In computer networks, a proxy server is a server (a computer system or an application program) that acts as an intermediary for requests from clients seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a different server. The proxy server evaluates the request according to its filtering rules. For example, it may filter traffic by IP address or protocol. If the request is validated by the filter, the proxy provides the resource by connecting to the relevant server and requesting the service on behalf of the client. A proxy server may optionally alter the client's request or the server's response, and sometimes it may serve the request without contacting the specified server. In this case, it 'caches' responses from the remote server, and returns subsequent requests for the same content directly.
A proxy server has many potential purposes, including:
- To keep machines behind it anonymous (mainly for security).
- To speed up access to resources (using caching). Web proxies are commonly used to cache web pages from a web server.
- To apply access policy to network services or content, e.g. to block undesired sites.
- To log / audit usage, i.e. to provide company employee Internet usage reporting.
- To bypass security/ parental controls.
- To scan transmitted content before delivery for malware.
- To scan outbound content, e.g. for data leak protection.
- To circumvent regional restrictions.
A proxy server that passes requests and replies unmodified is usually called a gateway or sometimes tunneling proxy .
A proxy server can be placed in the user's local computer or at various points between the user and the destination servers on the Internet.
A reverse proxy is a (usually) Internet-facing proxy used as a front-end to control and protect access to a server on a private network, commonly also performing tasks such as load-balancing, authentication, decryption or caching.
Types and functions
Proxy servers implement one or more of the following functions:
Caching proxy server
A caching proxy server accelerates service requests by retrieving content saved from a previous request made by the same client or even other clients. 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. Most ISPs and large businesses have a caching proxy. These machines are built to deliver superb file system performance (often with RAID and journaling) and also contain hot-rodded versions of TCP. Caching proxies were the first kind of proxy server.
Some poorly-implemented caching proxies have had downsides (e.g., an inability to use user authentication). Some problems are described in RFC 3143 (Known HTTP Proxy/Caching Problems).
Another important use of the proxy server is to reduce the hardware cost. An organization may have many systems on the same network or under control of a single server, prohibiting the possibility of an individual connection to the Internet for each system. In such a case, the individual systems can be connected to one proxy server, and the proxy server connected to the main server.
Web proxy
A proxy that focuses on World Wide Web traffic is called a "web proxy". The most common use of a web proxy is to serve as a web cache. Most proxy programs provide a means to deny access to URLs specified in a blacklist, thus providing content filtering. This is often used in a corporate, educational or library environment, and anywhere else where content filtering is desired. Some web proxies reformat web pages for a specific purpose or audience, such as for cell phones and PDAs.
AOL dialup customers used to have their requests routed through an extensible proxy that 'thinned' or reduced the detail in JPEG pictures. This sped up performance but caused problems, either when more resolution was needed or when the thinning program produced incorrect results. This is why in the early days of the web many web pages would contain a link saying "AOL Users Click " to bypass the web proxy and to avoid the bugs in the thinning software.
Content-filtering web proxy
Further information: Content-control softwareA content-filtering web proxy server provides administrative control over the content that may be relayed through the proxy. It is commonly used in both commercial and non-commercial organizations (especially schools) to ensure that Internet usage conforms to acceptable use policy. In some cases users can circumvent the proxy, since there are services designed to proxy information from a filtered website through a non filtered site to allow it through the users proxy.
Some common methods used for content filtering include: URL or DNS blacklists, URL regex filtering, MIME filtering, or content keyword filtering. Some products have been known to employ content analysis techniques to look for traits commonly used by certain types of content providers.
A content filtering proxy will often support user authentication, to control web access. It also usually produces logs, either to give detailed information about the URLs accessed by specific users, or to monitor bandwidth usage statistics. It may also communicate to daemon-based and/or ICAP-based antivirus software to provide security against virus and other malware by scanning incoming content in real time before it enters the network.
Anonymizing proxy server
An anonymous proxy server (sometimes called a web proxy) generally attempts to anonymize web surfing. There are different varieties of anonymizers. One of the more common variations is the open proxy. Because they are typically difficult to track, open proxies are especially useful to those seeking online anonymity, from political dissidents to computer criminals. Some users are merely interested in anonymity for added security, hiding their identities from potentially malicious websites for instance, or on principle, to facilitate constitutional human rights of freedom of speech, for instance. The server receives requests from the anonymizing proxy server, and thus does not receive information about the end user's address. However, the requests are not anonymous to the anonymizing proxy server, and so a degree of trust is present between that server and the user. Many of them are funded through a continued advertising link to the user.
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.
Some anonymizing proxy servers may forward data packets with header lines such as HTTP_VIA, HTTP_X_FORWARDED_FOR, or HTTP_FORWARDED, which may reveal the IP address of the client. Other anonymizing proxy servers, known as elite or high anonymity proxies, only include the REMOTE_ADDR header with the IP address of the proxy server, making it appear that the proxy server is the client. A website could still suspect a proxy is being used if the client sends packets which include a cookie from a previous visit that did not use the high anonymity proxy server. Clearing cookies, and possibly the cache, would solve this problem.
Hostile proxy
Proxies can also be installed in order to eavesdrop upon the dataflow between client machines and the web. All accessed pages, as well as all forms submitted, can be captured and analyzed by the proxy operator. For this reason, passwords to online services (such as webmail and banking) should always be exchanged over a cryptographically secured connection, such as SSL.
Intercepting proxy server
An intercepting proxy combines a proxy server with a gateway or router (commonly with NAT capabilities). Connections made by client browsers through the gateway are diverted to the proxy without client-side configuration (or often knowledge). Connections may also be diverted from a SOCKS server or other circuit-level proxies.
Intercepting proxies are also commonly referred to as "transparent" proxies, or "forced" proxies, presumably because the existence of the proxy is transparent to the user, or the user is forced to use the proxy regardless of local settings.
Purpose
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. This second reason however is mitigated by features such as Active Directory group policy, or DHCP and automatic proxy detection.
Intercepting proxies are also commonly used by ISPs in some countries to save upstream bandwidth and improve customer response times by caching. This is more common in countries where bandwidth is more limited (e.g. island nations) or must be paid for.
Issues
The diversion / interception of a TCP connection creates several issues. Firstly the original destination IP and port must somehow be communicated to the proxy. This is not always possible (e.g. where the gateway and proxy reside on different hosts). There is a class of cross site attacks which depend on cert
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