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Proxy server

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File:Proxyserverdiagram.PNG
A diagram of a standard proxy

A proxy server is a computer that offers a computer network service to allow clients to make indirect network connections to other network services. A client connects to the proxy server, then requests a connection, file, or other resource available on a different server. The proxy provides the resource either by connecting to the specified server or by serving it from a cache. In some cases, the proxy may alter the client's request or the server's response for various purposes. It is frequently used in schools to bypass the block list of certain sites.

Types

Web

A common proxy application is a web proxy. This provides a nearby cache of web pages and files available on remote web servers, allowing local network clients to access them more efficiently, quickly or reliably.

When it receives a request for a web resource (specified by a URL), a caching proxy looks for the resulting URL in its local cache. If found, it returns the document immediately. Otherwise it fetches it from the remote server, returns it to the requester and saves a copy in the cache. The cache usually uses an expiry algorithm to remove documents from the cache, according to their age, size, and access history. Two simple cache algorithms are Least Recently Used (LRU) and Least Frequently Used (LFU). LRU removes the documents that have been left the longest, while LFU removes the least popular documents. The algorithms can also be combined.

Some censorware applications — which attempt to block offensive web content — are implemented as web proxies. Other web proxies reformat web pages for a specific purpose or audience; for example, Skweezer reformats web pages for cell phones and PDAs. Network operators can also deploy proxies to intercept computer viruses and other hostile content served from remote web pages.

A special case of web proxies are "CGI proxies." These are web sites which allow a user to access a site through them. They generally use PHP or CGI to implement the proxying functionality. CGI 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."

SSL

A growing subset of Web traffic uses HTTPS to create an encrypted tunnel. Regulations often require all Intranet and ASP applications handling sensitive data to use encryption. Traditional (HTTP) proxies can not cache encrypted traffic; hence provide no acceleration, control or bandwidth savings for SSL applications.

HTTPS Proxies are specifically designed to decrypt, apply policy, cache and re-encrypt SSL traffic. As a result, HTTPS Proxies can transparently monitor, control and accelerate SSL traffic. An HTTPS Proxy can also apply malware scanning and content filtering to inhibit phishing, spyware and computer viruses hiding inside encrypted tunnels.

There are privacy concerns with SSL proxies. In essence, the IT department is conducting a Man-in-the-middle attack, potentially exposing sensitive corporate information, personal online banking information, etc. Countries such as Sweden have very strong privacy laws, which may require the proxy (*) to handle corporate information with different policy than personal traffic. Content filtering can differentiate this traffic. Additional compromise policy options include caching GIF and JPEG objects (which typically are user interface elements and lack confidential data), but exclude from caching HTML and TXT traffic.

Sometimes the term "SSL proxy" also refers to CGI web proxies that are accessible via encrypted SSL connections. In this case, SSL adds an extra layer of security on top of the CGI proxy system, lessening the chance of data interception.

(*) In Sweden, privacy laws apply to non-SSL proxies too. PUL (Personuppgiftslagen) doesn't differ between encrypted and unencrypted data. But as long as no details about the identity of the user is stored or collected, PUL (a Swedish privacy law, see above) doesn't apply, and unrestricted scanning of SSL information is allowed, as long as you don't arrange it in a way that may look like that you are trying to steal bank information. Then its called "preparing to fraud" by swedish law, and is punishable. (Example: Making sure the SSL-proxy only "activates" itself when a specific internetbank URL is accessed will be considered "preparing to fraud")

Intercepting

Many organizations — including corporations, schools, and families — use a proxy server to enforce acceptable network use policies (see censorware) or to provide security, anti-malware and/or caching services. A traditional web proxy is not transparent to the client application, which must be configured to use the proxy (manually or with a configuration script). In some cases, where alternative means of connection to the Internet are available (e.g. a SOCKS server or NAT connection), the user may be able to avoid policy control by simply resetting the client configuration and bypassing the proxy. Furthermore administration of browser configuration can be a burden for network administrators.

An intercepting proxy, often incorrectly called transparent proxy (also known as a forced proxy) combines a proxy server with NAT. Connections made by client browsers through the NAT are intercepted and redirected to 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.

Intercepting proxies are also commonly used by Internet Service Providers in many countries in order to reduce upstream link bandwidth requirements by providing a shared cache to their customers.

It is often possible to detect the use of a 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.

The term transparent proxy, which is often incorrectly used instead of intercepting proxy to describe the same behaviour, is defined in RFC 2616 (which defines the HTTP/1.1 protocol) this way:

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

Open

An open proxy is a proxy server which will accept client connections from any IP address and make connections to any Internet resource. Abuse of open proxies is currently implicated in a significant portion of e-mail spam delivery. Spammers frequently install open proxies on unwitting end users' operating systems by means of computer viruses designed for this purpose. Internet Relay Chat (IRC) abusers also frequently use open proxies to cloak their identities.

Because proxies might be used for abuse, system administrators have developed a number of ways to refuse service to open proxies. IRC networks such as the Blitzed network automatically test client systems for known types of open proxy [1]. Likewise, an email server may be configured to automatically test e-mail senders for open proxies, using software such as Michael Tokarev's proxycheck [2].

Groups of IRC and electronic mail operators run DNSBLs publishing lists of the IP addresses of known open proxies, such as AHBL, CBL [3], NJABL [4], 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. [5] Others consider the client to have solicited the scan by connecting to a server whose terms of service include testing.

Reverse

A reverse proxy is a proxy server that is installed in the neighborhood of one or more webservers. All traffic coming from the Internet and with a destination of one of the webservers is going through the proxy server. There are several reasons for installing reverse proxy servers:

  • Security: the proxy server is an additional layer of defense and therefore protects the webservers further up the chain
  • Encryption / SSL acceleration: when secure websites are created, the SSL encryption is often not done by the webserver itself, but by a reverse proxy that is equipped with SSL acceleration hardware. See Secure Sockets Layer.
  • Load balancing: the reverse proxy can distribute the load to several webservers, each webserver serving its own application area. In such a case, the reverse proxy may need to rewrite the URLs in each webpage (translation from externally known URLs to the internal locations)
  • Serve/cache static content: A reverse proxy can offload the webservers 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: if a program is producing the webpage on the webservers, the webservers can produce it, serve it to the reverse-proxy, which can spoon-feed it however slowly the clients need and then close the program rather than having to keep it open while the clients insist on being spoon fed.

Split

A split proxy is essentially a pair of proxies installed across two computers. Since they are effectively two parts of the same program, they can communicate with each other in a more efficient way than they can communicate with a more standard resource or tool such as a website or browser. This is ideal for compressing data over a slow link, such as a wireless or mobile data service and also for reducing the issues regarding high latency links (such as satellite internet) where establishing a TCP connection is time consuming. Taking the example of web browsing, the user's browser is pointed to a local proxy which then communicates with its other half at some remote location. This remote server fetches the requisite data, repackages it and sends it back to the user's local proxy, which unpacks the data and presents it to the browser in the standard fashion .

See Google's Web Accelerator

Circumventor

A circumventor is a web-based page that takes a site that is blocked and "circumvents" it through to an unblocked website, 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.

The most common use is in schools, many blocking programs block circumventors as loopholes, only by site not by code. That means that every now and then someone can get around it. Circumventors are also used by people who have been blocked from a website.

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.

Anonymous

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 possible, and has been demonstrated (see, for example, Sugarcane) 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 requestor, 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 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 properly encrypted) through the proxy.

More of an inconvenience than a risk, proxy users may find themselves being blocked from certain websites, as numerous forums and websites block IP addresses from proxies known to have spammed or trolled the site.

  • The Squid cache is a popular HTTP proxy server in the UNIX/Linux world.*
  • The HTTP-Tunnel is a popular HTTP proxy server and Client for Windows.*
  • 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.
  • 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. It also provides firewall, VPN and mail server functionality.
  • Privoxy is a free, open source web proxy with privacy features
  • Microsoft Internet Security and Acceleration Server is a product that runs on Windows 2000/2003 servers and combines the functions of both a proxy server and a firewall.
  • Tor - A proxy-based anonymyzing Internet communication system
  • Proxomitron - User-configurable web proxy used to re-write webpages on the fly. Most noted for blocking ads, but has many other useful features.
  • PHProxy is a Web HTTP proxy programmed in PHP to bypass firewalls and other proxy restrictions through a Web interface very similar to the popular CGIProxy.
  • SJSWebProxy (SunMicrosystems) is a proxy server for HTTP and HTTPS (CONNECT) requests. It can also serve as a gateway for Ftp and Gopher traffic. It is also free for download.
  • Nginx Web and Reverse proxy server, that can act as POP3 proxy server.
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