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Versions: 00

Internet Area Working Group                                   V. Olteanu
Internet-Draft                                              D. Niculescu
Intended status: Experimental        University Politehnica of Bucharest
Expires: December 30, 2017                                 June 28, 2017


                        SOCKS Protocol Version 6
                    draft-olteanu-intarea-socks-6-00

Abstract

   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 http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 30, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements language . . . . . . . . . . . . . . . . . . . .   3
   3.  Mode of operation . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Connection Requests . . . . . . . . . . . . . . . . . . . . .   5
   5.  SOCKS Options . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Authentication options  . . . . . . . . . . . . . . . . .   7
   6.  Authentication Replies  . . . . . . . . . . . . . . . . . . .   8
   7.  Operation Replies . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Handling CONNECT  . . . . . . . . . . . . . . . . . . . .  10
     7.2.  Handling BIND . . . . . . . . . . . . . . . . . . . . . .  10
     7.3.  Handling UDP ASSOCIATE  . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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.



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   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:

   o  The client sends as much information upfront as possible, and does
      not wait for the authentication process to conclude before
      requesting the creation of a socket.

   o  The connection request also mimics the semantics of TCP Fast Open
      [RFC7413].  As part of the connection request, the client can
      supply the payload for the initial SYN that is sent out to the
      server.

   o  The protocol can be extended via options without breaking
      backward-compatibility.

   o  The protocol can leverage the aforementioned options to support
      0-RTT authentication schemes.

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


















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    CLIENT                                                        PROXY

            +------------------------+
            | Authentication methods | Request
    --------> Command code           +------------------------------>
            | TFO                    |
            | 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.





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   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.  Connection Requests

   The client starts by sending a request to the proxy.

   +---------------+-----------+----------+
   |    Version    | Number of | Methods  |
   | Major | Minor |  Methods  |          |
   +-------+-------+-----------+----------+
   |   1   |   1   |     1     | Variable |
   +-------+-------+-----------+----------+
   +---------+-----+---------+----------+------+
   | Command | TFO | Address | Address  | Port |
   |  Code   |     |  Type   |          |      |
   +---------+-----+---------+----------+------+
   |    1    |  1  |    1    | Variable |  2   |
   +---------+-----+---------+----------+------+
   +-----------+----------+--------------+--------------+
   | Number of | Options  | Initial Data | Initial Data |
   |  Options  |          |     Size     |              |
   +-----------+----------+--------------+--------------+
   |     1     | Variable |      2       |   Variable   |
   +-----------+----------+--------------+--------------+


                         Figure 2: SOCKS 6 Request

   o  Version: The major byte MUST be set to 0x06, and the minor byte
      MUST be set to 0x00.

   o  Number of Methods: The number of supported authentication methods
      that the client wishes to advertise.

   o  Methods: One byte per advertised method.  Method numbers are
      assigned by IANA.

   o  Command Code:

      *  0x00 AUTH: authenticate the client and do nothing.




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      *  0x01 CONNECT: requests the establishment of a TCP connection.

      *  0x02 BIND: requests the establishment of a TCP port binding.

      *  0x03 UDP ASSOCIATE: requests a UDP port association.

   o  TFO:

      *  0x00 indicates that 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 an AUTH or UDP ASSOCIATE command, this
         field MUST be set to 0x00.

      *  0x01 indicates that the proxy SHOULD attempt to use TFO in case
         of a CONNECT command, or accept TFO in case of a BIND command.

   o  Address Type:

      *  0x01: IPv4

      *  0x03: Domain Name

      *  0x04: IPv6

   o  Address: this field's format depends on the address type:

      *  IPv4: a 4-byte IPv4 address

      *  Domain Name: one byte that contains the length of the FQDN,
         followed by the FQDN itself.  The string is not NUL-terminated.

      *  IPv6: a 16-byte IPv6 address

   o  Port: the port in network byte order.

   o  Number of Options: the number of SOCKS options that appear in the
      Options field.

   o  Options: see section Section 5.

   o  Initial Data Size: A two-byte number in network byte order.  In
      case of AUTH, BIND or UDP ASSOCIATE, this field MUST be set to 0.
      In case of CONNECT, this is the number of bytes of initial data
      that are supplied in the following field.

   o  Initial Data: The first octets of the data stream.





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   Clients MUST support the "No authentication required" method.
   Clients MAY omit advertising the "No authentication required" option.

   Clients SHOULD NOT issue AUTH commands unless they advertise
   authentication methods with support for 0-RTT authentication.

   The server MAY truncate the initial data to an arbitrary size and
   disregard the rest.

5.  SOCKS Options

   SOCKS options have the following format:

   +---------------+-------------+
   | Kind | Length | Option Data |
   +------+--------+-------------+
   |  1   |   1    |   Variable  |
   +------+--------+-------------+


                         Figure 3: SOCKS 6 Option

   o  Kind: MUST be allocated by IANA.  (See section Section 9.)

   o  Length: The length of the option data.

   o  Option Data: the contents are specific to each option kind.

5.1.  Authentication options

   Authentication options have the following format:

   +---------------+--------+---------------------+
   | Kind | Length | Method | Authentication Data |
   +------+--------+--------+---------------------+
   |  1   |   1    |   1    |       Variable      |
   +------+--------+--------+---------------------+


                      Figure 4: Authentication Option

   o  Kind: MUST be allocated by IANA.  (See section Section 9.)

   o  Length: the length of the option data.

   o  Method: the number of the authentication method.  These numbers
      are assigned by IANA.




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   o  Authentication Data: the contents are specific to each method.

   All proxy implementations MUST support authentication method options.
   Clients MAY omit advertising authentication methods for which they
   have included at least an authentication option.

6.  Authentication Replies

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

   +---------------+------+--------+-----------+----------+
   |    Version    | Type | Method | Number of | Options  |
   | Major | Minor |      |        |  Options  |          |
   +-------+-------+------+--------+-----------+----------+
   |   1   |   1   |  1   |   1    |     1     | Variable |
   +-------+-------+------+--------+-----------+----------+


                  Figure 5: SOCKS 6 Authentication Reply

   o  Version: The major byte MUST be set to 0x06, and the minor byte
      MUST be set to 0x00.

   o  Type:

      *  0x00: authentication successful.

      *  0x01: further authentication needed.

   o  Method: The chosen authentication method.

   o  Number of Options: the number of SOCKS options that appear in the
      Options field.

   o  Options: see section Section 5.

   Multihomed clients SHOULD cache the chosen method on a per-interface
   basis and SHOULD NOT include authentication 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



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   option.  Descriptions of such negotiations are beyond the scope of
   this memo.

   If the cliend issued an AUTH command, the client MUST close the
   connection after the negociation is complete.

7.  Operation Replies

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

   +---------------+-------+---------+----------+------+
   |    Version    | Reply | Address |   Bind   | Bind |
   | Major | Minor | Code  |  Type   | Address  | Port |
   +-------+-------+-------+---------+----------+------+
   |   1   |   1   |   1   |    1    | Variable |  2   |
   +-------+-------+-------+---------+----------+------+
   +-----------+----------+--------------+
   | Number of | Options  | Initial Data |
   |  Options  |          |    Offset    |
   +-----------+----------+--------------+
   |     1     | Variable |      2       |
   +-----------+----------+--------------+


                     Figure 6: SOCKS 6 Operation Reply

   o  Version: The major byte MUST be set to 0x06, and the minor byte
      MUST be set to 0x00.

   o  Reply Code:

      *  0x00: Succes

      *  0x01: General SOCKS server failure

      *  0x02: Connection not allowed by ruleset

      *  0x03: Network unreachable

      *  0x04: Host unreachable

      *  0x05: Connection refused

      *  0x06: TTL expired

      *  0x07: Command not supported




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      *  0x08: Address type not supported

   o  Address Type:

      *  0x01: IPv4

      *  0x03: Domain Name

      *  0x04: IPv6

   o  Bind Address: the proxy bound address in the following format:

      *  IPv4: a 4-byte IPv4 address

      *  Domain Name: one byte that contains the length of the FQDN,
         followed by the FQDN itself.  The string is not NUL-terminated.

      *  IPv6: a 16-byte IPv6 address

   o  Bind Port: the proxy bound port in network byte order.

   o  Number of Options: the number of SOCKS options that appear in the
      Options field.

   o  Options: see section Section 5

   o  Initial Data Offset: A two-byte number in network byte order.  In
      case of BIND or UDP ASSOCIATE, this field MUST be set to 0.  In
      case of CONNECT, it represents the offset in the plain data stream
      from which the client is expected to continue sending data.

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

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.




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7.3.  Handling UDP ASSOCIATE

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

8.  Security Considerations

   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:

   o  the request is not fully received after a certain timeout, or

   o  the number of options exceeds an imposed hard cap, or

   o  the total size of the options exceeds an imposed hard cap, or

   o  the size of the initial data excedes a hard cap.

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

9.  IANA Considerations

   This document requests that IANA allocate option codes for SOCKS 6
   options.  Further, this document requests an option code for
   authentication options.

10.  Acknowledgements

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

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.







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11.2.  Informative References

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-20 (work in progress),
              April 2017.

   [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,
              <http://www.rfc-editor.org/info/rfc1928>.

   [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,
              <http://www.rfc-editor.org/info/rfc6824>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <http://www.rfc-editor.org/info/rfc7413>.

Authors' Addresses

   Vladimir Olteanu
   University Politehnica of Bucharest

   Email: vladimir.olteanu@cs.pub.ro


   Dragos Niculescu
   University Politehnica of Bucharest

   Email: dragos.niculescu@cs.pub.ro


















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