View source for SOCKS

{{Short description|Internet protocol that uses a proxy server}}
{{About|the internet protocol|other uses|Socks (disambiguation)}}
{{IPstack}}
'''SOCKS''' is an [[Internet]] [[Protocol (computing)|protocol]] that exchanges [[Packet (information technology)|network packets]] between a [[client (computing)|client]] and [[server (computing)|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.{{Ref RFC|1928}}<ref>{{cite web|url=https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=1080|title=Service Name and Transport Protocol Port Number Registry|publisher=[[Internet Assigned Numbers Authority]]|date=19 May 2017|access-date=23 May 2017}}</ref>

==History==
The protocol was originally designed and 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,<ref name="usenix">{{cite conference |last1=Koblas |first1=David |last2=Koblas |first2=Michelle R. |title=SOCKS |conference=USENIX UNIX Security Symposium III | conference-url=https://www.usenix.org/legacy/publications/library/proceedings/sec92/ | url=https://www.usenix.org/legacy/publications/library/proceedings/sec92/full_papers/koblas.pdf |access-date=16 November 2019}}</ref> making SOCKS publicly available.<ref>{{Cite journal|journal=[[;login:]]|author=Darmohray, Tina.|url=https://www.usenix.org/system/files/login/issues/login_february_2005.pdf|title=Firewalls and fairy tales|volume=30|number=1|access-date=2025-03-03}}</ref> The protocol was extended to version 4 by Ying-Da Lee of [[NEC]].

The SOCKS reference architecture and client are owned by [[Permeo Technologies]],<ref>{{webarchive |url=https://web.archive.org/web/*/http://www.socks.permeo.com/ |date=* }}</ref> 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 [[Firewall (computing)|firewalls]] and other security products easier to administer. It was approved by the [[Internet Engineering Task Force|IETF]] in 1996.{{Ref RFC|1928}} The protocol was developed in collaboration with [[Aventail Corporation]], which markets the technology outside of Asia.<ref>[http://www.cnet.com/tech/tech-industry/cyberspace-from-outer-space/ CNET: Cyberspace from outer space]</ref>

==Acronym==
SOCKS is sometimes defined as an acronym for "socket secure" from at least 2001,<ref>{{cite patent|country = US|number= US8984268B2}}</ref><ref>{{cite patent|country = US|number= US20210058367A1}}</ref><ref>{{cite patent|country = US|number= US11190374B2}}</ref><ref>{{cite patent|country = JP|number= JP6761452B2}}</ref><ref>{{cite patent|country = US|number= US11425565B2}}</ref> although it was not originally defined as such in the SOCKS Protocol Version 5 RFC in 1996{{Ref RFC|1928}} or the UNIX Security Symposium III paper in 1992<ref name="usenix"/> but simply referred to a specific proxy protocol designed to facilitate communication between clients and servers through a firewall.

==Usage==
''SOCKS'' is a [[de facto standard|''de facto'' standard]] for [[circuit-level gateway]]s (level 5 gateways).<ref>{{cite book |last1=Oppliger |first1=Rolf |title=Security technologies for the World Wide Web |publisher=Artech House |isbn=1580533485 |edition=2nd |url=https://flylib.com/books/en/4.179.1.22/1/ |access-date=21 January 2020 |chapter=Circuit-level gateways|year=2003 }}</ref>

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 circumvention tool, allowing traffic to bypass Internet filtering to access content otherwise blocked, e.g., by governments, workplaces, schools, and country-specific web services.<ref>{{cite web|title=2010 Circumvention Tool Usage Report |url=http://cyber.law.harvard.edu/sites/cyber.law.harvard.edu/files/2010_Circumvention_Tool_Usage_Report.pdf |publisher=The Berkman Center for Internet & Society at Harvard University |date=October 2010}}</ref> Since SOCKS is very detectable, a common approach is to present a SOCKS interface for more sophisticated protocols:
** The [[Tor (anonymity network)|Tor]] onion proxy software presents a SOCKS interface to its clients.<ref>{{cite web | title=Tor FAQ |url=https://www.torproject.org/docs/faq.html.en#TBBSocksPort}}</ref>
* 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.<ref>{{cite web |title=OpenSSH FAQ |url=http://www.openssh.org/faq.html#2.11 |archive-url=https://web.archive.org/web/20020201204826/http://www.openssh.org/faq.html#2.11 |url-status=dead |archive-date=2002-02-01 }}</ref> This can free the user from the limitations of connecting only to a predefined remote port and server.

==Protocol==

===SOCKS4===

A typical SOCKS4 connection request looks like this:
{| class="wikitable"
|+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|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 string|null-terminated]].

{| class="wikitable"
|+Response packet from server
!
!VN
!REP
!DSTPORT
!DSTIP
|-
!Byte Count
|1
|1
|2
|4
|}
;VN: reply version, null byte
;REP: reply code
:{| class="wikitable"
!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 &ndash; the ip:port the client should bind to

For example, this is a SOCKS4 request to connect Fred to ''66.102.7.99:80'', the server replies with an "OK":

*Client: <code>0x04 | 0x01 | 0x00 0x50 | 0x42 0x66 0x07 0x63 | 0x46 0x72 0x65 0x64 0x00</code>
**The last field is "Fred" in [[ASCII]], followed by a null byte.
*Server: <code>0x00 | 0x5A | 0xXX 0xXX | 0xXX 0xXX 0xXX 0xXX</code>
**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 [[File Transfer Protocol|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.<ref>{{cite web |url=http://www.openssh.org/txt/socks4a.protocol |title=SOCKS 4A: A Simple Extension to SOCKS 4 Protocol |publisher=OpenSSH |author=Ying-Da Lee |access-date=2013-04-03}}</ref>

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:
{| class="wikitable"
|+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 , null (0x00) terminated

Server to SOCKS client: (Same as SOCKS4)

A server using protocol SOCKS4a must check the DSTIP in the request [[packet (information technology)|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 {{IETF RFC|1928}}.  It is an incompatible extension of the SOCKS4 protocol; it offers more choices for authentication and adds support for [[IPv6]] and [[User Datagram Protocol|UDP]], the latter of which can be used for [[DNS lookup]]s. 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:
{| class="wikitable"
|+Client greeting
!
!VER
!NAUTH
!AUTH
|-
!scope=row|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]] ({{IETF RFC|1961}})
:*0x02: Username/password ({{IETF RFC|1929}})
:*0x03–0x7F: methods assigned by [[Internet Assigned Numbers Authority|IANA]]<ref>[https://www.iana.org/assignments/socks-methods IANA.org]</ref>
:**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

{| class="wikitable"
|+Server choice
!
!VER
!CAUTH
|-
!scope=row|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 {{IETF RFC|1929}}:

{| class="wikitable"
|+Client authentication request, 0x02
!
!VER
!IDLEN
!ID
!PWLEN
!PW
|-
!scope=row|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

{| class="wikitable"
|+Server response, 0x02
!
!VER
!STATUS
|-
!scope=row|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:
{| class=wikitable
|+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

{| class="wikitable"
|+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]]

{| class="wikitable"
|+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: [[Time to live|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.<ref>{{cite web |title=CURLOPT_PROXY |url=https://curl.se/libcurl/c/CURLOPT_PROXY.html |website=curl.se |access-date=20 January 2020}}</ref>

==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.
* [http://www.socksgate5.it/ 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.
* [http://www.inet.no/dante/ 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.<ref>{{Cite web|title=Products developed by Inferno Nettverk A/S|url=https://www.inet.no/en/products.html|access-date=2021-03-20|website=www.inet.no}}</ref>
* [https://github.com/heiher/hev-socks5-server HevSocks5Server] is a high-performance and low-overhead SOCKS server for Unix (Linux/BSD/macOS). It supports standard TCP-CONNECT and UDP-ASSOCIATE methods and multiple username/password authentication.

====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.
* [https://secureshellfish.app/ Secure ShellFish] is a SSH client for iOS and macOS that includes a SOCKS server.
* [[ShimmerCat]]<ref>{{cite web|title=Easy Net with SOCKS5|archive-url=https://web.archive.org/web/20180913040145/https://www.shimmercat.com/en/docs/1.5/socks5-swift/|archive-date=2018-09-13|url=https://www.shimmercat.com/en/info/articles/socks5-swift/|website=shimmercat.com|publisher=ShimmerCat|access-date=20 April 2016}}</ref> is a web server that uses SOCKS5 to simulate an internal network, allowing web developers to test their local sites without modifying their [[Hosts (file)|/etc/hosts]] file. 
* [[Tor (anonymity network)|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.
* [[netcat]] implementations, as Ncat and socat.

===Clients===

Client software must have native SOCKS support in order to connect through SOCKS.

====Browser====
*Chrome: support SOCKS4, SOCKS4a and SOCKS5.<ref>{{Cite web |title=Configuring a SOCKS proxy server in Chrome |url=https://www.chromium.org/developers/design-documents/network-stack/socks-proxy/ |access-date=2024-03-19 |website=www.chromium.org}}</ref>
*Firefox: support SOCKS4, SOCKS4a and SOCKS5.
*Internet Explorer and EdgeHTML-based Microsoft Edge: support SOCKS4 only.
*Chromium-based Microsoft Edge: support SOCKS4, SOCKS4a and SOCKS5.

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.
*[https://win2socks.com 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, capable of UDP if supported on another end. Works on Linux and Windows,<ref>{{cite web |last1=Bizjak |first1=Ambroz |title=ambrop72/badvpn: NCD scripting language, tun2socks proxifier, P2P VPN |url=https://github.com/ambrop72/badvpn |website=GitHub |access-date=20 January 2020 |date=20 January 2020}}</ref> has a macOS port and reimplementation in [[Golang]].<ref>{{cite web|title= xjasonlyu/tun2socks: tun2socks - powered by gVisor TCP/IP stack |url=https://github.com/xjasonlyu/tun2socks |website=GitHub}}</ref> Another written in C with good-performance, works on Linux/Android/BSD/macOS and iOS<ref>{{cite web|title= heiher/hev-socks5-tunnel: A high-performance tun2socks |url=https://github.com/heiher/hev-socks5-tunnel |website=GitHub}}</ref>
*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.<ref>{{cite web |last1=Hamsik |first1=Adam |title=proxychains: a tool that forces any TCP connection made by any given application to follow through proxy like TOR or any other SOCKS4, SOCKS5 or HTTP(S) proxy |url=https://github.com/haad/proxychains |website=GitHub |access-date=20 January 2020 |date=20 January 2020}}</ref>

====Translating proxies====
* [[Polipo]], a discontinued forwarding and caching HTTP/1.1 proxy server with [[IPv4]] support. Open Source running on [[Linux]], [[OpenWrt]], [[Microsoft Windows|Windows]], [[Mac OS X]], and [[FreeBSD]]. Almost any Web browser can use it.
* [[Privoxy]], a non-caching SOCKS-to-HTTP proxy.
* [https://tinyproxy.github.io/ Tinyproxy], a light-weight HTTP/HTTPS proxy daemon for POSIX operating systems. Designed from the ground up to be fast and yet small. It presents an http proxy interface and can connect to SOCKS4/5 and http upstream proxies.

== Security ==
{{unreferenced-section|date=January 2022}}
Due to lack of request and packets exchange encryption it makes SOCKS practically vulnerable to [[man-in-the-middle attack]]s and IP addresses eavesdropping which in consequence clears a way to censorship by governments.

==References==
{{Reflist}}

==External links==
* {{IETF RFC|1929}}: Username/Password Authentication for SOCKS V5
* {{IETF RFC|1961}}: GSS-API Authentication Method for SOCKS Version 5
* {{IETF RFC|3089}}: A SOCKS-based IPv6/IPv4 Gateway Mechanism
* [http://www.tools.ietf.org/html/draft-ietf-aft-socks-chap Draft-ietf-aft-socks-chap], Challenge-Handshake Authentication Protocol for SOCKS V5
* [http://ftp.icm.edu.pl/packages/socks/socks4/SOCKS4.protocol SOCKS: A protocol for TCP proxy across firewalls],  SOCKS Protocol Version 4 ([[NEC]])

{{DEFAULTSORT:Socks}}
[[Category:Internet protocols]]
[[Category:Internet privacy software]]
[[Category:Session layer protocols]]