[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-olteanu-socks-6) 00 01 02 03 04 05 06 07

Internet Area Working Group                                   V. Olteanu
Internet-Draft                                              D. Niculescu
Intended status: Experimental        University Politehnica of Bucharest
Expires: April 26, 2019                                 October 23, 2018


                        SOCKS Protocol Version 6
                    draft-olteanu-intarea-socks-6-05

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 April 26, 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



Olteanu & Niculescu      Expires April 26, 2019                 [Page 1]


Internet-Draft                   SOCKS 6                    October 2018


   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Revision log  . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements language . . . . . . . . . . . . . . . . . . . .   7
   3.  Mode of operation . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Requests  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Version Mismatch Replies  . . . . . . . . . . . . . . . . . .  10
   6.  Authentication Replies  . . . . . . . . . . . . . . . . . . .  11
   7.  Operation Replies . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Handling CONNECT  . . . . . . . . . . . . . . . . . . . .  13
     7.2.  Handling BIND . . . . . . . . . . . . . . . . . . . . . .  14
     7.3.  Handling UDP ASSOCIATE  . . . . . . . . . . . . . . . . .  14
       7.3.1.  Proxying UDP servers  . . . . . . . . . . . . . . . .  16
   8.  SOCKS Options . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Stack options . . . . . . . . . . . . . . . . . . . . . .  16
       8.1.1.  IP TOS options  . . . . . . . . . . . . . . . . . . .  18
       8.1.2.  TFO options . . . . . . . . . . . . . . . . . . . . .  18
       8.1.3.  Multipath TCP options . . . . . . . . . . . . . . . .  19
       8.1.4.  MPTCP Scheduler options . . . . . . . . . . . . . . .  19
       8.1.5.  Listen Backlog options  . . . . . . . . . . . . . . .  20
     8.2.  Authentication Method options . . . . . . . . . . . . . .  21
     8.3.  Authentication Data options . . . . . . . . . . . . . . .  22
     8.4.  Idempotence options . . . . . . . . . . . . . . . . . . .  22
       8.4.1.  Requesting a fresh token window . . . . . . . . . . .  23
       8.4.2.  Spending a token  . . . . . . . . . . . . . . . . . .  24
       8.4.3.  Handling Token Window Advertisements  . . . . . . . .  26
   9.  Username/Password Authentication  . . . . . . . . . . . . . .  26
   10. TCP Fast Open on the Client-Proxy Leg . . . . . . . . . . . .  26
   11. False Starts  . . . . . . . . . . . . . . . . . . . . . . . .  27
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  27
     12.1.  Large requests . . . . . . . . . . . . . . . . . . . . .  27
     12.2.  Replay attacks . . . . . . . . . . . . . . . . . . . . .  28
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     15.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29







Olteanu & Niculescu      Expires April 26, 2019                 [Page 2]


Internet-Draft                   SOCKS 6                    October 2018


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:

   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.



Olteanu & Niculescu      Expires April 26, 2019                 [Page 3]


Internet-Draft                   SOCKS 6                    October 2018


1.1.  Revision log

   Typos and minor clarifications are not listed.

   draft-05

   o  Limited the "slow" authentication negociations to one (and
      Authentication Replies to 2)

   o  Revamped the handling of the first bytes in the application data
      stream

      *  False starts are now recommended.  (Added the "False Start"
         section.)

      *  Initial data is only available to clients willing to do "slow"
         authentication.  Moved the "Initial data size" field from
         Requests to Authentication Method options.

      *  Initial data size capped at 2^13.  Initial data can no longer
         be dropped by the proxy.

      *  The TFO option can hint at the desired SYN payload size.

   o  Request: clarified the meaning of the Address and Port fields.

   o  Better reverse TCP proxy support: optional listen backlog for TCP
      BIND

   o  TFO options can no longer be placed inside Operation Replies.

   o  IP TOS stack option

   o  Suggested a range for vendor-specific options.

   o  Revamped UDP functionality

      *  Now using fixed UDP ports

      *  DTLS support

   o  Stack options: renamed Proxy-Server leg to Proxy-Remote leg

   draft-04

   o  Moved Token Expenditure Replies to the Authentication Reply.





Olteanu & Niculescu      Expires April 26, 2019                 [Page 4]


Internet-Draft                   SOCKS 6                    October 2018


   o  Shifted the Initial Data Size field in the Request, in order to
      make it easier to parse.

   draft-03

   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.




Olteanu & Niculescu      Expires April 26, 2019                 [Page 5]


Internet-Draft                   SOCKS 6                    October 2018


         +  Tweaked token expenditure response codes.

         +  Support no longer mandatory on the proxy side.

      *  Revamped Socket options

         +  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




Olteanu & Niculescu      Expires April 26, 2019                 [Page 6]


Internet-Draft                   SOCKS 6                    October 2018


   o  Added this section.

   o  Support for idempotent commands.

   o  Removed version numbers from operation replies.

   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




























Olteanu & Niculescu      Expires April 26, 2019                 [Page 7]


Internet-Draft                   SOCKS 6                    October 2018


    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 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



Olteanu & Niculescu      Expires April 26, 2019                 [Page 8]


Internet-Draft                   SOCKS 6                    October 2018


   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  |
   |  Options  |          |
   +-----------+----------+
   |     1     | 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  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.




Olteanu & Niculescu      Expires April 26, 2019                 [Page 9]


Internet-Draft                   SOCKS 6                    October 2018


      *  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.

   The Address and Port fields have different meanings based on the
   Command Code: * NOOP: The fields have no meaning.  The Address Type
   field MUST be either 0x01 (IPv4) or 0x04 (IPv6).  The Address and
   Port fields MUST be 0.  * CONNECT: The fields signify the address and
   port to which the client wishes to connect.  * BIND, UDP ASSOCIATE:
   The fields indicate the desired bind address and port.  If the client
   does not require a certain address, it can set the Address Type field
   to 0x01 (IPv4) or 0x04 (IPv6), and the Address field to 0.  Likewise,
   if the client does not require a certain port, it can set the Port
   field to 0.

   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:







Olteanu & Niculescu      Expires April 26, 2019                [Page 10]


Internet-Draft                   SOCKS 6                    October 2018


   +---------------+
   |    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.

   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



Olteanu & Niculescu      Expires April 26, 2019                [Page 11]


Internet-Draft                   SOCKS 6                    October 2018


   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 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   | Number of | Options  |
   | Code  | Port |  Type   | Address  |  Options  |          |
   +-------+------+---------+----------+-----------+----------+
   |   1   |  2   |    1    | Variable |     1     | Variable |
   +-------+------+---------+----------+-----------+----------+


                     Figure 5: SOCKS 6 Operation Reply

   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




Olteanu & Niculescu      Expires April 26, 2019                [Page 12]


Internet-Draft                   SOCKS 6                    October 2018


      *  0x06: TTL expired

      *  0x07: Command not supported

      *  0x08: Address type not supported

      *  0x09: Connection attempt timed out

   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.

   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.







Olteanu & Niculescu      Expires April 26, 2019                [Page 13]


Internet-Draft                   SOCKS 6                    October 2018


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

   o  Association ID: the identifier of the UDP association

   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

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



Olteanu & Niculescu      Expires April 26, 2019                [Page 14]


Internet-Draft                   SOCKS 6                    October 2018


   o  Association ID: the identifier of the UDP 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.

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

   o  the 5-tuple of the UDP conversation, if the datagram was received
      over plain UDP, or

   o  the DTLS connection, if the datagram was received over DTLS.  The
      DTLS connection is identified either by its 5-tuple, or some other
      mechanism, like [I-D.ietf-tls-dtls-connection-id].

   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

   o  Status: MUST be 0x00




Olteanu & Niculescu      Expires April 26, 2019                [Page 15]


Internet-Draft                   SOCKS 6                    October 2018


   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   |   1    |   Variable  |
   +------+--------+-------------+


                         Figure 9: SOCKS 6 Option

   o  Kind: MUST be allocated by IANA.  (See Section 13.)

   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
   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,



Olteanu & Niculescu      Expires April 26, 2019                [Page 16]


Internet-Draft                   SOCKS 6                    October 2018


   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.

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


                          Figure 10: Stack Option

   o  Kind: 0x01 (Stack option)

   o  Length: The length of the option.

   o  Leg:

      *  0x1: Client-Proxy Leg

      *  0x2: Proxy-Remote Leg

      *  0x3: Both Legs

   o  Level:

      *  0x01: IP

      *  0x02: IPv4

      *  0x03: IPv6

      *  0x04: TCP

      *  0x05: UDP




Olteanu & Niculescu      Expires April 26, 2019                [Page 17]


Internet-Draft                   SOCKS 6                    October 2018


   o  Code: Option code

   o  Data: Option-specific data

8.1.1.  IP TOS options

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


                         Figure 11: IP TOS Option

   o  Kind: 0x01 (Stack option)

   o  Length: 4

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

   o  Level: 0x04 (TCP).

   o  Code: 0x01

   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   |   1    | 2 bits | 6 bits |  1   |      2       |
   +------+--------+--------+--------+------+--------------+


                           Figure 12: TFO Option

   o  Kind: 0x01 (Stack option)

   o  Length: 4

   o  Leg: 0x2 (Proxy-Remote leg).

   o  Level: 0x04 (TCP).



Olteanu & Niculescu      Expires April 26, 2019                [Page 18]


Internet-Draft                   SOCKS 6                    October 2018


   o  Code: 0x01

   o  Payload Size: The desired payload size of the TFO SYN.  MUST be 0
      in case of a BIND command.

   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.  The proxy MAY use a different 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   |   1    | 2 bits | 6 bits |  1   |
   +------+--------+--------+--------+------+


                      Figure 13: Multipath TCP Option

   o  Kind: 0x01 (Stack option)

   o  Length: 4

   o  Leg: 0x2 (Proxy-Remote leg)

   o  Level: 0x04 (TCP).

   o  Code: 0x02

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.





Olteanu & Niculescu      Expires April 26, 2019                [Page 19]


Internet-Draft                   SOCKS 6                    October 2018


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


                     Figure 14: MPTCP Scheduler Option

   o  Kind: 0x01 (Stack option)

   o  Length: 5

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

   o  Level: 0x04 (TCP)

   o  Code: 0x03

   o  Scheduler:

      *  0x01: Default

      *  0x02: Round-Robin

      *  0x03: Redundant

8.1.5.  Listen Backlog options

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


                     Figure 15: Listen Backlog Option

   o  Kind: 0x01 (Stack option)

   o  Length: 5

   o  Leg: 0x02 (Proxy-Remote leg)

   o  Level: 0x04 (TCP)

   o  Code: 0x04



Olteanu & Niculescu      Expires April 26, 2019                [Page 20]


Internet-Draft                   SOCKS 6                    October 2018


   o  Backlog: The length of the listen backlog.  MUST be greater than
      1.

   The default behavior of the BIND does not allow a client to
   simultaneously handle multiple connections to the same bind address.
   An authenticated client can alter BIND's behavior by adding a TCP
   Listen Backlog Option to a BIND Request.

   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.

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   |   1    |          2          | Variable |
   +------+--------+---------------------+----------+


                  Figure 16: Authentication Method Option

   o  Kind: 0x02 (Authentication Method option)

   o  Length: The length of the option.

   o  Initial Data Size: A two-byte number in network byte order.  In
      case of CONNECT, this is the number of bytes of initial data that
      are supplied by the client immediately following the Request.
      This number MUST NOT be larger than 2^13.




Olteanu & Niculescu      Expires April 26, 2019                [Page 21]


Internet-Draft                   SOCKS 6                    October 2018


   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:

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


                   Figure 17: 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.





Olteanu & Niculescu      Expires April 26, 2019                [Page 22]


Internet-Draft                   SOCKS 6                    October 2018


   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 18: Idempotence Option

   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 19: Token Request

   o  Kind: MUST be allocated by IANA.  (See Section 13.)




Olteanu & Niculescu      Expires April 26, 2019                [Page 23]


Internet-Draft                   SOCKS 6                    October 2018


   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:

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


                   Figure 20: 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 21: Token Expenditure




Olteanu & Niculescu      Expires April 26, 2019                [Page 24]


Internet-Draft                   SOCKS 6                    October 2018


   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.

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

   +------+--------+------+---------------+
   | Kind | Length | Type | Response Code |
   +------+--------+------+---------------+
   |  1   |   1    |  1   |       1       |
   +------+--------+------+---------------+


                   Figure 22: 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



Olteanu & Niculescu      Expires April 26, 2019                [Page 25]


Internet-Draft                   SOCKS 6                    October 2018


   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.

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
   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, provided that it
   is no longer than 252 bytes.

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


           Figure 23: Password authentication via a SOCKS Option

   o  Kind: MUST be allocated by IANA.  (See Section 13.)

   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




Olteanu & Niculescu      Expires April 26, 2019                [Page 26]


Internet-Draft                   SOCKS 6                    October 2018


   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:

   o  The protocol running on top of SOCKS tolerates the risks of TFO,
      or

   o  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

   o  The client uses Idempotence Options, making replays impossible, or

   o  SOCKS is running on top of TLS and Early Data is not used.

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:

   o  If the Request does not contain an Authentication Method option,
      the authentication phase is guaranteed not to happen.  In this
      case, application data MAY be sent immediately after the Request.

   o  Application data MAY be sent immediately after receiving an
      Authentication Reply indicating success.

   o  When performing a method-specific authentication sequence,
      application data MAY be sent immediately after the last client
      message.

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 80 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:



Olteanu & Niculescu      Expires April 26, 2019                [Page 27]


Internet-Draft                   SOCKS 6                    October 2018


   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.

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

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 when
   sending a Token Request.

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:

   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: 0xC0-0xFF

   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







Olteanu & Niculescu      Expires April 26, 2019                [Page 28]


Internet-Draft                   SOCKS 6                    October 2018


15.1.  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>.

15.2.  Informative References

   [I-D.ietf-tls-dtls-connection-id]
              Rescorla, E., Tschofenig, H., Fossati, T., and T. Gondrom,
              "Connection Identifiers for DTLS 1.2", draft-ietf-tls-
              dtls-connection-id-02 (work in progress), October 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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

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



Olteanu & Niculescu      Expires April 26, 2019                [Page 29]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/