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Versions: (draft-olteanu-socks-6) 00 01 02 03

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
                    draft-olteanu-intarea-socks-6-03

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 https://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 January 3, 2019.

Copyright Notice

   Copyright (c) 2018 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
   (https://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
     1.1.  Revision log  . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements language . . . . . . . . . . . . . . . . . . . .   6
   3.  Mode of operation . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Requests  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Version Mismatch Replies  . . . . . . . . . . . . . . . . . .   9
   6.  Authentication Replies  . . . . . . . . . . . . . . . . . . .   9
   7.  Operation Replies . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Handling CONNECT  . . . . . . . . . . . . . . . . . . . .  12
     7.2.  Handling BIND . . . . . . . . . . . . . . . . . . . . . .  12
     7.3.  Handling UDP ASSOCIATE  . . . . . . . . . . . . . . . . .  12
   8.  SOCKS Options . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Stack options . . . . . . . . . . . . . . . . . . . . . .  13
       8.1.1.  TFO options . . . . . . . . . . . . . . . . . . . . .  14
       8.1.2.  Multipath TCP options . . . . . . . . . . . . . . . .  14
       8.1.3.  MPTCP Scheduler options . . . . . . . . . . . . . . .  15
     8.2.  Authentication Method options . . . . . . . . . . . . . .  16
     8.3.  Authentication Data options . . . . . . . . . . . . . . .  16
     8.4.  Idempotence options . . . . . . . . . . . . . . . . . . .  17
       8.4.1.  Requesting a fresh token window . . . . . . . . . . .  18
       8.4.2.  Spending a token  . . . . . . . . . . . . . . . . . .  19
       8.4.3.  Handling Token Window Advertisements  . . . . . . . .  21
   9.  Username/Password Authentication  . . . . . . . . . . . . . .  21
   10. TCP Fast Open on the Client-Proxy Leg . . . . . . . . . . . .  21
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  22
     11.1.  Large requests . . . . . . . . . . . . . . . . . . . . .  22
     11.2.  Replay attacks . . . . . . . . . . . . . . . . . . . . .  22
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     14.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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



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   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:

   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 potential 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.

1.1.  Revision log

   Typos and minor clarifications are not listed.

   draft-03




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   o  Shifted some fields in the Operation Reply to make it easier to
      parse.

   o  Added connection attempt timeout response code to Operation
      Replies.

   o  Proxies send an additional Authentication Reply after the
      authentication phase.  (Useful for token window advertisements.)

   o  Renamed the section "Connection Requests" to "Requests"

   o  Clarified the fact that proxies don't need to support any command
      in particular.

   o  Added the section "TCP Fast Open on the Client-Proxy Leg"

   o  Options:

      *  Added constants for option kinds

      *  Salt options removed, along with the relevant section from
         Security Considerations.  (TLS 1.3 Makes AEAD mandatory.)

      *  Limited Authentication Data options to one per method.

      *  Relaxed proxy requirements with regard to handling multiple
         Authentication Data options.  (When the client violates the
         above bullet point.)

      *  Removed interdependence between Authentication Method and
         Authentication Data options.

      *  Clients SHOULD omit advertising the "No authentication
         required" option.  (Was MAY.)

      *  Idempotence options:

         +  Token Window Advertisements are now part of successful
            Authentication Replies (so that the proxy-server RTT has no
            impact on their timeliness).

         +  Proxies can't advetise token windows of size 0.

         +  Tweaked token expenditure response codes.

         +  Support no longer mandatory on the proxy side.

      *  Revamped Socket options



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         +  Renamed Socket options to Stack options.

         +  Banned contradictory socket options.

         +  Added socket level for generic IP.  Removed the "socket"
            socket level.

         +  Stack options no longer use option codes from setsockopt().

         +  Changed MPTCP Scheduler constants.

   draft-02

   o  Made support for Idempotence options mandatory for proxies.

   o  Clarified what happens when proxies can not or will not issue
      tokens.

   o  Limited token windows to 2^31 - 1.

   o  Fixed definition of "less than" for tokens.

   o  NOOP commands now trigger Operation Replies.

   o  Renamed Authentication options to Authentication Data options.

   o  Authentication Data options are no longer mandatory.

   o  Authentication methods are now advertised via options.

   o  Shifted some Request fields.

   o  Option range for vendor-specific options.

   o  Socket options.

   o  Password authentication.

   o  Salt options.

   draft-01

   o  Added this section.

   o  Support for idempotent commands.

   o  Removed version numbers from operation replies.




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   o  Request port number for SOCKS over TLS.  Deprecate encryption/
      encapsulation within SOCKS.

   o  Added Version Mismatch Replies.

   o  Renamed the AUTH command to NOOP.

   o  Shifted some fields to make requests and operation replies easier
      to parse.

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



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   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

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



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   o  Command Code:

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

      *  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  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 8.

   o  Initial Data Size: A two-byte number in network byte order.  In
      case of NOOP, 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.

   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



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   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

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

   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

   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.



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

   o  Options: see Section 8.

   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

   o  Reply Code:

      *  0x00: Succes




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      *  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

      *  0x08: Address type not supported

      *  0x09: Connection attempt timed out

   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.

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

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

   o  Options: see Section 8.





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   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

   o  Kind: MUST be allocated by IANA.  (See Section 12.)

   o  Length: The length of the option.

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

   Unless otherwise noted, client and proxy implementations MAY omit
   supporting any of the options described in this document.  Upon



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   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

   o  Kind: 0x01 (Stack option)

   o  Length: The length of the option.

   o  Leg:

      *  0x1: Client-Proxy Leg

      *  0x2: Proxy-Server Leg

      *  0x3: Both Legs

   o  Level:

      *  0x01: IP

      *  0x02: IPv4

      *  0x03: IPv6



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      *  0x04: TCP

      *  0x05: UDP

   o  Code: Option code

   o  Data: Option-specific data

8.1.1.  TFO options

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


                           Figure 8: TFO Option

   o  Kind: 0x01 (Stack option)

   o  Length: MUST be 4.

   o  Leg: MUST be 0x2 (Proxy-Server Leg).

   o  Level: 0x04 (TCP).

   o  Code: 0x01

   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.







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


                      Figure 9: Multipath TCP Option

   o  Kind: 0x01 (Stack option)

   o  Length: 4.

   o  Leg: MUST be 0x2 (Proxy-Server Leg).

   o  Level: 0x04 (TCP).

   o  Code: 0x02

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

   o  Kind: 0x01 (Stack option)

   o  Length: MUST be 5.

   o  Leg: Either 0x01, 0x02, or 0x03 (Client-Proxy, Proxy-Client or
      Both legs).

   o  Level: 0x04 (TCP).

   o  Code: 0x03




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   o  Scheduler:

      *  0x01: Default

      *  0x02: Round-Robin

      *  0x03: Redundant

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

   o  Kind: 0x02 (Authentication Method option)

   o  Length: The length of the option.

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

   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:











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


                   Figure 12: Authentication Data Option

   o  Kind: 0x03 (Authentication Data option)

   o  Length: The length of the option.

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

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

   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



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   o  Kind: 0x04 (Idempotence option)

   o  Length: The length of the option.

   o  Type:

      *  0x00: Token Request

      *  0x01: Token Window Advertisement

      *  0x02: Token Expenditure

      *  0x03: Token Expenditure Reply

   o  Option Data: The contents are specific to each type.

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

   o  Kind: MUST be allocated by IANA.  (See Section 12.)

   o  Length: 7

   o  Type: 0x00 (Token Request)

   o  Window Size: The requested window size.

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










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   +---------------+------+-------------+-------------+
   | Kind | Length | Type | Window Base | Window Size |
   +------+--------+------+-------------+-------------+
   |  1   |   1    |  1   |      4      |      4      |
   +------+--------+------+-------------+-------------+


                   Figure 15: Token Window Advertisement

   o  Kind: 0x04 (Idempotence option)

   o  Length: 11

   o  Type: 0x01 (Token Grant)

   o  Window Base: The first token in the window.

   o  Window Size: The window size.  This value SHOULD be lower or equal
      to the requested window size.  Window sizes MUST be less than
      2^31.  Window sizes MUST NOT be 0.

   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

   o  Kind: 0x04 (Idempotence option)

   o  Length: 7

   o  Type: 0x02 (Token Expenditure)

   o  Token: The token being spent.

   Clients SHOULD prioritize spending the smaller tokens.




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   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

   o  Kind: 0x04 (Idempotence option)

   o  Length: 4

   o  Type: 0x03 (Token Expenditure Response)

   o  Response Code:

      *  0x01: Success: The token was spent successfully.

      *  0x02: No Window: The proxy does not have a token window
         associated with the client.

      *  0x03: Out of Window: The token is not within the window.

      *  0x04: Duplicate: The token has already been spent.

   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:

   o  the token is out of window, or

   o  by the proxy's internal logic, successfully spending the token
      caused the window to shift.

   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.




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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

   o  Kind: MUST be allocated by IANA.  (See Section 12.)

   o  Length: The length of the option.

   o  Method: 0x02 (Username/Password).

   o  Username/Password request: The Username/Password request, as
      described in [RFC1929].

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.



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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:

   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 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:

   o  Stack options: 0x01

   o  Authentication Method options: 0x02

   o  Authentication Data options: 0x03

   o  Idempotence options: 0x04

   o  A range for vendor-specific options



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   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,
              <https://www.rfc-editor.org/info/rfc1929>.

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

14.2.  Informative References

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              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,
              <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc7413>.

Authors' Addresses

   Vladimir Olteanu
   University Politehnica of Bucharest

   Email: vladimir.olteanu@cs.pub.ro



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   Dragos Niculescu
   University Politehnica of Bucharest

   Email: dragos.niculescu@cs.pub.ro















































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