SOCKS

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

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.

SOCKS performs at Layer 5 of the OSI model (the session layer, an intermediate layer between the presentation layer and the transport layer). A SOCKS server accepts incoming client connection on TCP port 1080.

History

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.)

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 as RFC 1928 (authored by: M. Leech, M. Ganis, Y. Lee, R. Kuris, D. Koblas, and L. Jones). The protocol was developed in collaboration with Aventail Corporation, which markets the technology outside of Asia.

Usage

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 a good interface for all types of routing tools. It can be used as:

• A circumvention tool, allowing traffic to bypass Internet filtering to access content otherwise blocked, e.g., by governments, workplaces, schools, and country-specific web services. Since SOCKS is very detectable, a common approach is to present a SOCKS interface for more sophiscated protocols:
  • The Tor onion proxy software presents a SOCKS interface to its clients.
• Providing similar functionality to a virtual private network, allowing connections to be forwarded to a server's "local" network:
  • Some SSH suites, such as OpenSSH, support dynamic port forwarding that allows the user to create a local SOCKS proxy. This can free the user from the limitations of connecting only to a predefined remote port and server.

Comparison to HTTP proxying

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 then acts as transparently as possible, whereas a regular proxy may interpret and rewrite headers (say, to employ another underlying protocol, such as FTP; however, an HTTP proxy simply forwards an HTTP request to the desired HTTP server). Though HTTP proxying has a different usage model in mind, the CONNECT method allows for forwarding TCP connections; however, SOCKS proxies can also forward UDP traffic and work in reverse, while HTTP proxies cannot. HTTP proxies are traditionally more aware of the HTTP protocol, performing higher-level filtering (though that usually only applies to GET and POST methods, not the CONNECT method).

SOCKS

Bill, or any client for that matter, wishes to communicate with Chris over the internet, but a firewall between them exists on his network, where Bill is not authorized to communicate with Chris directly. So, Bill connects to the SOCKS proxy on his network, informing it about the connection he wishes to make to Chris; the SOCKS proxy opens a connection through the firewall and facilitates the communication between Bill and Chris.

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 that it would directly with Jane's server if that were possible; that is, it sends a standard HTTP request header. The HTTP proxy connects to Jane's server, and then transmits back to Bill any data that Jane's server returns.

Protocol

SOCKS4

A typical SOCKS4 connection request looks like this:

First packet to server

	VER	CMD	DSTPORT	DSTIP	ID
Byte Count	1	1	2	4	Variable

VER

SOCKS version number, 0x04 for this version

CMD

command code:

• 0x01 = establish a TCP/IP stream connection
• 0x02 = establish a TCP/IP port binding

DSTPORT

2-byte port number (in network byte order)

DESTIP

IPv4 Address, 4 bytes (in network byte order)

ID

the user ID string, variable length, null-terminated.

Response packet from server

	VN	REP	DSTPORT	DSTIP
Byte Count	1	1	2	4

VN

reply version, null byte

REP

reply code

Byte	Meaning
0x5A	Request granted
0x5B	Request rejected or failed
0x5C	Request failed because client is not running identd (or not reachable from server)
0x5D	Request failed because client's identd could not confirm the user ID in the request

DSTPORT

destination port, meaningful if granted in BIND, otherwise ignore

DSTIP

destination IP, as above – the ip:port the client should bind to

For example, this a SOCKS4 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 SOCKS4 protocol specifies that the values of these bytes should be ignored.

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

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:

First packet to server

	SOCKS4_C	DOMAIN
Byte Count	8+variable	variable

SOCKS4_C

SOCKS4 client handshake packet (above)

DOMAIN

the domain name of the host to contact, variable length, null (0x00) terminated

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.

SOCKS5

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:

• Client connects and sends a greeting, which includes a list of authentication methods supported.
• Server chooses one of the methods (or sends a failure response if none of them 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 SOCKS4.
• Server responds similar to SOCKS4.

The initial greeting from the client is:

Client greeting

	VER	NAUTH	AUTH
Byte count	1	1	variable

VER

SOCKS version (0x05)

NAUTH

Number of authentication methods supported, uint8

AUTH

Authentication methods, 1 byte per method supported

The authentication methods supported are numbered as follows:

• 0x00: No authentication
• 0x01: GSSAPI
• 0x02: Username/password
• 0x03–0x7F: methods assigned by IANA
  • 0x03: Challenge-Handshake Authentication Protocol
  • 0x04: Unassigned
  • 0x05: Challenge-Response Authentication Method
  • 0x06: Secure Sockets Layer
  • 0x07: NDS Authentication
  • 0x08: Multi-Authentication Framework
  • 0x09: JSON Parameter Block
  • 0x0A–0x7F: Unassigned
• 0x80–0xFE: methods reserved for private use

Server choice

	VER	CAUTH
Byte count	1	1

VER

SOCKS version (0x05)

CAUTH

chosen authentication method, or 0xFF if no acceptable methods were offered

The subsequent authentication is method-dependent. Username and password authentication (method 0x02) is described in RFC 1929:

Client authentication request, 0x02

	VER	IDLEN	ID	PWLEN	PW
Byte count	1	1	(1-255)	1	(1-255)

VER

0x01 for current version of username/password authentication

IDLEN, ID

Username length, uint8; username as bytestring

PWLEN, PW

Password length, uint8; password as bytestring

Server response, 0x02

	VER	STATUS
Byte count	1	1

VER

0x01 for current version of username/password authentication

STATUS

0x00 success, otherwise failure, connection must be closed

After authentication the connection can proceed. We first define an address datatype as:

SOCKS5 address

	TYPE	ADDR
Byte Count	1	variable

TYPE

type of the address. One of:

• 0x01: IPv4 address
• 0x03: Domain name
• 0x04: IPv6 address

ADDR

the address data that follows. Depending on type:

• 4 bytes for IPv4 address
• 1 byte of name length followed by 1–255 bytes for the domain name
• 16 bytes for IPv6 address

Client connection request

	VER	CMD	RSV	DSTADDR	DSTPORT
Byte Count	1	1	1	Variable	2

VER

SOCKS version (0x05)

CMD

command code:

• 0x01: establish a TCP/IP stream connection
• 0x02: establish a TCP/IP port binding
• 0x03: associate a UDP port

RSV

reserved, must be 0x00

DSTADDR

destination address, see the address structure above.

DSTPORT

port number in a network byte order

Response packet from server

	VER	STATUS	RSV	BNDADDR	BNDPORT
Byte Count	1	1	1	variable	2

VER

SOCKS version (0x05)

STATUS

status code:

• 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

RSV

reserved, must be 0x00

BNDADDR

server bound address, in the "SOCKS5 address" format specified above

BNDPORT

server bound port number in a network byte order

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.

Software

Servers

SOCKS proxy server implementations

• Sun Java System Web Proxy Server is a caching proxy server running on Solaris, Linux and Windows servers that support HTTPS, NSAPI I/O filters, dynamic reconfiguration, SOCKSv5 and reverse proxy.
• WinGate is a multi-protocol proxy server and SOCKS server for Microsoft Windows which supports SOCKS4, SOCKS4a and SOCKS5 (including UDP-ASSOCIATE and GSSAPI auth). It also supports handing over SOCKS connections to the HTTP proxy, so can cache and scan HTTP over SOCKS.
• Socksgate5 SocksGate5 is an application-SOCKS firewall with inspection feature on Layer 7 of the OSI model, the Application Layer. Because packets are inspected at 7 OSI Level the application-SOCKS firewall may search for protocol non-compliance and blocking specified content.
• Dante is a circuit-level SOCKS server that can be used to provide convenient and secure network connectivity, requiring only the host Dante runs on to have external network connectivity.

Other programs providing SOCKS server interface

• 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.
• ShimmerCat is a web server that uses SOCKS5 to simulate an internal network, allowing web developers to test their local sites without modifying their /etc/hosts file.
• Tor is a system intended to enable online anonymity. Tor offers a TCP-only SOCKS server interface to its clients.
• Shadowsocks is a circumvent censorship tool. It provides a SOCKS5 interface.

Clients

Client software must have native SOCKS support in order to connect through SOCKS. There are programs that allow users to circumvent such limitations:

Socksifiers

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.

• Win2Socks, which enables applications to access the network through SOCKS5, HTTPS or Shadowsocks.
• tun2socks, an open source tool that creates virtual TCP TUN adapters from a SOCKS proxy. Works on Linux and Windows, has a macOS port and a UDP-capable reimplementation in Golang.
• proxychains, a Unix program that forces TCP traffic through SOCKS or HTTP proxies on (dynamically-linked) programs it launches. Works on various Unix-like systems.

Translating proxies

• Polipo, a forwarding and caching HTTP/1.1 proxy server with IPv4 support. Open Source running on GNU/Linux, OpenWrt, Windows, Mac OS X, and FreeBSD. Almost any Web browser can use it.
• Privoxy, a non-caching SOCKS-to-HTTP proxy.

External links

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