In computer networks, a proxy server is a server (a computer system or an application program) which services the requests of its clients by forwarding requests to 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 provides the resource by connecting to the specified 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 would 'cache' the first request to the remote server, so it could save the information for later, and make everything as fast as possible.
A proxy server that passes all 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.
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.
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).
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 trouble, either when more resolution was needed or when the thinning program produced incorrect results. This is why in the early days of the Internet 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.
Content-filtering web proxy
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
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. These can easily be overridden by site administrators, and thus rendered useless in some cases. There are different varieties of anonymizers
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.
Proxies can also be installed by online criminals, in order to eavesdrop
upon the dataflow between the client machine 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 TLS.
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.
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".
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.
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. Others consider the client to have solicited the scan by connecting to a server whose terms of service include testing.
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 hoster 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 feeds" 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 it's web application is exposed to attack from the Internet.
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.
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.
Many work places and schools 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, made by a company such as WebSense
, 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 unacceptable 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.
Unforuntately, 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 are "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 [
] 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 possible, as has been demonstrated, for a malicious proxy server to record everything sent to the proxy: 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.
- 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 use with Flash Remoting (AMF0 and AMF3).
- EZproxy is a URL-rewriting web proxy designed primarily for providing remote access to sites that authenticate users by IP address.
- ffproxy is a filtering HTTP/HTTPS proxy server.
- Glype is a free proxy server written in PHP.
- ISA Server - A User-configurable web proxy and firewall / VPN product that also offers SOCKS4.3A proxy; used in forward and reverse proxy scenarios.
- JAP - A local proxy, web anonymizer software connecting to proxy server chains of different organisations
- Mavis - A peer-to-peer (P2P) proxy sharing program.
- m0n0wall - lightweight firewall solution along with NAT proxy server, based on UNIX system.
- 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.
- 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/S, NSAPI I/O filters, dynamic reconfiguration, SOCKSv5 and reverse proxy.
- Squid is a popular HTTP proxy server in the UNIX/Linux world.
- tinyproxy is a fast light-weight HTTP proxy ideal for embedded use on POSIX operating systems.
- Tor - A proxy-based anonymizing Internet communication system.
- 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.
- Ziproxy is a non-caching proxy for acceleration purposes. It recompresses pictures and optimizes HTML code.