In computer networks A computer network, often simply referred to as a network, is a collection of computers and devices connected by communications channels that facilitates communications among users and allows users to share resources with other users. Networks may be classified according to a wide variety of characteristics. This article provides a general, a proxy server is a server In computing, a server is any combination of hardware or software designed to provide services to clients. When used alone, the term typically refers to a computer which may be running a server operating system, but is also used to refer to any software or dedicated hardware capable of providing services (a computer system or an application program) that acts as an intermediary for requests from clients A client is an application or system that accesses a remote service on another computer system, known as a server, by way of a network. The term was first applied to devices that were not capable of running their own stand-alone programs, but could interact with remote computers via a network. These dumb terminals were clients of the time-sharing 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 An Internet Protocol address is a numerical label that is assigned to devices participating in a computer network, that uses the Internet Protocol for communication between its nodes. An IP address serves two principal functions: host or network interface identification and location addressing. Its role has been characterized as follows: "A or protocol In computing, a protocol is a set of rules which is used by computers to communicate with each other across a network. A protocol is a convention or standard that controls or enables the connection, communication, and data transfer between computing endpoints. In its simplest form, a protocol can be defined as the rules governing the syntax,. 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 In computer science, a cache is a component that improves performance by transparently storing data such that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere. If requested data is contained in the' responses from the remote server, and returns subsequent requests for the same content directly.

A proxy server has a large variety of potential purposes, including:

• To keep machines behind it anonymous (mainly for security Computer security is a branch of computer technology known as information security as applied to computers and networks. The objective of computer security includes protection of information and property from theft, corruption, or natural disaster, while allowing the information and property to remain accessible and productive to its intended).^[1]
• To speed up access to resources (using caching). Web proxies are commonly used to cache In computer science, a cache is a component that improves performance by transparently storing data such that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere. If requested data is contained in the web pages from a web server.^[2]
• 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 for malware before delivery.
• 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 Gateways, also called protocol converters, can operate at any layer of the OSI model. The job of a gateway is much more complex than that of a router or switch. Typically, a gateway must convert one protocol stack into another 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 A reverse proxy is a proxy server that is installed in a server network. Typically, reverse proxies are used in front of Web servers. All connections coming from the Internet addressed to one of the Web servers are routed through the proxy server, which may either deal with the request itself or pass the request wholly or partially to the main web is (usually) an 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. 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 The World Wide Web, abbreviated as WWW and commonly known as the Web, is a system of interlinked hypertext documents accessed via the Internet. With a web browser, one can view web pages that may contain text, images, videos, and other multimedia and navigate between them by using hyperlinks. Using concepts from earlier hypertext systems, British traffic is called a "web proxy". The most common use of a web proxy is to serve as a web cache Web caching is the caching of web documents to reduce bandwidth usage, server load, and perceived lag. A web cache stores copies of documents passing through it; subsequent requests may be satisfied from the cache if certain conditions are met. Most proxy programs provide a means to deny access to URLs specified in a blacklist In computing, a blacklist or block list is a basic access control mechanism that allows everyone access, except for the members of the black list . The opposite is a whitelist, which means allow nobody, except members of the white list. As a sort of middle ground, a greylist, contains entries that are temporarily blocked or temporarily allowed, 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 AOL Inc. , formerly known as America Online and logo typeset as "Aol.", is an American global Internet services and media company. AOL is headquartered at 770 Broadway in New York. Founded in 1983 as Quantum Computer Services, it has franchised its services to companies in several nations around the world or set up international versions 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

A content-filtering Content filtering is the most commonly used group of methods to filter spam. Content filters act either on the content, the information contained in the mail body, or on the mail headers to either classify, accept or reject a message 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 An acceptable use policy is a set of rules applied by the owner/manager of a network, website or large computer system that restrict the ways in which the network site or system may be used. AUP documents are written for corporations, businesses, universities, schools, internet service providers, and website owners often to reduce the potential. 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 user's proxy.

Some common methods used for content filtering include: URL A blacklist is a list or register of persons who, for one reason or another, are being denied a particular privilege, service, mobility, access or recognition. As a verb, to blacklist can mean to deny someone work in a particular field, or to ostracize a person from a certain social circle. Conversely, a whitelist is a list or compilation or DNS blacklists A DNSBL is list of IP addresses published through the Internet Domain Name Service in a particular format. DNSBLs are most often used to publish the addresses of computers or networks linked to spamming; most mail server software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists, URL In computing, a Uniform Resource Locator is a Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. In popular usage and in many technical documents and verbal discussions it is often incorrectly used as a synonym for URI,. The best-known example of a URL is the " regex filtering, MIME MIME's use, however, has grown beyond describing the content of e-mail to describing content type in general, including for the web 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 Authentication is the act of establishing or confirming something (or someone) as authentic, that is, that claims made by or about the subject are true ("authentification" is a French language variant of this word). This might involve confirming the identity of a person, tracing the origins of an artifact, ensuring that a product is what, to control web access. It also usually produces logs A server log is a log file automatically created and maintained by a server of activity performed by it, either to give detailed information about the URLs accessed by specific users, or to monitor bandwidth In computer networking and computer science, digital bandwidth, network bandwidth or just bandwidth is a measure of available or consumed data communication resources expressed in bit/s or multiples of it usage statistics. It may also communicate to daemon In Unix and other computer multitasking operating systems, a daemon is a computer program that runs in the background, rather than under the direct control of a user; they are usually initiated as background processes. Typically daemons have names that end with the letter "d": for example, syslogd, the daemon that handles the system log,-based and/or ICAP The Internet Content Adaptation Protocol is a lightweight HTTP based protocol specified in RFC 3507 designed to off-load specific content to dedicated servers, thereby freeing up resources and standardizing the way in which features are implemented. ICAP is generally used in proxy servers to integrate with third party products like antivirus-based antivirus software Antivirus software is used to prevent, detect, and remove malware, including computer viruses, worms, and trojan horses. Such programs may also prevent and remove adware, spyware, and other forms of malware to provide security against virus and other malware Malware, short for malicious software, is software designed to infiltrate a computer system without the owner's informed consent. The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code. The term "computer virus" is sometimes used as a catch- 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 An anonymizer or an anonymous proxy is a tool that attempts to make activity on the Internet untraceable. It accesses the Internet on the user's behalf, protecting personal information by hiding the source computer's identifying information. One of the more common variations is the open proxy An open proxy is a proxy server that is accessible by any Internet user. Generally, a proxy server allows users within a network group to store and forward Internet services such as DNS or web pages to reduce and control the bandwidth used by the group. With an open proxy, however, any user on the Internet is able to use this forwarding service. Because they are typically difficult to track, open proxies are especially useful to those seeking online anonymity, from political dissidents A dissident, broadly defined, is a person who actively challenges an established doctrine, policy, or institution. When individual dissidents unite in a common cause they may become known as a dissident movement to computer criminals Computer crime encompass a broad range of potentially illegal activities. Generally, however, it may be divided into one of two types of categories: crimes that target computer networks or devices directly; (2) crimes facilitated by computer networks or devices, the primary target of which is independent of the computer network or device. 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 Freedom of speech is the freedom to speak without censorship and/or limitation. The synonymous term freedom of expression is sometimes used to indicate not only freedom of verbal speech but any act of seeking, receiving and imparting information or ideas, regardless of the medium used. In practice, the right to freedom of speech is not absolute in, 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 World Wide Web, abbreviated as WWW and commonly known as the Web, is a system of interlinked hypertext documents accessed via the Internet. With a web browser, one can view web pages that may contain text, images, videos, and other multimedia and navigate between them by using hyperlinks. Using concepts from earlier hypertext systems, British. 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 Eavesdropping is the act of secretly listening to the private conversation of others without their consent, as defined by Black's Law Dictionary. This is commonly thought to be unethical and there is an old adage that eavesdroppers seldom hear anything good of themselves...eavesdroppers always try to listen to matters that concern them . upon the data-flow 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 Gateways, also called protocol converters, can operate at any layer of the OSI model. The job of a gateway is much more complex than that of a router or switch. Typically, a gateway must convert one protocol stack into another or router A router is a device that interconnects two or more computer networks, and selectively interchanges packets of data between them. Each data packet contains address information that a router can use to determine if the source and destination are on the same network, or if the data packet must be transferred from one network to another. Where (commonly with NAT In computer networking, network address translation is the process of modifying network address information in datagram (IP) packet headers while in transit across a traffic routing device for the purpose of remapping a given address space into another 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 Cross-site scripting is a type of computer security vulnerability typically found in web applications that enables malicious attackers to inject client-side script into web pages viewed by other users. An exploited cross-site scripting vulnerability can be used by attackers to bypass access controls such as the same origin policy. Cross-site which depend on certain behaviour of intercepting proxies that do not check or have access to information about the original (intercepted) destination. This problem can 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 NTLM (not to be confused with LAN Manager) is a Microsoft authentication protocol. MS-CHAP is similar and is used for authentication with Microsoft remote access protocols. It is the successor of LANMAN (Microsoft LAN Manager), an older Microsoft authentication protocol, and attempted to be backwards compatible with LANMAN. NTLM was followed by, since 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, since some requests and responses become uncacheble by a shared cache.

Therefore intercepting connections is generally discouraged. However due to the simplicity of deploying such systems, they are in widespread use.

Implementation Methods

Interception can be performed using Cisco's WCCP Web Cache Communication Protocol is a Cisco-developed content-routing protocol that provides a mechanism to redirect traffic flows in real-time. It has built-in load balancing, scaling, fault tolerance, and service-assurance (failsafe) mechanisms. Cisco IOS Release 12.1 and later releases allow the use of either Version 1 (WCCPv1) or Version 2 ( (Web Cache Control Protocol). 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: GRE Tunneling (OSI Layer 3) or MAC rewrites (OSI Layer 2).

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. Recent releases of Linux and some BSD provide TPROXY (Transparent Proxy) which performs IP-level (OSI Layer 3) transparent interception and Spoofing of outbound traffic. Hiding the proxy IP address from other network devices.

Detecting

It is often possible to detect the use of an intercepting proxy server by comparing the client's external IP address to the address seen by an external web server, or sometimes by examining the HTTP headers received by a server. A number of sites have been created to address this issue, by reporting the user's IP address as seen by the site back to the user in a web page.

Transparent and non-transparent proxy server

The term "transparent proxy" is most often used incorrectly to mean "intercepting proxy" (because the client does not need to configure a proxy and cannot directly detect that its requests are being proxied).

However, RFC 2616 (Hypertext Transfer Protocol—HTTP/1.1) offers different definitions:

"A 'transparent proxy' is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification".

"A 'non-transparent proxy' is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering".

A security flaw in the way that transparent proxies operate was published by Robert Auger in 2009 [1] and advisory by the Computer Emergency Response Team [2] was issued listing dozens of affected transparent, and intercepting proxy servers.

Forced proxy

The term "forced proxy" is ambiguous. It means both "intercepting proxy" (because it filters all traffic on the only available gateway to the Internet) and its exact opposite, "non-intercepting proxy" (because the user is forced to configure a proxy in order to access the Internet).

Forced proxy operation is sometimes necessary due to issues with the interception of TCP connections and HTTP. For instance, interception of HTTP requests can affect the usability of a proxy cache, and can greatly affect certain authentication mechanisms. This is primarily because the client thinks it is talking to a server, and so request headers required by a proxy are unable to be distinguished from headers that may be required by an upstream server (esp authorization headers). Also the HTTP specification prohibits caching of responses where the request contained an authorization header.

Suffix proxy

A suffix proxy server allows a user to access web content by appending the name of the proxy server to the URL of the requested content (e.g. "en.wikipedia.org.example.com").

Suffix proxy servers are easier to use than regular proxy servers. The concept appeared in 2003 in form of the IPv6Gate and in 2004 in form of the Coral Content Distribution Network The Coral Content Distribution Network, sometimes called Coral Cache or Coral, is a free peer-to-peer content distribution network designed and operated by Michael Freedman. Coral uses the bandwidth of volunteers to mirror web content, often to avoid the Slashdot Effect or to reduce the load on websites in general, but the term suffix proxy was only coined in October 2008 by "6a.nl"^{[citation needed]}.

Open proxy server

Because proxies might be used to abuse, system administrators have developed a number of ways to refuse service to open proxies. Many IRC Internet Relay Chat is a form of real-time Internet text messaging (chat) or synchronous conferencing. It is mainly designed for group communication in discussion forums, called channels, but also allows one-to-one communication via private message as well as chat and data transfers via Direct Client-to-Client networks automatically test client systems for known types of open proxy. Likewise, an email server Within Internet message handling services , a message transfer agent or mail transfer agent (MTA) or mail relay is a computer process or software agent that transfers electronic mail messages from one computer to another, in single hop application-level transactions. An MTA implements both the client (sending) and server (receiving) portions of may be configured to automatically test e-mail Electronic mail, most commonly abbreviated email or e-mail, is a method of exchanging digital messages. E-mail systems are based on a store-and-forward model in which e-mail server computer systems accept, forward, deliver and store messages on behalf of users, who only need to connect to the e-mail infrastructure, typically an e-mail server, with senders for open proxies.

Groups of IRC and electronic mail operators run DNSBLs A DNSBL is list of IP addresses published through the Internet Domain Name Service in a particular format. DNSBLs are most often used to publish the addresses of computers or networks linked to spamming; most mail server software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists publishing lists of the IP addresses An Internet Protocol address is a numerical label that is assigned to devices participating in a computer network, that uses the Internet Protocol for communication between its nodes. An IP address serves two principal functions: host or network interface identification and location addressing. Its role has been characterized as follows: "A of known open proxies, such as AHBL The Abusive Hosts Blocking List is an internet abuse tracking and filtering system developed by The Summit Open Source Development Group, and based on the original Summit Blocking List, CBL, NJABL NJABL maintains a list of known and potential spam sources for the purpose of being able to tag or refuse e-mail and thereby block spam from certain sources. NJABL automatically retests only listed open relays every 90 days, and SORBS.

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. [3] Others consider the client to have solicited the scan by connecting to a server whose terms of service include testing.

Forward proxy

The terms "forward proxy" and "forwarding proxy" are a general description of behaviour (forwarding traffic) and thus ambiguous. It is used to refer to a proxy able to retrieve from a wide range of sources (in most cases anywhere on Internet). Except for Reverse proxy the types described on this article are more specialized sub-types of the general forward proxy concept.

Reverse proxy server

A reverse proxy is a proxy server that is installed in the neighborhood of one or more web servers. All traffic coming from the Internet and with a destination of one of the 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:

• Encryption / SSL acceleration: when secure web sites are created, the SSL encryption is often not done by the web server itself, but by a reverse proxy that is equipped with SSL acceleration hardware. See Secure Sockets Layer. Furthermore, a host can provide a single "SSL proxy" to provide SSL encryption for an arbitrary number of hosts; removing the need for a separate SSL Server Certificate for each host, with the downside that all hosts behind the SSL proxy have to share a common DNS name or IP address for SSL connections. This problem can partly be overcome by using the SubjectAltName feature of X.509 certificates.
• Load balancing: the reverse proxy can distribute the load to several web servers, each web server serving its own application area. In such a case, the reverse proxy may need to rewrite the URLs in each web page (translation from externally known URLs to the internal locations).
• Serve/cache static content: A reverse proxy can offload the web servers by caching static content like pictures and other static graphical content.
• Compression: the proxy server can optimize and compress the content to speed up the load time.
• Spoon feeding: reduces resource usage caused by slow clients on the web servers by caching the content the web server sent and slowly "spoon feeding" it to the client. This especially benefits dynamically generated pages.
• Security: the proxy server is an additional layer of defense and can protect against some OS and WebServer specific attacks. However, it does not provide any protection to attacks against the web application or service itself, which is generally considered the larger threat.
• Extranet Publishing: a reverse proxy server facing the Internet can be used to communicate to a firewalled server internal to an organization, providing extranet access to some functions while keeping the servers behind the firewalls. If used in this way, security measures should be considered to protect the rest of your infrastructure in case this server is compromised, as its web application is exposed to attack from the Internet.

Tunneling proxy server

A tunneling proxy server is a method of defeating blocking policies implemented using proxy servers. Tunneling proxy servers are used by people who have been blocked from viewing a particular web site. Most tunneling proxy servers are also proxy servers, of varying degrees of sophistication, which effectively implement "bypass policies".

A tunneling proxy server is a web-based page that takes a site that is blocked and "tunnels" it, allowing the user to view blocked pages. A famous example is elgooG, which allowed users in China to use Google after it had been blocked there. elgooG differs from most tunneling proxy servers in that it circumvents only one block.

A September 2007 report from Citizen Lab and BBC.co.uk recommended secure based proxies HTTP Tunnel, StupidCensorship, and CGIProxy. Alternatively, users could partner with individuals outside the censored network running Psiphon or Peacefire/tunneling proxy server. A more elaborate approach suggested was to run free tunneling software such as FreeGate, or pay services Anonymizer and Ghost Surf. Also listed were free application tunneling software PaperBus, Gpass and HTTP Tunnel, and pay application software Relakks and Guardster. Lastly, anonymous communication networks JAP ANON, Tor, and I2P offer a range of possibilities for secure publication and browsing.^[3]

Other options include Garden and GTunnel by Garden Networks.

Students are able to access blocked sites (games, chatrooms, messenger, offensive material, internet pornography, social networking, etc.) through a tunneling proxy server. As fast as the filtering software blocks tunneling proxy servers, others spring up. However, in some cases the filter may still intercept traffic to the tunneling proxy server, thus the person who manages the filter can still see the sites that are being visited.

Another use of a tunneling proxy server is to allow access to country-specific services, so that Internet users from other countries may also make use of them. An example is country-restricted reproduction of media and webcasting.

The use of tunneling proxy servers is usually safe with the exception that tunneling proxy server sites run by an untrusted third party can be run with hidden intentions, such as collecting personal information, and as a result users are typically advised against running personal data such as credit card numbers or passwords through a tunneling proxy server.

In some network configurations, clients attempting to access the proxy server are given different levels of access privilege on the grounds of their computer location or even the MAC address of the network card. However, if one has access to a system with higher access rights, one could use that system as a proxy server for which the other clients use to access the original proxy server, consequently altering their access privileges.

Content filter

Many work places, schools, and colleges restrict the web sites and online services that are made available in their buildings. 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.

Requests made to the open internet must first pass through an outbound proxy filter. The web-filtering company provides a database of URL patterns (regular expressions) with associated content attributes. This database is updated weekly by site-wide subscription, much like a virus filter subscription. The administrator instructs the web filter to ban broad classes of content (such as sports, pornography, online shopping, gambling, or social networking). Requests that match a banned URL pattern are rejected immediately.

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 is returned and nothing is cached.

Most web filtering companies use an internet-wide crawling robot that assesses the likelihood that a content is a certain type (e.g. "This content is 70% chance of porn, 40% chance of sports, and 30% chance of news" could be the outcome for one web page). The resultant database is then corrected by manual labor based on complaints or known flaws in the content-matching algorithms.

Web filtering proxies are not able to peer inside secure sockets HTTP transactions, assuming the chain-of-trust of SSL/TLS has not been tampered with. As a result, users wanting to bypass web filtering will typically search the internet for an open and anonymous HTTPS transparent proxy. They will then program their browser to proxy all requests through the web filter to this anonymous proxy. Those requests will be encrypted with https. The web filter cannot distinguish these transactions from, say, a legitimate access to a financial website. Thus, content filters are only effective against unsophisticated users.

As mentioned above, the SSL/TLS chain-of-trust does rely on trusted root certificate authorities; 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. Concretely, a root certificate generated by the proxy is installed into the browser CA list by IT staff. In such scenarios, proxy analysis of the contents of a SSL/TLS transaction becomes possible. The proxy is effectively operating a man-in-the-middle attack, allowed by the client's trust of a root certificate the proxy owns.

A special case of web proxies is "CGI proxies". These are web sites that allow a user to access a site through them. They generally use PHP or CGI to implement the proxy functionality. These types of 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, called "Proxy Avoidance".

Risks of using anonymous proxy servers

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 a feasible risk that a malicious proxy server may record everything sent: including unencrypted logins and passwords.

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.

See also

• Application layer firewall
• Captive portal
• HTTP
• ICAP
• Internet privacy
• Proxy list
• Reverse proxy
• SOCKS
• Transparent SMTP proxy
• Web cache

References

1. ^ "How-to". Linux.org. http://www.linux.org/docs/ldp/howto/Firewall-HOWTO-11.html#ss11.4. "The proxy server is, above all, a security device."
2. ^ Thomas, Keir (2006). Beginning Ubuntu Linux: From Novice to Professional. Apress. "A proxy server helps speed up Internet access by storing frequently accessed pages"
3. ^ "Everyone's Guide to By-Passing Internet Censorship". http://civisec.org/guides/everyones-guides.

External links

• Proxy software and scripts at the Open Directory Project
• Free web-based proxy services at the Open Directory Project
• Free http proxy servers at the Open Directory Project

Categories: Computer networking | Network performance | Internet architecture | Internet privacy | Computer security software | Proxy servers

See Also:

• Java Anon Proxy: Encyclopedia
• Computers: Internet: Proxying and Filtering: Hosted Proxy Services: Directory
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