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Introduction to Proxy Servers in Computer Networking

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Proxy servers work as a intermediary between the two ends of a client/server network connection. Proxy servers interface with network applications, most commonly Web browsers and servers. Inside corporate networks, proxy servers are installed on specially-designated internal (intranet) devices. Some Internet Service Providers (ISPs) also utilize proxy servers as part of providing online services to their customers. Finally, a category of third-party hosted Web sites called Web proxy servers is available to end users on the Internet for their Web browsing sessions.

Key Features of Proxy Servers

Proxy servers traditionally provide three main functions:
  1. firewall and network data filtering support
  2. network connection sharing
  3. data caching

Proxy Servers, Firewalls and Content Filtering

Proxy servers work at the Application layer (layer 7) of the OSI model. They differ from traditional network firewalls that work at lower OSI layers and support application-independent filtering. Proxy servers are also more difficult to install and maintain than firewalls, as proxy functionality for each application protocol like HTTP, SMTP, or SOCKS must be configured individually. However, a properly configured proxy server improves network security and performance for the target protocols.

Network administrators often deploy both firewall and proxy server software to work in tandem, installing both firewall and proxy server software on network gateway server.

Because they function at the OSI Application layer, the filtering capability of proxy servers is relatively more sophisticated compared to that of ordinary routers. For example, proxy Web servers can check the URL of an outgoing requests for Web pages by inspecting HTTP messages. Network administrators can use this feature bar access to illegal domains but allow access to other sites. Ordinary network firewalls, in contrast, cannot see the Web domain names inside HTTP request messages. Likewise for incoming data traffic, ordinary routers can filter by port number or IP address, but proxy servers can also filter based on application content inside the messages.

Connection Sharing with Proxy Servers

Many years ago, third-party software products were commonly used on home networks to share the Internet connection of one PC with other computers. Home broadband routers now provide Internet connection sharing functions in most homes instead. On corporate networks, however, proxy servers are still commonly employed to distribute Internet connections across multiple routers and local intranet networks.

Proxy Servers and Caching

The caching of Web pages by proxy servers can improve a network's user experience in three ways. First, caching may conserve bandwidth on the network, increasing its scalability. Next, caching can improve response time experienced by clients. With an HTTP proxy cache, for example, Web pages can load more quickly into the browser. Finally, proxy server caches increase content availability. Copies of Web pages and other static content in the cache remain accessible even if the original source or an intermediate network link goes offline. With the trend of Web sites to dynamic database drive content, the benefit of proxy caching has declined somewhat compared to the years ago.

Web Proxy Servers

While many businesses deploy proxy servers physically connected to their internal networks, most home networks don't use them because home broadband routers supply the essential firewall and connection sharing capabilities. A separate class of proxy servers called Web proxies exists that allows users to take advantage of some proxy server benefits even when their own local network doesn't support them. Internet users most commonly seek out Web proxy services as a means to increase their privacy while surfing online, although these services offer other benefits too including caching. Some Web proxy servers are free to use, while other charge service fees.

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