Science Fair Project Encyclopedia
A proxy server is a computer network service which allows 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, possibly 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.
A common proxy application is a caching 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 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 least-recently used documents, and LFU removes the least-frequently used documents.
Web proxies can also filter the content of Web pages served. 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.
Network address translation proxies
Many organizations — including corporations, schools, and families — use proxy servers to enforce network use policies (see censorware) or provide security and caching services. Usually, the web or NAT proxy is not transparent to the client application: it must be configured to use the proxy, manually or with a configuration script. Thus, the user can evade the proxy by simply resetting the client configuration. A transparent proxy or transproxy combines a proxy server with NAT so that connections are routed into the proxy without client-side configuration.
The term proxy is also used in a different sense in SIP, the Session Initiation Protocol used in many modern voice over IP systems. A SIP Proxy, unlike a Web proxy, does not handle the content of client data.
Open proxies, abuse, and detection
Both Web and other network proxies have been abused by spammers and other network abusers. 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' Microsoft Windows computers 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 are implicated in 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 . Likewise, an email server may be configured to automatically test e-mail senders for open proxies, using software such as Michael Tokarev's
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.
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 destination 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 distribution: 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 have 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
Popular proxy servers
- The Squid cache is the popular HTTP proxy server in UNIX/Linux world
- The Apache HTTP Server can be configured to act as a proxy server.
- Wingate is a small Internet Protocol proxy server that can be used to redirect any kind traffic on a Microsoft Windows host. A more free proxy server is AnalogX's proxy
- Windows 98se and other versions of Windows released later have Internet Connection Sharing (ICS) built-in. This gives NAT functionality to any Windows computer with an Internet connection and a LAN connection.
- The Linux kernel contains modules for NAT functionality as part of the ipchains and iptables packet filtering systems.
- OpenBSD's Packet Filter is an extremely flexible interface for inspecting network traffic that can provide, among many other things, NAT functionality.
- The elhttp server is a lightweight HTTP proxy for UNIX and Windows (GPL license)
- Microsoft's ISA Server is a product that runs on Windows 2000/2003 servers and combines the functions of both a proxy server and a firewall.
- Anonymity and Internet privacy
- Web Proxy Autodiscovery Protocol WPAD
- Firewall (networking)
Web-based proxy servers
- Free Proxy Server
- Anonymous Browsing Quick-Start Page
- Free proxies
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