A SOCKS based server operates at the Session layer of the OSI model.
SOCKS is an Internet protocol that allows client-server applications to transparently use the services of a network firewall. SOCKS is an abbreviation for "SOCKetS". As of Version 5 of SOCK, both UDP and TCP is supported.
One of the best known circuit-level proxies is SOCKS proxy server. The basic purpose of the protocol is to enable hosts on one side of a SOCKS server to gain access to hosts on the other side of a SOCKS Server, without requiring direct “IP-reachability”
The protocol was originally developed by David Koblas, a system administrator of MIPS Computer Systems. After MIPS was taken over by Silicon Graphics in 1992, Koblas presented a paper on SOCKS at that year's Usenix Security Symposium and SOCKS became publicly available. The protocol was extended to version 4 by Ying-Da Lee of NEC.
SOCKS includes two components, the SOCKS server and the SOCKS client.
The SOCKS protocol performs four functions:
Making connection requests
Setting up proxy circuits
Relaying application data
Performing user authentication (optional)
Source:
KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 3: Telecommunications and Network Security (page 96).
and
http://en.wikipedia.org/wiki/SOCKS
and
http://www.faqs.org/rfcs/rfc1928.html
and
The ISC2 OIG on page 619
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