Internet Area Working Group V. Olteanu
Internet-Draft D. Niculescu
Intended status: Experimental University Politehnica of Bucharest
Expires: January 3, 2019 July 02, 2018

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

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This Internet-Draft will expire on January 3, 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 [I-D.ietf-tls-tls13] 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)------------------>

     Operation reply   | Reply code            |
  <--------------------+ Bind address          <------------------
                       | Bind port             |
                       | Options               |
                       | Initial data offset   |

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 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 |
| Number of | Options  | Initial Data | Initial Data |
|  Options  |          |     Size     |              |
|     1     | Variable |      2       |   Variable   |

Figure 2: SOCKS 6 Request

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

The server MAY truncate the initial data to an arbitrary size and disregard the rest. This is will be communicated later to the client, should the authentication process be successful (see Section 7). As such, server implementations do not have to buffer the initial data while waiting for the (potentially malicious) client to authenticate.

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 | Number of | Options  |
| Major | Minor |      |        |  Options  |          |
|   1   |   1   |  1   |   1    |     1     | 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. The client and proxy SHOULD NOT negotiate the encryption of the application data. Descriptions of such negotiations are beyond the scope of this memo. When the negotiation is complete (either successfully or unsuccessfully), the proxy sends another Authentication Reply.

7. Operation Replies

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

| Reply | Initial Data | Bind | Address |   Bind   |
| Code  |    Offset    | Port |  Type   | Address  |
|   1   |      2       |  2   |    1    | Variable |
| Number of | Options  |
|  Options  |          |
|     1     | 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. The client MUST resume the data stream at the offset indicated by the Initial Data Offset field.

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

The relay of UDP packets is handled exactly as in SOCKS 5 [RFC1928].

8. SOCKS Options

SOCKS options have the following format:

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

Figure 6: 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 server (i.e. IP, TCP, UDP). A Stack option can affect either the proxy’s protocol on the client-proxy leg or on the proxy-server 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.

Stack options that are part of the same message MUST NOT contradict one another.

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

Figure 7: Stack Option

8.1.1. TFO options

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

Figure 8: 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 proxy MAY include a TFO option in the Operation reply if TFO was attempted, the operation succeded and the remote server supports TFO. In case of a BIND command, the proxy MAY include a TFO option in the first Operation reply to signal that it will accept an incoming TFO connection.

8.1.2. 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   |   1    | 2 bits | 6 bits |  1   |

Figure 9: Multipath TCP Option

8.1.3. 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   |   1    | 2 bits | 6 bits |  1   |     1     |

Figure 10: MPTCP Scheduler Option

8.2. Authentication Method options

Authentication Method options are used by clients to advertise supported authentication methods. They can be part of SOCKS Requests.

| Kind | Length | Methods  |
|  1   |   1    | Variable | 

Figure 11: 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   |   1    |   1    |       Variable      |

Figure 12: 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. Idempotence options

To protect against duplicate SOCKS Requests, authenticated 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   |   1    |  1   |   Variable  |

Figure 13: Idempotence Option

8.4.1. 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   |   1    |  1   |      4      |

Figure 14: 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   |   1    |  1   |      4      |      4      |

Figure 15: Token Window Advertisement

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

8.4.2. 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   |   1    |  1   |   4   |

Figure 16: Token Expenditure

Clients SHOULD prioritize spending the smaller tokens.

The server responds by sending a Token Expenditure Reply option as part of the Operation Reply:

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

Figure 17: Token Expenditure Response

If eligible, the token is spent as soon as the client authenticates. If the token is not eligible for spending, the proxy MUST NOT attempt to honor the client’s SOCKS 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.

8.4.3. 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 unsollicited 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, provided that it is no longer than 252 bytes.

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

Figure 18: 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: * The protocol running on top of SOCKS tolerates the risks of TFO, or * The SYN’s payload does not contain any application data (so that no data is replayed to the server, even though duplicate connections are still possible), or * The client uses Idempotence Options, making replays impossible, or * SOCKS is running on top of TLS and Early Data is not used.

11. Security Considerations

11.1. Large requests

Given the format of the request message, a malicious client could craft a request that is in excess of 100 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:

Further, the server MAY choose not to buffer any initial data beyond what would be expected to fit in a TFO SYN’s payload.

11.2. 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 when sending a Token Request.

12. 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 port for SOCKS over TLS.

13. Acknowledgements

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

14. References

14.1. 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.

14.2. Informative References

[I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-28, March 2018.
[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.

Authors' Addresses

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