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In computer networks , a proxy server is a server a computer system or an application that acts as an intermediary for requests from clients seeking resources from other servers. Proxies were invented to add structure and encapsulation to distributed systems. A proxy server may reside on the user's local computer, or at various points between the user's computer and destination servers on the Internet. An open proxy is a forwarding proxy server that is accessible by any Internet user.
Gordon Lyon estimates there are "hundreds of thousands" of open proxies on the Internet. There are varying degrees of anonymity however, as well as a number of methods of 'tricking' the client into revealing itself regardless of the proxy being used. A reverse proxy or surrogate is a proxy server that appears to clients to be an ordinary server.
Reverse proxies forward requests to one or more ordinary servers which handle the request. The response from the proxy server is returned as if it came directly from the original server, leaving the client with no knowledge of the origin servers. All traffic coming from the Internet and with a destination of one of the neighborhood's web servers goes through the proxy server.
The use of "reverse" originates in its counterpart "forward proxy" since the reverse proxy sits closer to the web server and serves only a restricted set of websites. There are several reasons for installing reverse proxy servers:. A content-filtering web proxy server provides administrative control over the content that may be relayed in one or both directions 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.
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. Many work places, schools and colleges restrict the web sites and online services that are accessible and available in their buildings. Governments also censor undesirable content.
This is done either with a specialized proxy, called a content filter both commercial and free products are available , or by using a cache-extension protocol such as ICAP , that allows plug-in extensions to an open caching architecture.
Ironically, websites commonly used by students to circumvent filters and access blocked content often include a proxy, from which the user can then access the websites that the filter is trying to block. Some products have been known to employ content analysis techniques to look for traits commonly used by certain types of content providers.
Assuming the requested URL is acceptable, the content is then fetched by the proxy. At this point a dynamic filter may be applied on the return path.
For example, JPEG files could be blocked based on fleshtone matches, or language filters could dynamically detect unwanted language. If the content is rejected then an HTTP fetch error may be returned to the requester. Most web filtering companies use an internet-wide crawling robot that assesses the likelihood that a content is a certain type. The resultant database is then corrected by manual labor based on complaints or known flaws in the content-matching algorithms.
Some proxies scan outbound content, e. In a workplace setting where the client is managed by the organization, trust might be granted to a root certificate whose private key is known to the proxy. Consequently, a root certificate generated by the proxy is installed into the browser CA list by IT staff. The proxy is effectively operating a man-in-the-middle attack , allowed by the client's trust of a root certificate the proxy owns.
If the destination server filters content based on the origin of the request, the use of a proxy can circumvent this filter. For example, a server using IP -based geolocation to restrict its service to a certain country can be accessed using a proxy located in that country to access the service. In some cases users can circumvent proxies which filter using blacklists using services designed to proxy information from a non-blacklisted location.
Proxies can be installed in order to eavesdrop upon the data-flow between client machines and the web. For this reason, passwords to online services such as webmail and banking should always be exchanged over a cryptographically secured connection, such as SSL. 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. 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.
Proxy bouncing can be used to maintain privacy. 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 costs, while significantly increasing performance. Most ISPs and large businesses have a caching proxy. Caching proxies were the first kind of proxy server.
Web proxies are commonly used to cache web pages from a web server. A proxy that is designed to mitigate specific link related issues or degradations is a Performance Enhancing Proxy PEPs. These typically are used to improve TCP performance in the presence of high round-trip times or high packet loss such as wireless or mobile phone networks ; or highly asymmetric links featuring very different upload and download rates.
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. A translation proxy is a proxy server that is used to localize a website experience for different markets. Traffic from global audiences is routed through the translation proxy to the source website. As visitors browse the proxied site, requests go back to the source site where pages are rendered. Original language content in the response is replaced by translated content as it passes back through the proxy.
The translations used in a translation proxy can be either machine translation, human translation, or a combination of machine and human translation.
Different translation proxy implementations have different capabilities. Some allow further customization of the source site for local audiences such as excluding source content or substituting source content with original local content.
An anonymous proxy server sometimes called a web proxy generally attempts to anonymize web surfing. There are different varieties of anonymizers. The destination server the server that ultimately satisfies the web request receives requests from the anonymizing proxy server, and thus does not receive information about the end user's address. The requests are not anonymous to the anonymizing proxy server, however, and so a degree of trust is present between the proxy server and the user.
Many proxy servers are funded through a continued advertising link to the user. 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. Other anonymizing proxy servers, known as elite or high-anonymity proxies, make 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. Advertisers use proxy servers for validating, checking and quality assurance of geotargeted ads.
A geotargeting ad server checks the request source IP address and uses a geo-IP database to determine the geographic source of requests. A proxy can keep the internal network structure of a company secret by using network address translation , which can help the security of the internal network.
Proxies can also be combined with firewalls. An incorrectly configured proxy can provide access to a network otherwise isolated from the Internet. Proxies allow web sites to make web requests to externally hosted resources e. Proxies also allow the browser to make web requests to externally hosted content on behalf of a website when cross-domain restrictions in place to protect websites from the likes of data theft prohibit the browser from directly accessing the outside domains.
Not to be confused with secondary market , secondary market brokers use web proxy servers to buy large stocks of limited products such as limited sneakers [12] or tickets. Web proxies forward HTTP requests. This request is sent to the proxy server, the proxy makes the request specified and returns the response. Also known as an intercepting proxy , inline proxy , or forced proxy , a transparent proxy intercepts normal communication at the network layer without requiring any special client configuration.
Clients need not be aware of the existence of the proxy. A transparent proxy is normally located between the client and the Internet, with the proxy performing some of the functions of a gateway or router. In a security flaw in the way that transparent proxies operate was published by Robert Auger, [15] and the Computer Emergency Response Team issued an advisory listing dozens of affected transparent and intercepting proxy servers.
Intercepting proxies are commonly used in businesses to enforce acceptable use policy, and to ease administrative overheads, 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. Firstly the original destination IP and port must somehow be communicated to the proxy. This is not always possible e. There is a class of cross site attacks that depend on certain behaviour of intercepting proxies that do not check or have access to information about the original intercepted destination.
This problem may be resolved by using an integrated packet-level and application level appliance or software which is then able to communicate this information between the packet handler and the proxy. Intercepting also creates problems for HTTP authentication, especially connection-oriented authentication such as NTLM , as the client browser believes it is talking to a server rather than a proxy.
This can cause problems where an intercepting proxy requires authentication, then the user connects to a site which also requires authentication. Finally intercepting connections can cause problems for HTTP caches, as some requests and responses become uncacheable by a shared cache. This proprietary protocol resides on the router and is configured from the cache, allowing the cache to determine what ports and traffic is sent to it via transparent redirection from the router.
This redirection can occur in one of two ways: Once traffic reaches the proxy machine itself interception is commonly performed with NAT Network Address Translation. Such setups are invisible to the client browser, but leave the proxy visible to the web server and other devices on the internet side of the proxy.
There are several methods that can often be used to detect the presence of an intercepting proxy server:.