Internet Area Working Group V. Olteanu
Internet-Draft D. Niculescu
Intended status: Experimental University Politehnica of Bucharest
Expires: September 12, 2019 March 11, 2019

SOCKS Protocol Version 6


The SOCKS protocol is used primarily to proxy TCP connections to arbitrary destinations via the use of a proxy server. Under the latest version of the protocol (version 5), it takes 2 RTTs (or 3, if authentication is used) before data can flow between the client and the server.

This memo proposes SOCKS version 6, which reduces the number of RTTs used, takes full advantage of TCP Fast Open, and adds support for 0-RTT authentication.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on September 12, 2019.

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Table of Contents

1. Introduction

Versions 4 and 5 [RFC1928] of the SOCKS protocol were developed two decades ago and are in widespread use for circuit level gateways or as circumvention tools, and enjoy wide support and usage from various software, such as web browsers, SSH clients, and proxifiers. However, their design needs an update in order to take advantage of the new features of transport protocols, such as TCP Fast Open [RFC7413], or to better assist newer transport protocols, such as MPTCP [RFC6824].

One of the main issues faced by SOCKS version 5 is that, when taking into account the TCP handshake, method negotiation, authentication, connection request and grant, it may take up to 5 RTTs for a data exchange to take place at the application layer. This is especially costly in networks with a large delay at the access layer, such as 3G, 4G, or satelite.

The desire to reduce the number of RTTs manifests itself in the design of newer security protocols. TLS version 1.3 [RFC8446] defines a zero round trip (0-RTT) handshake mode for connections if the client and server had previously communicated.

TCP Fast Open [RFC7413] is a TCP option that allows TCP to send data in the SYN and receive a response in the first ACK, and aims at obtaining a data response in one RTT. The SOCKS protocol needs to concern itself with at least two TFO deployment scenarios: First, when TFO is available end-to-end (at the client, at the proxy, and at the server); second, when TFO is active between the client and the proxy, but not at the server.

This document describes the SOCKS protocol version 6. The key improvements over SOCKS version 5 are:

1.1. Revision log

Typos and minor clarifications are not listed.







2. Requirements language

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

3. Mode of operation

 CLIENT                                                        PROXY 

         | Authentication methods | Request
 --------> Command code           +------------------------------>
         | Address                |         
         | Port                   |         
         | Options                |         
 --------> Initial data           +------------------------------>

                Authentication Reply | Type                  |
  <----------------------------------+ Method                <-----
                                     | Options               |
  <-------------------(Authentication protocol)------------------>

                Authentication Reply | Type = Success        |
  <----------------------------------+ Method                <-----
                                     | Options               |
     Operation Reply   | Reply code            |
  <--------------------+ Bind address          <------------------
                       | Bind port             |
                       | Options               |

Figure 1: The SOCKS version 6 protocol message exchange

When a TCP-based client wishes to establish a connection to a server, it must open a TCP connection to the appropriate SOCKS port on the SOCKS proxy. The client then enters a negotiation phase, by sending the request in Figure 1, that contains, in addition to fields present in SOCKS 5 [RFC1928], fields that facilitate low RTT usage and faster authentication negotiation.

Next, the server sends an authentication reply. If the request did not contain the necessary authentication information, the proxy indicates an authentication method that must proceed. This may trigger a longer authentication sequence that could include tokens for ulterior faster authentications. The part labeled “Authentication protocol” is specific to the authentication method employed and is not expected to be employed for every connection between a client and its proxy server. The authentication protocol typically takes up 1 RTT or more.

If the authentication is successful, an operation reply is generated by the proxy. It indicates whether the proxy was successful in creating the requested socket or not.

In the fast case, when authentication is properly set up, the proxy attempts to create the socket immediately after the receipt of the request, thus achieving an operational conection in one RTT (provided TFO functionality is available at the client, proxy, and server).

4. Requests

The client starts by sending a request to the proxy.

|    Version    | Command | Port | Address | Address  |
| Major | Minor |  Code   |      |  Type   |          |
|   1   |   1   |    1    |  2   |    1    | Variable |
| Options | Options  |
| Length  |          |
|    2    | Variable |

Figure 2: SOCKS 6 Request

The Address and Port fields have different meanings based on the Command Code:

Clients can advertise their supported authentication methods by including an Authentication Method option (see Section 8.2).

5. Version Mismatch Replies

Upon receipt of a request starting with a version number other than 6.0, the proxy sends the following response:

|    Version    |
| Major | Minor |
|   1   |   1   |

Figure 3: SOCKS 6 Version Mismatch Reply

A client MUST close the connection after receiving such a reply.

6. Authentication Replies

Upon receipt of a valid request, the proxy sends an Authentication Reply:

|    Version    | Type | Method | Options | Options  |
| Major | Minor |      |        | Length  |          |
|   1   |   1   |  1   |   1    |    2    | Variable |

Figure 4: SOCKS 6 Authentication Reply

Multihomed clients SHOULD cache the chosen method on a per-interface basis and SHOULD NOT include Authentication Data options related to any other methods in further requests originating from the same interface.

If the server signals that further authentication is needed and selects “No Acceptable Methods”, the client MUST close the connection.

The client and proxy begin a method-specific negotiation. During such negotiations, the proxy MAY supply information that allows the client to authenticate a future request using an Authentication Data option. Application data is not subject to any encryption negotiated during this phase. Descriptions of such negotiations are beyond the scope of this memo.

When the negotiation is complete (either successfully or unsuccessfully), the proxy sends a second Authentication Reply. The second Authentication Reply MUST either signal success or that there are no more acceptable authentication methods.

7. Operation Replies

After the authentication negotiations are complete, the proxy sends an Operation Reply:

| Reply | Bind | Address |   Bind   | Options | Options  |
| Code  | Port |  Type   | Address  | Length  |          |
|   1   |  2   |    1    | Variable |    2    | Variable |

Figure 5: SOCKS 6 Operation Reply

Proxy implementations MAY support any subset of the client commands listed in Section 4.

If the proxy returns a reply code other than “Success”, the client MUST close the connection.

If the client issued an NOOP command, the client MUST close the connection after receiving the Operation Reply.

7.1. Handling CONNECT

In case the client has issued a CONNECT request, data can now pass.

7.2. Handling BIND

In case the client has issued a BIND request, it must wait for a second Operation reply from the proxy, which signifies that a host has connected to the bound port. The Bind Address and Bind Port fields contain the address and port of the connecting host. Afterwards, application data may pass.

7.3. Handling UDP ASSOCIATE

Proxies offering UDP functionality must be configured with a UDP port used for relaying UDP datagrams to and from the client, and/or a port used for relaying datagrams over DTLS.

Following a successful Operation Reply, the proxy sends a UDP Association Initialization message:

| Association ID |
|        4       |

Figure 6: UDP Association Initialization

Proxy implementations SHOULD generate Association IDs randomly or pseudo-randomly.

Clients may start sending UDP datagrams to the proxy either in plaintext, or over an established DTLS session, using the proxy’s configured UDP ports. A client’s datagrams are prefixed by a SOCKS Datagram Header, indicating the remote host’s address and port:

|    Version    | Association | Port | Address | Address  |
| Major | Minor |      ID     |      |  Type   |          |
|   1   |   1   |      4      |  2   |    1    | Variable |

Figure 7: SOCKS 6 Datagram Header

Following the receipt of the first datagram from the client, the proxy makes a one-way mapping between the Association ID and:

Further datagrams carrying the same Association ID, but not matching the established mapping, are silently dropped.

The proxy then sends an UDP Association Confirmation message over the TCP connection with the client:

| Status |
|   1    |

Figure 8: UDP Association Confirmation

Following the confirmation message, UDP packets bound for the proxy’s bind address and port are relayed to the client, also prefixed by a Datagram Header.

The UDP association remains active for as long as the TCP connection between the client and the proxy is kept open.

7.3.1. Proxying UDP servers

Under some circumstances (e.g. when hosting a server), the SOCKS client expects the remote host to send UDP datagrams first. As such, the SOCKS client must trigger a UDP Association Confirmation without having the proxy relay any datagrams on its behalf.

To that end, it sends an empty datagram prefixed by a Datagram Header with an IP address and port consisting of zeroes. The client SHOULD resend the empty datagram if an UDP Association Confirmation is not received after a timeout.

8. SOCKS Options

SOCKS options have the following format:

| Kind | Length | Option Data |
|  1   |   2    |   Variable  |

Figure 9: SOCKS 6 Option

Unless otherwise noted, client and proxy implementations MAY omit supporting any of the options described in this document. Upon encountering an unsupported option, a SOCKS endpoint MUST silently ignore it.

8.1. Stack options

Stack options can be used by clients to alter the behavior of the protocols on top of which SOCKS is running, as well the protcols used by the proxy to communicate with the remote host (i.e. IP, TCP, UDP). A Stack option can affect either the proxy’s protocol on the client-proxy leg or on the proxy-remote leg. Clients can only place Stack options inside SOCKS Requests.

Proxies MAY include Stack options in their Operation Replies to signal their behavior. Said options MAY be unsolicited, i. e. the proxy MAY send them to signal behaviour that was not explicitly requested by the client.

In case of UDP ASSOCIATE, the stack options refer to the UDP traffic relayed by the proxy.

Stack options that are part of the same message MUST NOT contradict one another or contain duplicate information.

| Kind | Length |  Leg   | Level  | Code |   Data   |
|  1   |   2    | 2 bits | 6 bits |  1   | Variable |

Figure 10: Stack Option

8.1.1. IP TOS options

| Kind | Length |  Leg   | Level  | Code | TOS |
|  1   |   2    | 2 bits | 6 bits |  1   |  1  |

Figure 11: IP TOS Option

The client can use IP TOS options to request that the proxy use a certain value for the IP TOS field. Likewise, the proxy can use IP TOS options to advertise the TOS values being used.

8.1.2. TFO options

| Kind | Length |  Leg   | Level  | Code | Payload Size |
|  1   |   2    | 2 bits | 6 bits |  1   |      2       |

Figure 12: TFO Option

If a SOCKS Request contains a TFO option, the proxy SHOULD attempt to use TFO in case of a CONNECT command, or accept TFO in case of a BIND command. Otherwise, the proxy MUST NOT attempt to use TFO in case of a CONNECT command, or accept TFO in case of a BIND command.

In case of a CONNECT command, the client can indicate the desired payload size of the SYN. If the field is 0, the proxy can use an arbitrary payload size. If the field is non-zero, the proxy MUST NOT use a payload size larger than the one indicated. The proxy MAY use a smaller payload size than the one indicated.

8.1.3. Multipath TCP options

In case of a CONNECT command, the proxy can inform the client that the connection to the server is an MPTCP connection.

| Kind | Length |  Leg   | Level  | Code |
|  1   |   2    | 2 bits | 6 bits |  1   |

Figure 13: Multipath TCP Option

8.1.4. MPTCP Scheduler options

In case of a CONNECT or BIND command, a client can use an MPTCP Scheduler option to indicate its preferred scheduler for the connection.

A proxy can use an MPTCP Scheduler option to inform the client about what scheduler is in use.

| Kind | Length |  Leg   | Level  | Code | Scheduler |
|  1   |   2    | 2 bits | 6 bits |  1   |     1     |

Figure 14: MPTCP Scheduler Option

8.1.5. Listen Backlog options

| Kind | Length |  Leg   | Level  | Backlog |
|  1   |   2    | 2 bits | 6 bits |    2    |

Figure 15: Listen Backlog Option

The default behavior of the BIND does not allow a client to simultaneously handle multiple connections to the same bind address. A client can alter BIND’s behavior by adding a TCP Listen Backlog Option to a BIND Request, provided that the Request is part of a Session.

In response, the proxy sends a TCP Listen Backlog Option as part of the Operation Reply, with the Backlog field signalling the actual backlog used. The proxy SHOULD NOT use a backlog longer than requested.

Following the successful negotiation of a backlog, the proxy listens for incoming connections for as long as the initial connection stays open. The initial connection is not used to relay data between the client and a remote host.

To accept connections, the client issues further BIND Requests using the bind address and port supplied by the proxy in the initial Operation Reply. Said BIND requests must belong to the same Session as the original Request.

If a proxy can not or will not honor a Listen Backlog option, it does so silently.

8.2. Authentication Method options

Authentication Method options are placed in SOCKS Requests to advertise supported authentication methods. In case of a CONNECT Request, they are also used to specify the amount of initial data supplied before any method-specific authentication negotiations take place.

| Kind | Length | Initial Data Length | Methods  |
|  1   |   2    |          2          | Variable | 

Figure 16: Authentication Method Option

Clients MUST support the “No authentication required” method. Clients SHOULD omit advertising the “No authentication required” option.

8.3. Authentication Data options

Authentication Data options carry method-specific authentication data. They can be part of SOCKS Requests and Authentication Replies.

Authentication Data options have the following format:

| Kind | Length | Method | Authentication Data |
|  1   |   2    |   1    |       Variable      |

Figure 17: Authentication Data Option

Clients SHOULD only place one Authentication Data option per authentication method. Server implementations MAY silently ignore all Authentication Data options for the same method aside from an arbitrarily chosen one.

8.4. Session options

Clients and proxies can establish SOCKS sessions, which span one or more Requests. All session-related negotiations are done via Session Options, which are placed in Requests and Authentication Replies by the client and, respectively, by the proxy.

Session Options have the following format:

| Kind | Length | Type | Session Option Data |
|  1   |   2    |  1   |       Variable      |

Figure 18: Session Option

Client and proxy implementations MUST either support all Session Option Types, or none.

8.4.1. Session initiation

A client can initiate a session by sending a Session Request Option:

| Kind | Length | Type |
|  1   |   2    |  1   |

Figure 19: Session Request Option

The proxy then replies with a Session ID Option in the successful Operation Reply:

| Kind | Length | Type | Session ID |
|  1   |   2    |  1   |  Variable  |

Figure 20: Session ID Option

The Session ID serves to identify the session and is opaque to the client.

The credentials, or lack thereof, used to initiate the session are tied to the session. If authentication is to be performed for further Requests, the session is deemed “untrusted”, and the proxy also places a Session Untrusted option in the Authentication Reply:

| Kind | Length | Type |
|  1   |   2    |  1   |

Figure 21: Session Untrusted Option

The SOCKS Request that initiated the session is considered part of the session. A client MUST NOT attempt to initiate a session from within a different session.

If the proxy can not or will not honor the Session Request, it does so silently.

8.4.2. Further SOCKS Requests

Any further SOCKS Requests that are part of the session MUST include a Session ID Option (as seen in Figure 20).

The authentication procedure is altered based on the Session ID’s validity and whether or not the Session is untrusted.

For valid Session IDs:

The proxy then replies by placing a Session OK option in the successful Authentication Reply:

| Kind | Length | Type |
|  1   |   2    |  1   |

Figure 22: Session OK Option

If the ticket is invalid, the first Authentication Reply MUST signal that authentication failed and can not continue (by setting the Type field to 0x01 and the Method field to 0xff). Further, it SHALL contain a Session Invalid option:

| Kind | Length | Type |
|  1   |   2    |  1   |

Figure 23: Session Invalid Option

8.4.3. Tearing down the session

Proxies can, at their discretion, tear down a session and free all associated state. Proxy implementations SHOULD feature a timeout mechanism that destroys sessions after a period of inactivity.

Clients can signal that a session is no longer needed, and can be torn down, by sending a Session Teardown option in addition to the Session ID option:

| Kind | Length | Type |
|  1   |   2    |  1   |

Figure 24: Session Teardown Option

After sending such an option, the client MUST assume that the session is no longer valid.

8.5. Idempotence options

To protect against duplicate SOCKS Requests, clients can request, and then spend, idempotence tokens. A token can only be spent on a single SOCKS request.

Tokens are 4-byte unsigned integers in a modular 4-byte space. Therefore, if x and y are tokens, x is less than y if 0 < (y - x) < 2^31 in unsigned 32-bit arithmetic.

Proxies grant contiguous ranges of tokens called token windows. Token windows are defined by their base (the first token in the range) and size. Windows can be shifted (i. e. have their base increased, while retaining their size) unilaterally by the proxy.

Requesting and spending tokens is done via Idempotence options:

| Kind | Length | Type | Option Data |
|  1   |   2    |  1   |   Variable  |

Figure 25: Idempotence Option

Idempotence options are only valid in the context of a SOCKS Session. If a SOCKS Request is not part of a Session (either by supplying a valid Session ID or successfully initiating one via a Session Request), the proxy MUST silently ignore any Idempotence options.

Token windows are tracked by the proxy on a per-session basis. There can be at most one token window for every session and its tokens can only be spent from within said session.

Client and proxy implementations MUST either support all Idempotence Option Types, or none.

8.5.1. Requesting a fresh token window

A client can obtain a fresh window of tokens by sending a Token Request option as part of a SOCKS Request:

| Kind | Length | Type | Window Size |
|  1   |   2    |  1   |      4      |

Figure 26: Token Request

If a token window is issued, the proxy then includes a Token Window Advertisement option in the corresponding successful Authentication Reply:

| Kind | Length | Type | Window Base | Window Size |
|  1   |   2    |  1   |      4      |      4      |

Figure 27: Token Window Advertisement

If no token window is issued, the proxy MUST silently ignore the Token Request.

8.5.2. Spending a token

The client can attempt to spend a token by including a Token Expenditure option in its SOCKS request:

| Kind | Length | Type | Token |
|  1   |   2    |  1   |   4   |

Figure 28: Token Expenditure

Clients SHOULD prioritize spending the smaller tokens.

The proxy responds by sending a Token Expenditure Reply option as part of the successful Authentication Reply:

| Kind | Length | Type | Response Code |
|  1   |   2    |  1   |       1       |

Figure 29: Token Expenditure Response

If eligible, the token is spent before attempting to honor the Request. If the token is not eligible for spending, the proxy MUST NOT attempt to honor the client’s Request; further, it MUST indicate a General SOCKS server failure in the Operation Reply.

Proxy implementations SHOULD also send a Token Window Advertisement if:

Proxy implementations SHOULD NOT shift the window’s base beyond the highest unspent token.

Proxy implementations MAY include a Token Window Advertisement in any Authentication Reply that indicates success.

8.5.3. Handling Token Window Advertisements

Even though the proxy increases the window’s base monotonically, there is no mechanism whereby a SOCKS client can receive the Token Window Advertisements in order. As such, clients SHOULD disregard unsolicited Token Window Advertisements with a Window Base less than the previously known value.

9. Username/Password Authentication

Username/Password authentication is carried out as in [RFC1929].

Clients can also attempt to authenticate by placing the Username/Password request in an Authentication Data Option.

| Kind | Length | Method | Username/Password request |
|  1   |   2    |   1    |          Variable         |

Figure 30: Password authentication via a SOCKS Option

10. TCP Fast Open on the Client-Proxy Leg

TFO breaks TCP semantics, causing replays of the data in the SYN’s payload under certain rare circumstances [RFC7413]. A replayed SOCKS Request could itself result in a replayed connection on behalf of the client.

As such, client implementations SHOULD NOT use TFO on the client-proxy leg unless:

11. False Starts

In case of CONNECT Requests, the client MAY start sending application data as soon as possible, as long as doing so does not incur the risk of breaking the SOCKS protocol.

Clients must work around the authentication phase by doing any of the following:

12. Security Considerations

12.1. Large requests

Given the format of the request message, a malicious client could craft a request that is in excess of 16 KB and proxies could be prone to DDoS attacks.

To mitigate such attacks, proxy implementations SHOULD be able to incrementally parse the requests. Proxies MAY close the connection to the client if:

12.2. Replay attacks

In TLS 1.3, early data (which is likely to contain a full SOCKS request) is prone to replay attacks.

While Token Expenditure options can be used to mitigate replay attacks, the initial Token Request is still vulnerable. As such, client implementations SHOULD NOT make use of TLS early data unless the Request attempts to spend a token.

12.3. Resource exhaustion

Malicious clients can issue a large number of Session Requests, forcing the proxy to keep large amounts of state.

To mitigate this, the proxy MAY implement policies restricting the number of concurrent sessions on a per-IP or per-user basis, or barring unauthenticated clients from establishing sessions.

13. IANA Considerations

This document requests that IANA allocate 1-byte option kinds for SOCKS 6 options. Further, this document requests the following option kinds:

This document also requests that IANA allocate a TCP and UDP port for SOCKS over TLS and DTLS, respectively.

14. Acknowledgements

The protocol described in this draft builds upon and is a direct continuation of SOCKS 5 [RFC1928].

15. References

15.1. Normative References

[RFC1929] Leech, M., "Username/Password Authentication for SOCKS V5", RFC 1929, DOI 10.17487/RFC1929, March 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.

15.2. Informative References

[I-D.ietf-tls-dtls-connection-id] Rescorla, E., Tschofenig, H. and T. Fossati, "Connection Identifiers for DTLS 1.2", Internet-Draft draft-ietf-tls-dtls-connection-id-04, March 2019.
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and L. Jones, "SOCKS Protocol Version 5", RFC 1928, DOI 10.17487/RFC1928, March 1996.
[RFC6824] Ford, A., Raiciu, C., Handley, M. and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S. and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018.

Authors' Addresses

Vladimir Olteanu University Politehnica of Bucharest EMail:
Dragos Niculescu University Politehnica of Bucharest EMail: