Proxy Server - SOCKS, HTTP proxy

Types of proxy

Proxy server can be placed in the user's local computer or at various points between the user and the destination servers on the Internet.

• A proxy server that passes requests and responses unmodified is usually called a gateway or sometimes tunneling proxy.
• A forward proxy is an Internet-facing proxy used to retrieve from a wide range of sources (in most cases anywhere on the Internet).
• A reverse proxy is (usually) an Internet-facing proxy used as a front-end to control and protect access to a server on a private network, commonly also performing tasks such as load-balancing, authentication, decryption or caching. 
• Performance Enhancing Proxies (PEPs) are network agents designed to improve the end-to-end performance of some communications protocol. Performance Enhancing Proxies standards are defined in RFC 3135 (Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations) and RFC 3449 (TCP Performance Implications of Network Path Asymmetry).

SOCKet Secure (SOCKS) is an Internet protocol that routes network packets between a client and server through a proxy server. SOCKS5 additionally provides authentication so only authorized users may access a server. Practically, a SOCKS server will proxy TCP connections to an arbitrary IP address as well as providing a means for UDP packets to be forwarded.

Comparison

SOCKS operates at a lower level than HTTP proxying: SOCKS uses a handshake protocol to inform the proxy software about the connection that the client is trying to make and may be used for any form of TCP or UDP socket connection, whereas an HTTP proxy works for HTTP protocol and HTTP only (and HTTP is usually transported over a TCP connection). HTTP proxies take an HTTP request and forward it to an HTTP server. Though HTTP proxying has a different use-case in mind, the CONNECT^[8] method allows one to forward TCP connections^{[citation needed]}, there is however no mechanism for UDP proxying. The following examples show the difference between the two methods:

SOCKS

Bill wishes to communicate with Jane over the internet, but a firewall exists on his network between them. Bill is not authorized to communicate through it himself. He connects to the SOCKS proxy on his network and sends it information about the connection he wishes to make to Jane. The SOCKS proxy opens a connection through the firewall and facilitates the communication between Bill and Jane. For more information on the technical specifics of the SOCKS protocol, see the sections below.

HTTP

Bill wishes to download a web page from Jane, who runs a web server. Bill cannot directly connect to Jane's server, as a firewall has been put in place on his network. In order to communicate with the server, Bill connects to his network's HTTP proxy. His web browser communicates with the proxy in exactly the same way it would with the target server—it sends a standard HTTP request header. The HTTP proxy reads the request and looks for the host header. It then connects to the server specified in the header and transmits any data the server replies with back to Bill.^[9]

Interaction with firewalls

Many company and university network administrators set firewall rules that prevent users from connecting to any internet service apart from webpages. Both the SOCKS and HTTP proxy protocols can be used to pierce these firewalls. SOCKS is usually used to create a raw TCP connection, and the HTTP proxy protocol can do the same with the CONNECT method. In both cases a TCP connection is created from the client to the proxy server, and the IP address and port to which the client requests a connection is communicated over the connection. In both cases the proxy server can grant, reject, redirect and alter connection requests as it likes. HTTP proxies are traditionally more HTTP protocol aware and do more high level filtering (even though that usually only applies to GET and POST methods, not CONNECT). SOCKS proxies can also forward UDP traffic and work in reverse: HTTP proxies cannot.

http://en.wikipedia.org/wiki/SOCKS
HTTP proxying
Performance Enhancing Proxy
rfc2616 - HTTP