Essential Concepts - Proxy Server

The concept of proxy is similar to the concept of firewall but it differs by its use. Both are a part of Network Security. So I recommend you to read my post on Firewall before you start understanding proxy.

In a computer network, a proxy server is any computer system offering a service that acts as an intermediary between the two communicating parties, the client and the server. A proxy server is a computer that offers a computer network service to allow clients to make indirect network connections to other network services.

How Proxy Server works?

In the presence of a proxy server, there is no direct communication between the client and the server. Instead, the client connects to the proxy server and sends requests for resources such as a document, web page or a file that resides on a remote server.

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.

As shown in the above example, whenever the client connects to a web proxy server and makes a request for the resources (in this case, “Sample.html”) that reside on a remote server (in this case, xyz.com), the proxy server forwards this request to the target server on behalf of the client so as to fetch the requested resource and deliver it back to the client. An example of client can be a user operated computer that is connected to the Internet.

Functions of a proxy server :

Proxy servers are used for many different purposes depending on its function

• A proxy server is most widely used to conceal the IP address or the origin of the Internet users during their activity. Since it the proxy server which handles the requests between the client and the target, only the IP address of the proxy server is exposed to the outside world and not the actual one. Therefore, most hackers use a proxy server during the attacks on their target so that it would be hard to trace back to them. 
• Proxies may also cache web pages. Each time an internal user requests a URL from outside, a temporary copy is stored locally. The next time an internal user requests the same URL, the proxy can serve the local copy instead of retrieving the original across the network, improving performance.
• 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.
• Web proxies can also reformat web pages for a specific purpose or audience; for example, Opera mini (browser uses its own proxy server) reformats web pages for mobile devices in order to provide faster speed.
• A special case of web proxies are "CGI proxies." These are web sites which allow a user to access a site through them. 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.