Introduction to Proxy Servers in Computer Networking

Privacy and speed are among the benefits

PROXY server

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A proxy server acts as an 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 employ proxy servers as part of their services. Finally, a category of third-party hosted websites called web proxy servers is available to internet users for their web browsing sessions.

Key Features of Proxy Servers

Proxy servers traditionally provide three main functions:

  • Firewall and network data filtering support
  • Network connection sharing
  • Data caching

Proxy Servers, Firewalls, and Content Filtering

Proxy servers work at the application layer (layer 7) of the Open Systems Interconnection (OSI) model. They differ from traditional network firewalls that work at lower OSI layers, and they support application-independent filtering. Proxy servers are also more difficult to install and maintain than firewalls, because the administrator must configure proxy functionality for each application protocol (e.g., HTTP, SMTP, and SOCKS) 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 servers.

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 outgoing requests for web pages by inspecting HTTP messages. Network administrators can use this feature to 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 also can filter based on application content inside the messages.

Connection Sharing With Proxy Servers

Many years ago, third-party software products were common in home networks to share the internet connection of one computer with others. Now, however, home broadband routers provide internet connection sharing functions as a matter of course. Not so with corporate networks, though: They typically employ proxy servers to distribute internet connections across multiple routers and local intranet networks.

Proxy Servers and Caching

Web page caching by proxy servers can improve a network's user experience in three ways:

  • Can conserve bandwidth on the network, increasing its scalability.
  • Can improve response time. With an HTTP proxy cache, for example, web pages can load more quickly into the browser.
  • Can 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 websites to dynamic, database-driven content, the benefit of proxy caching has declined somewhat over the years.

Web Proxy Servers

Many businesses deploy proxy servers that are physically connected to their internal networks. Most home networks don't use them, however, because home broadband routers supply the essential firewall and connection-sharing capabilities.

Web proxies, a separate class of proxy servers, allow 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. These services offer other benefits, too, including caching. Some web proxy servers are free to use, while other charge service fees.