A proxy server is a kind of buffer between your computer and the Internet resources you are accessing (Web sites, FTP archives etc.). The data you request come to the proxy first, and only then it transmits the data to you.

HTTP proxy server: what is it?

HTTP proxy server is a proxy, allowing working on the Internet with HTTP and (not always) FTP protocols. It (as well as other proxy servers) can carry out caching of information downloaded from the Internet. Let's have a more detailed look over its abilities.

Anonymity of HTTP Proxy

HTTP proxy servers have several anonymity levels. It depends on purposes, which a proxy is used for, so an anonymity requirement is not always an essential one. Conditionally HTTP proxy servers by their anonymity degree could be divided onto:

Transparent - these proxies are not anonymous. They, first, let a web server know that there is used a proxy server and, secondly, "give away" an IP-address of a client. The task of such proxies, as a rule, is information caching and/or support of Internet access for several computers via single connection.
Anonymous - these proxy servers let a remote computer (web-server) know, that there is used a proxy, however, they do not pass an IP-address of a client.
Distorting - unlike the previous type, they transfer an IP-address to a remote web-server, however, this address is a phantom: randomly generated by a proxy server or any fixed (not your) IP. So, these proxies distort your IP address from the point of view of a web-server.
High anonymous (elite) – they do not send your IP-address to a remote computer. Also, they do not inform that there is used any proxy server! So, a web-server "thinks” that it works directly with a client.

This anonymity classification is rather conventional. There are many other classifications, specifying / expanding / generalizing this one.

Using HTTP Proxies

Now HTTP proxy servers are the most widespread. Their support (ability to use them) is included into many programs: browsers, download managers etc. However, their support is not realized at a level of an operating system – in order to use them, you should configure all programs, which should use proxies, in an appropriate way

HTTP Proxy Chaining

HTTP proxies could be organized into a chain and this will significantly improve your anonymity on the Internet. However, the construction of such chain is related to a number of difficulties. The main difficulty is that for connection into a chain you should use special software. Browsers (and majority of programs) do not allow to create and use chains of proxy servers. To organize proxies into a chain, it is necessary to organize tunneling of requests: there is created a virtual tunnel, which passes through an HTTP proxy and, using this tunnel, the program can "make a path" through some proxy servers to a specific web server.
Besides that, in order to carry out tunneling of requests, an HTTP proxy should support SSL (Secure Sockets Layer). This is an additional feature designed for protection of connections from interception and decoding. Except for pure protection, the SSL allows to organize "a virtual tunnel" through a chain of proxy servers. To find out whether a proxy supports a SSL server is quite simple: access using this proxy any site, where set the HTTPS protocol (for example mail server HotMail.com). Make sure that in an address line before a server name there is the https:// prefix and if you could see this page through a proxy server, it means your proxy supports the SSL.
An example of a program, which is capable to create such virtual tunnel, is SocksChain (http://www.ufasoft.com). This program allows to work with any Internet service through a chain of SOCKS or HTTP proxies.
Other example is the HTTPort program (http://www.htthost.com). It can organize a chain from not more than 2 proxies, but it has some advantages. It allows creating a port mapping through HTTP proxy.

Connection of various types proxies into a chain

You can unite not only HTTP proxies into a chain. In a chain you can "mix" them with other types of proxy servers - SOCKS and CGI. By the level of its abilities (support of various protocols, transparency in usage etc.) HTTP proxy occupies an intermediate place between SOCKS and CGI proxies. In a chain they also should occupy an intermediate place: after SOCKS proxy (if those are available in a chain) and before CGI proxy (if they present).
So, the chains can look like:
SOCKS proxy >>>> HTTP proxy >>>> CGI proxy SOCKS proxy >>>> HTTP proxy HTTP proxy >>>> CGI proxy
possible look like:
HTTP proxy >>>> SOCKS proxy
and cannot look like:
CGI proxy >>>> HTTP proxy

What should you do, if there is already a corporate proxy server?

If an organization uses a corporate proxy and the Internet access is possible only through it, you can build a chain, being based on a type of your corporate proxy:

if it is a SOCKS proxy, there is no problem, you can build any chains
if it is an HTTP proxy, you can create a chain, using HTTP and CGI proxies (possible and SOCKS proxies)

How to check mail in mail programs (such as Outlook) from a corporate proxy?

For this purpose you need to organize a port mapping through your proxy server. With this purpose you can take advantage of the HTTPort program (http://www.htthost.com). You must customize both the HTTPort program, and your mail program. After start of the HTTPort program you need to customize port mapping on your mail server (for example to connect with your local port 9110 the pop.mail.yahoo.com port 110 server and smtp.mail.yahoo.com port 25 - with your port 9025). Besides it, in your program setting, you must specify a name of a server and port of a corporate proxy and activate the program, having pressed the "start" button. After that you need to customize the mail program for getting mail from the