The English used in this article may not be easy for everybody to understand. (April 2012)
In computer networks, a proxy server is a server (computer) which clients (people or computers) use to access other computers. A proxy server that passes information to its clients without changing it is usually called a gateway or sometimes tunneling proxy.
Proxy servers get their name because they act like a proxy (a stand-in) and act on behalf of a Client's computer.
A client that connects to the proxy server requests some service, such as a file, connection, web page, or other resource, that is available on a different server. The proxy server then goes to the other server and requests what the client wants for them.
A proxy server can change the information that it gives to the client, and if the same information is accessed many times or by many different clients it can use what is called a cache to make things faster. A cache is the term for information that has been accessed and saved for future use; if a proxy server has what the client is accessing in its cache, it will make it faster because it does not need to go and access the other server to provide what the client wants.
A proxy server can be placed anywhere in the connection between the client and the server, which could include software on the client computer itself or on any computer between.
Some proxy servers use Secure Sockets Layer (SSL) to secure the connection between the client and the remote server. This security layer helps to make sure that no other computers can read or understand what the client is asking from the server.
Benefits of proxy servers
There are many benefits of using a proxy server. First, the client machine can exchange data with the remote server without making a direct connection. This way, the client's real internet address will not be known to the remote server. This is sometimes called anonymitizing because it makes the client anonymous. A second advantage is that when the proxy server itself is able to serve the request made by the client, it will not contact the remote server any more. So the load on the remote server will be reduced by using a proxy server. This type of proxy servers are called caching servers.
Big organizations (or even countries) sometimes use proxy servers to control access to the Internet. A large bank may use a proxy server that only allows connections to other websites relevant to banking. The proxy server might however block access to Websites offering free email or serving pornographic material. It might also block access to file sharing applications. Limiting access to specific content on the internet is also called internet filtering.
Types and functions
A proxy server may have one or more of the functions described below:
Caching proxy server
A caching proxy server can service requests from clients, without contacting the remote server; instead, it sends the data which it has stored from a previous request. This is called caching.
Caching proxy servers reduce the work load of the remote server. However, they their own problems, especially if they are not configured well. Some problems are described in RFC 3143.
A web proxy is a proxy server that focuses on the traffic over the World Wide Web. It may be used to block offensive web content, or to control access of clients to online content. They may be used by corporations or countries.
Web proxies can also be used to track how different individuals have used internet access.
Anonymizing proxy server
An anonymizing proxy server removes identifying information from the clients' requests, for the purpose of anonymity. They may also be used to break through filtered contents on the internet.
A proxy server is called an open proxy if everyone can connect to it and use it. Usually, open proxies are proxy servers which are badly configured. Open proxies may be abused easily; for example, a bad user may send a corrupting request to a remote server, but hide himself behind an open proxy, so the administrators of the remote server cannot stop him. Open proxies can also be used for spamming. For this reason, some websites do not allow connections to their web servers or to edit content on them through known open proxies.
A forced proxy server is a proxy server which handles all of the traffic from the client to the internet. The client will not know that the proxy exists, but all of the information passes through the proxy server. They are sometimes called "transparent" proxy servers, because the user does not know a proxy server is between the client and remote server.
A Transparent SMTP Proxy is a SMTP proxy server that is inserted in between a sending mail server and a receiving mail server. The main purpose of the SMTP proxy is to filter outbound spam. The proxy masquerades itself in such a way that the client and server believe that they are talking to each other even though there is a proxy in between.
There is a lot of software which can be used for running a proxy server. Some software can only work as a proxy server, while other software can also work as a firewall or caching server. Squid, Varnish and Microsoft Internet Security and Acceleration Server (ISA Server) are among the best known pieces of proxy server software. Some proxy software uses the SOCKS protocol. An example is the Java SOCKS Proxy Server.
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