Proxy server

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Schematic representation of a proxy server, where the computer in the middle acts as the proxy server between the other two.

In computer networks, a proxy server is a server (a computer system or an application program) that acts as a go-between 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 two purposes:

  • To keep machines behind it anonymous (mainly for security).[1]
  • To speed up access to a resource (via caching). It is commonly used to cache web pages from a web server.[2]

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 or the Internet. A reverse proxy is a proxy used as a front-end to accelerate and cache in-demand resources. (such as a web page)


[edit] Types and functions

Proxy servers implement one or more of the following functions:

[edit] 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.

The HTTP 1.0 and later protocols contain many types of headers for declaring static (cacheable):) content and verifying content freshness with an original server, e.g. ETAG (validation tags), If-Modified-Since (date-based validation), Expiry (timeout-based invalidation), etc. Other protocols such as DNS support expiry only and contain no support for validation.

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.

[edit] Web proxy

A proxy that focuses on WWW traffic is called a "web proxy". The most common use of a web proxy is to serve as a web cache. Most proxy programs (e.g. Squid) provide a means to deny access to certain URLs in a blacklist, thus providing content filtering. This is usually used in a corporate environment, though with the increasing use of Linux in small businesses and homes, this function is no longer confined to large corporations. Some web proxies reformat web pages for a specific purpose or audience (e.g., 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 Here" to bypass the web proxy and to avoid the bugs in the thinning software.

[edit] Content-filtering web proxy

A content-filtering web proxy server provides administrative control over the content that may be relayed through the proxy. It is commonly used in commercial and non-commercial organizations (especially schools) to ensure that Internet usage conforms to acceptable use policy.

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..

[edit] Anonymizing proxy server

An anonymous proxy server (sometimes called a web proxy) generally attempts to anonymize web surfing. These can easily be overridden by site administrators, and thus rendered useless in some cases. 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.

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.

[edit] 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.

[edit] Intercepting proxy server

An intercepting proxy (also known as a "transparent proxy") combines a proxy server with a gateway. Connections made by client browsers through the gateway are redirected through the proxy without client-side configuration (or often knowledge).

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.

It is often possible to detect the use of an intercepting proxy server by comparing the external IP address to the address seen by an external web server, or by examining the HTTP headers on the server side.

[edit] 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). Transparent proxies can be implemented using Cisco's WCCP (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).

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".

[edit] 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.

[edit] 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 networks automatically test client systems for known types of open proxy. Likewise, an email server may be configured to automatically test e-mail senders for open proxies.

Groups of IRC and electronic mail operators run DNSBLs publishing lists of the IP addresses of known open proxies, such as AHBL, CBL, NJABL, 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. [1] Others consider the client to have solicited the scan by connecting to a server whose terms of service include testing.

[edit] 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. 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.
  • 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.

[edit] Circumventor

A circumventor is a method of defeating blocking policies implemented using proxy servers. Ironically, most circumventors are also proxy servers, of varying degrees of sophistication, which effectively implement "bypass policies".

A circumventor is a web-based page that takes a site that is blocked and "circumvents" it through to an unblocked web site, 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 circumventors in that it circumvents only one block.

A September 2007 report from Citizen Lab recommended Web based proxies Proxify[2], StupidCensorship[3], and CGIProxy.[4] Alternatively, users could partner with individuals outside the censored country running Psiphon[5] or Peacefire/Circumventor.[6] A more elaborate approach suggested was to run free tunneling software such as UltraSurf[7], and FreeGate,[8] or pay services Anonymizer[9] and Ghost Surf.[10] Also listed were free application tunneling software Gpass[11] and HTTP Tunnel,[12] and pay application software Relakks[13] and Guardster.[3] Lastly, anonymous communication networks JAP ANON,[14] Tor,[15] and I2P[16] offer a range of possibilities for secure publication and browsing.[4]

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

Circumventors are also used by people who have been blocked from a web site.

Another use of a circumventor 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 circumventors is usually safe with the exception that circumventor 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 circumventor.

An example of one way to circumvent a content-filtering proxy server is by tunnelling through to another proxy server, usually controlled by the user, which has unrestricted access to the internet. This is often achieved by using a VPN type tunnel, such as VPN itself or SSH, through a port left open by the proxy server to be circumvented. Port 80 is almost always open to allow the use of HTTP, as is Port 443 to allow the use of HTTPS. Through the use of encryption, tunnelling to a remote proxy server, provided the remote proxy server is itself secure, is not only difficult to detect, but also difficult to intercept.

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, they could use that system as a proxy server for which the other clients use to access the original proxy server, consequently altering their access privelidges.

[edit] 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 (i.e. "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. 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.

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".

[edit] 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. "").

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, but the term suffix proxy was only coined in October 2008 by ""[citation needed].

[edit] 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.

The bottom line of this is to be wary when using anonymising 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 over an encrypted connection) through the proxy.

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.

[edit] Proxy software

  • 3proxy is a freeware proxy server, previously known as 3APA3A, and works on Windows and Linux/Unix.
  • AlchemyPoint is a user-programmable mashup proxy server that can be used to re-write web pages, emails, instant messenger messages, and other network transmissions on the fly.
  • The Apache HTTP Server can be configured to act as a proxy server.
  • Blue Coat's (formerly Cacheflow's) purpose-built SGOS proxies 15 protocols including HTTPS/SSL, has an extensive policy engine and runs on a range of appliances from branch-office to enterprise.
  • Charles is a java User-configurable HTTP proxy / HTTP monitor / Reverse Proxy that can be used with Flash Remoting (AMF0 and AMF3).
  • CGIProxy, web based proxy script written in Perl
  • EZproxy is a URL-rewriting web proxy designed primarily for providing remote access to sites that authenticate users by IP address.
  • Ideco Gateway - Linux based multi-protocol proxy server with GUI configuration tool and NAT/VPN support.
  • JAP - A local proxy, web anonymizer software connecting to proxy server chains of different organizations
  • ISA server is described by Microsoft as an "integrated edge security gateway"; it includes caching web proxy, caching reverse proxy, Firewall, VPN, NAT, content filter, SSL encryption, and other components.
  • Novell BorderManager web proxy server, reverse proxy, Firewall and VPN end point.
  • Nginx Web and Reverse proxy server, that can act as POP3 proxy server.
  • PHProxy is one of the oldest and most customized proxy scripts in use on the Internet. Many of the newer proxy scripts were inspired by PHPProxy.[5]
  • Privoxy is a free, open source web proxy with privacy and ad-blocking features.
  • Proxomitron - User-configurable web proxy used to re-write web pages on the fly. Most noted for blocking ads, but has many other useful features.
  • SafeSquid Linux based, complete content filtering HTTP1.1 proxy, allows distribution of 'profiled' internet access.
  • SSH Secure Shell can be configured to proxify a connection, by setting up a SOCKS proxy on the client, and tunneling the traffic through the SSH connection.
  • Sun Java System Web Proxy Server is a caching proxy server running on Solaris, Linux and Windows servers that supports http://https, NSAPI I/O filters, dynamic reconfiguration, SOCKSv5 and reverse proxy.
  • Squid is a popular HTTP proxy server in the UNIX/Linux world.
  • TcpCatcher is a free TCP and HTTP(S) proxy server for educational purposes. It allows you to modify packets on the fly.
  • Tinyproxy is a light-weight HTTP proxy daemon ideal for embedded use on POSIX operating systems.
  • Tor - A proxy-based anonymizing Internet communication system.
  • I2P - A proxy-like decentralized network for anonymizing Internet data transfers.
  • Varnish is designed to be a high-performance caching reverse proxy.
  • WinGate is a multi-protocol proxy server and NAT solution that can be used to redirect any kind of traffic on a Microsoft Windows host.
  • WWWOFFLE has been around since the mid-1990s, and was developed for storing online data for offline use.
  • yProxy is an NNTP proxy server that converts yEnc encoded message attachments to UUEncoding, complete with SSL client support.
  • Zeus functions as both a forward and reverse proxy server. It operates on Solaris, FreeBSD and Linux.
  • Ziproxy is a non-caching proxy for acceleration purposes. It recompresses pictures and optimizes HTML code.

[edit] References

  1. ^ "How-to". "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. ^ Site at
  4. ^ "Everyone's Guide to By-Passing Internet Censorship". 
  5. ^ "Proxies". Tech-FAQ. 

[edit] See also

[edit] External links

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