SOCKS

SOCKS is an Internet protocol that exchanges network packets between a client and server through a proxy server. SOCKS5 optionally provides authentication so only authorized users may access a server. Practically, a SOCKS server proxies TCP connections to an arbitrary IP address, and provides a means for UDP packets to be forwarded. A SOCKS server accepts incoming client connection on TCP port 1080.
The protocol was originally developed/designed 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, making SOCKS publicly available. The protocol was extended to version 4 by Ying-Da Lee of NEC.
The SOCKS reference architecture and client are owned by Permeo Technologies, a spin-off from NEC. (Blue Coat Systems bought out Permeo Technologies, and were in turn acquired by Symantec.)
The SOCKS5 protocol was originally a security protocol that made firewalls and other security products easier to administer. It was approved by the IETF in 1996. The protocol was developed in collaboration with Aventail Corporation, which markets the technology outside of Asia.
SOCKS is sometimes defined as an acronym for "socket secure" from at least 2001, although it was not originally defined as such in the SOCKS Protocol Version 5 RFC in 1996 or the UNIX Security Symposium III paper in 1992 but simply referred to a specific proxy protocol designed to facilitate communication between clients and servers through a firewall.
SOCKS is a de facto standard for circuit-level gateways (level 5 gateways).
The circuit/session level nature of SOCKS make it a versatile tool in forwarding any TCP (or UDP since SOCKS5) traffic, creating an interface for all types of routing tools. It can be used as:
A typical SOCKS4 connection request looks like this:
For example, this is a SOCKS4 request to connect Fred to 66.102.7.99:80, the server replies with an "OK":
From this point onwards, any data sent from the SOCKS client to the SOCKS server is relayed to 66.102.7.99, and vice versa.
The command field may be 0x01 for "connect" or 0x02 for "bind"; the "bind" command allows incoming connections for protocols such as active FTP.
SOCKS4a extends the SOCKS4 protocol to allow a client to specify a destination domain name rather than an IP address; this is useful when the client itself cannot resolve the destination host's domain name to an IP address. It was proposed by Ying-Da Lee, the author of SOCKS4.
The client should set the first three bytes of DSTIP to NULL and the last byte to a non-zero value. (This corresponds to IP address 0.0.0.x, with x nonzero, an inadmissible destination address and thus should never occur if the client can resolve the domain name.) Following the NULL byte terminating USERID, the client must send the destination domain name and terminate it with another NULL byte. This is used for both "connect" and "bind" requests.
Client to SOCKS server:
Server to SOCKS client: (Same as SOCKS4)
A server using protocol SOCKS4a must check the DSTIP in the request packet. If it represents address 0.0.0.x with nonzero x, the server must read in the domain name that the client sends in the packet. The server should resolve the domain name and make connection to the destination host if it can.
The SOCKS5 protocol is defined in RFC 1928. It is an incompatible extension of the SOCKS4 protocol; it offers more choices for authentication and adds support for IPv6 and UDP, the latter of which can be used for DNS lookups. The initial handshake consists of the following:
The initial greeting from the client is:
The subsequent authentication is method-dependent. Username and password authentication (method 0x02) is described in RFC 1929:
After authentication the connection can proceed. We first define an address datatype as:
Since clients are allowed to use either resolved addresses or domain names, a convention from cURL exists to label the domain name variant of SOCKS5 "socks5h", and the other simply "socks5". A similar convention exists between SOCKS4a and SOCKS4.
Client software must have native SOCKS support in order to connect through SOCKS.
There are programs that allow users to circumvent such limitations:
Socksifiers allow applications to access the networks to use a proxy without needing to support any proxy protocols. The most common way is to set up a virtual network adapter and appropriate routing tables to send traffic through the adapter.
Due to lack of request and packets exchange encryption it makes SOCKS practically vulnerable to man-in-the-middle attacks and IP addresses eavesdropping which in consequence clears a way to censorship by governments.