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. 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, Anonymous, Distorting, High anonymous (elite) (see my previous post). Now HTTP proxy servers are the most widespread. Their support (ability to use them) is included into many programs: browsers, download managers etc.
SOCKS is an Internet protocol that facilitates the routing of network packets between client-server applications via a proxy server. Though this protocol was developed long ago enough, it is rather new (compared to the HTTP proxy). SOCKS allows to work with any protocols. A socks proxy simply transfers data from a client to a server, not penetrating into this data contents (therefore it can work with HTTP, FTP, SMTP, POP3, NNTP, etc.).
As SOCKS (as it was already marked above) transfers all data from a client to a server, nothing adding from itself, from the point of view of a web-server, a socks proxy is a client. Therefore anonymity of this type of proxy servers is really always absolute.
At present time there exist 2 versions of the SOCKS protocol: 4 and 5. The 4th version, as it appeared earlier, is more widespread. However, now the 5th version is also supported by many popular programs such as ICQ (old ICQ versions, in general, could use only SOCKS proxies), Napster, AudioGalaxy, EeDoonkey2000 (MP3 files downloaders) etc. Generally speaking, as a SOCKS proxy can “proxy” any TCP / UDP protocol, it could be used also by mail programs, but they do not do it. Therefore, for such programs (which cannot work with socks) there was developed a special software automatically enabling TCP and UDP networking client applications to traverse a SOCKS server. The most popular program is the SocksCap. It allows to capture all traffic of a program and to route it onto a SOCKS proxy 4 or 5.
The Common Gateway Interface (CGI) is a standard protocol that defines how webserver software can delegate the generation of webpages to a console application. Such applications are known as CGI scripts; they can be written in any programming language, although scripting languages are often used. For use of a CGI proxy you do not need to change your browser settings, to install additional programs or to make something else more complex. It is enough, if one opens a web page of CGI proxy in a browser, enters a required URL into the address field and press the “Go” button, then you will get onto page, which URL you have specified to a CGI proxy. But address of this page (in the address line) from the point of view of your computer will be another, something like this: http://www.cgi-proxy.com/http/www.your-url.com/path/. CGI proxy supports HTTP and (sometimes) FTP and HTTPS protocols. As for anonymity, CGI proxies are the same, as the HTTP proxies. The disadvantages in usage of the CGI proxies are presence of additional advertising (which adds a CGI proxy itself, they simply exist at the expense of such advertising), a limited FTP / HTTPS support (not always supported) and sometimes a CGI proxy do not allow to view pictures.
Proxy types comparsion table:
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