SOCKS

This article is about an internet protocol. For other uses, see Socks (disambiguation).

SOCKS is an Internet protocol that facilitates the routing of network packets between client-server applications via a proxy server. SOCKS performs at Layer 5 of the OSI model - the Session Layer (an intermediate layer between the presentation layer and the transport layer).

History

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.

The SOCKS reference architecture and client are owned by Permeo Technologies. (Note that Permeo Technologies has been bought out by Blue Coat Systems a spin-off from NEC.)

Comparison between SOCKS and HTTP proxies

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 a HTTP proxy analyses the HTTP headers sent through it in order to deduce the address of the server and therefore may only be used for HTTP traffic. The following examples demonstrate the difference between the SOCKS and HTTP proxy protocols:

SOCKS

Bill wishes to communicate with Jane over the internet, but a firewall exists on his network between them and Bill is not authorised to communicate through it himself. Therefore, he connects to the SOCKS proxy on his network and sends 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 internet browser communicates with the proxy in exactly the same way it would 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.

SOCKS 4 protocol

A typical SOCKS 4 connection request looks like this (each number is one byte):

Client to SOCKS Server:

• field 1: SOCKS version number, 1 byte, must be 0x04 for this version
• field 2: command code, 1 byte:
  • 0x01 = establish a TCP/IP stream connection
  • 0x02 = establish a TCP/IP port binding
• field 3: network byte order port number, 2 bytes
• field 4: network byte order IP address, 4 bytes
• field 5: the user ID string, variable length, terminated with a null (0x00)

Server to SOCKS client:

• field 1: null byte
• field 2: status, 1 byte:
  • 0x5a = request granted
  • 0x5b = request rejected or failed
  • 0x5c = request failed because client is not running identd (or not reachable from the server)
  • 0x5d = request failed because client's identd could not confirm the user ID string in the request
• field 3: 2 arbitrary bytes, that should be ignored
• field 4: 4 arbitrary bytes, that should be ignored

Example:

This is a SOCKS 4 request to connect Fred to 66.102.7.99:80, the server replies with an "OK".

• Client: 0x04 | 0x01 | 0x00 0x50 | 0x42 0x66 0x07 0x63 | 0x46 0x72 0x65 0x64 0x00
  • The last field is 'Fred' in ASCII, followed by a null byte.
• Server: 0x00 | 0x5a | 0xXX 0xXX | 0xXX 0xXX 0xXX 0xXX
  • 0xXX can be any byte value. The Socks 4 protocol specifies the values of these bytes should be ignored.

From this point on any data sent from the SOCKS client to the SOCKS server will be relayed to 66.102.7.99 and vice versa.

The command field can be 0x01 for "connect" or 0x02 for "bind". "bind" allows incoming connections for protocols like active FTP.

SOCKS 4a protocol

SOCKS 4a is a simple extension to SOCKS 4 protocol that allows a client that cannot resolve the destination host's domain name to specify it.

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:

• field 1: SOCKS version number, 1 byte, must be 0x04 for this version
• field 2: command code, 1 byte:
  • 0x01 = establish a TCP/IP stream connection
  • 0x02 = establish a TCP/IP port binding
• field 3: network byte order port number, 2 bytes
• field 4: deliberate invalid IP address, 4 bytes, first three must be 0x00 and the last one must not be 0x00
• field 5: the user ID string, variable length, terminated with a null (0x00)
• field 6: the domain name of the host we want to contact, variable length, terminated with a null (0x00)

Server to SOCKS client:

• field 1: null byte
• field 2: status, 1 byte:
  • 0x5a = request granted
  • 0x5b = request rejected or failed
  • 0x5c = request failed because client is not running identd (or not reachable from the server)
  • 0x5d = request failed because client's identd could not confirm the user ID string in the request
• field 3: network byte order port number, 2 bytes
• field 4: network byte order IP address, 4 bytes

A server using protocol 4A 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.

SOCKS 5 protocol

The SOCKS 5 protocol is an extension of the SOCKS 4 protocol that is defined in RFC 1928. It offers more choices of authentication, adds support for IPv6 and UDP that can be used for DNS lookups. The initial handshake now consists of the following:

• Client connects and sends a greeting which includes a list of authentication methods supported.
• Server chooses one (or sends a failure response if none of the offered methods are acceptable).
• Several messages may now pass between the client and the server depending on the authentication method chosen.
• Client sends a connection request similar to SOCKS 4.
• Server responds similar to SOCKS 4.

The authentication methods supported are numbered as follows:

• 0x00 - No authentication
• 0x01 - GSSAPI
• 0x02 - Username/Password
• 0x03-0x7F - methods assigned by IANA
• 0x80-0xFE - methods reserved for private use

The initial greeting from the client is:

• field 1: SOCKS version number (must be 0x05 for this version)
• field 2: number of authentication methods supported, 1 byte
• field 3: authentication methods, variable length, 1 byte per method supported

The server's choice is communicated:

• field 1: SOCKS version, 1 byte (0x05 for this version)
• field 2: chosen authentication method, 1 byte, or 0xFF if no acceptable methods were offered

The subsequent authentication is method-dependent and described in RFC 1929:

The client's authentication request is:

• field 1: version number, 1 byte (must be 0x01)
• field 2: username length, 1 byte
• field 3: username
• field 4: password length, 1 byte
• field 5: password

Server response for authentication:

• field 1: version, 1 byte
• field 2: status code, 1 byte.
  • 0x00 = success
  • any other value = failure, connection must be closed

The client's connection request is:

• field 1: SOCKS version number, 1 byte (must be 0x05 for this version)
• field 2: command code, 1 byte:
  • 0x01 = establish a TCP/IP stream connection
  • 0x02 = establish a TCP/IP port binding
  • 0x03 = associate a UDP port
• field 3: reserved, must be 0x00
• field 4: address type, 1 byte:
  • 0x01 = IPv4 address
  • 0x03 = Domain name
  • 0x04 = IPv6 address
• field 5: destination address of
  • 4 bytes for IPv4 address
  • 1 byte of name length followed by the name for Domain name
  • 16 bytes for IPv6 address
• field 6: port number in a network byte order, 2 bytes

Server response:

• field 1: SOCKS protocol version, 1 byte (0x05 for this version)
• field 2: status, 1 byte:
  • 0x00 = request granted
  • 0x01 = general failure
  • 0x02 = connection not allowed by ruleset
  • 0x03 = network unreachable
  • 0x04 = host unreachable
  • 0x05 = connection refused by destination host
  • 0x06 = TTL expired
  • 0x07 = command not supported / protocol error
  • 0x08 = address type not supported
• field 3: reserved, must be 0x00
• field 4: address type, 1 byte:
  • 0x01 = IPv4 address
  • 0x03 = Domain name
  • 0x04 = IPv6 address
• field 5: destination address of
  • 4 bytes for IPv4 address
  • 1 byte of name length followed by the name for Domain name
  • 16 bytes for IPv6 address
• field 6: network byte order port number, 2 bytes

There are client programs that "socksify", which allows adaptation of any networked software to connect to external networks via SOCKS.

SOCKS software

• Kernel SOCKS Bouncer ksb26 (Kernel Socks Bouncer) is a Linux Kernel 2.6.x Loadable Kernel Module that redirects TCP connection (to user-defined target hosts) through socks 4/5 chains.
• SS5 Socks Server is an open-source SOCKS4/SOCKS5 server.
• Dante is an open-source SOCKS4/SOCKS5 implementation with commercial support developed by Inferno Nettverk A/S.
• OpenSSH allows dynamic creation of tunnels, specified via a subset of the SOCKS protocol, supporting the CONNECT command.
• PuTTy is a Win32 SSH client that supports local creation of SOCKS (dynamic) tunnels through remote SSH servers
• WinSocks is a light-weight SOCKS4/SOCKS5 server developed by Proxy Labs.
• SOcat Multipurpose relay (SOcket CAT): includes socks4, and socks4a functionality (Linux / Mac)
• FreeCap Socksifyer for Windows, any App can run its network traffic transparently via a SOCKS or HTTP proxy.
• Simple Socks Server for Perl - SSS is a Simple SOCKS Server written in perl that implements the SOCKS v5 protocol.
• Sun Java System Web Proxy Server is a caching proxy server running on Solaris, Linux and Windows servers that supports HTtp://S, NSAPI I/O filters, dynamic reconfiguration, SOCKSv5 and reverse proxy.
• BarracudaDrive Web Server - commercial SOCKS HTTP HTTPS tunnel/server, available for Windows, embedded Linux and Mac OS X (PowerPC)
• DeleGate is a multi-purpose application level gateway and proxy server which runs on multiple platforms. Beside SOCKS it also supports HTTP(S), FTP, NNTP, SMTP, POP, IMAP, LDAP, Telnet, DNS and many more.
• Freeproxy Is free proxy server software. It supports HTTP, SOCKS, and many other protocols.

External links

• draft-ietf-aft-socks-chap Challenge-Handshake Authentication Protocol for SOCKS V5
• RFC 3089 - A SOCKS-based IPv6/IPv4 Gateway Mechanism
• RFC 1961 - GSS-API Authentication Method for SOCKS Version 5
• RFC 1929 - Username/Password Authentication for SOCKS V5
• RFC 1928 - SOCKS Protocol Version 5
• SOCKS: A protocol for TCP proxy across firewalls - SOCKS Protocol Version 4 (NEC)
• Socks Servers Lists
• Socks Servers Lists information