, to control web access. It also usually produces logs, either to give de- Intercepting proxy server tailed information about the URLs accessed An intercepting proxy (also known as a by specific users, or to monitor bandwidth us- "transparent proxy") combines a proxy server age statistics. It may also communicate to with a gateway. Connections made by client daemon based and/or ICAP based antivirus browsers through the gateway are redirected software to provide security against virus and through the proxy without client-side config- other malware by scanning incoming content uration (or often knowledge). in real time before it enters the network.. Intercepting proxies are commonly used in businesses to prevent avoidance of accept- Anonymizing proxy server able use policy, and to ease administrative An anonymous proxy server (sometimes burden, since no client browser configuration called a web proxy) generally attempts to an- is required. onymize web surfing. There are different It is often possible to detect the use of an varieties of anonymizers. One of the more intercepting proxy server by comparing the common variations is the open proxy. external IP address to the address seen by an 2 From Wikipedia, the free encyclopedia Proxy server external web server, or by examining the prohibits caching of responses where the re- HTTP headers on the server side. quest contained an authorization header. Transparent and non-transpar- Open proxy server ent proxy server Because proxies might be used to abuse, sys- The term "transparent proxy" is most often tem administrators have developed a number used incorrectly to mean "intercepting proxy" of ways to refuse service to open proxies. (because the client does not need to config- Many IRC networks automatically test client ure a proxy and cannot directly detect that systems for known types of open proxy. Like- its requests are being proxied). Transparent wise, an email server may be configured to proxies can be implemented using Cisco’s automatically test e-mail senders for open WCCP (Web Cache Control Protocol). This proxies. proprietary protocol resides on the router Groups of IRC and electronic mail operat- and is configured from the cache, allowing ors run DNSBLs publishing lists of the IP ad- the cache to determine what ports and traffic dresses of known open proxies, such as is sent to it via transparent redirection from AHBL, CBL, NJABL, and SORBS. the router. This redirection can occur in one The ethics of automatically testing clients of two ways: GRE Tunneling (OSI Layer 3) or for open proxies are controversial. Some ex- MAC rewrites (OSI Layer 2). perts, such as Vernon Schryver, consider However, RFC 2616 (Hypertext Transfer such testing to be equivalent to an attacker Protocol -- HTTP/1.1) offers different portscanning the client host. [1] Others con- definitions: sider the client to have solicited the scan by "A ’transparent proxy’ is a proxy that connecting to a server whose terms of ser- does not modify the request or response vice include testing. beyond what is required for proxy authentication and identification".

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