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Versions: (draft-olteanu-socks-6) 00 01 02 03 04 05 06 07 08 09 10

Internet Area Working Group                                   V. Olteanu
Internet-Draft                                              D. Niculescu
Intended status: Experimental        University Politehnica of Bucharest
Expires: January 14, 2021                                  July 13, 2020


                        SOCKS Protocol Version 6
                    draft-olteanu-intarea-socks-6-10

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 14, 2021.

Copyright Notice

   Copyright (c) 2020 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Revision log  . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements language . . . . . . . . . . . . . . . . . . . .  10
   3.  Mode of operation . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Requests  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   5.  Version Mismatch Replies  . . . . . . . . . . . . . . . . . .  14
   6.  Authentication Replies  . . . . . . . . . . . . . . . . . . .  14
   7.  Operation Replies . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Handling CONNECT  . . . . . . . . . . . . . . . . . . . .  17
     7.2.  Handling BIND . . . . . . . . . . . . . . . . . . . . . .  17
     7.3.  Handling UDP ASSOCIATE  . . . . . . . . . . . . . . . . .  17
       7.3.1.  Proxying UDP servers  . . . . . . . . . . . . . . . .  20
       7.3.2.  Proxying multicast traffic  . . . . . . . . . . . . .  20
       7.3.3.  Reporting ICMP Errors . . . . . . . . . . . . . . . .  20
   8.  SOCKS Options . . . . . . . . . . . . . . . . . . . . . . . .  22
     8.1.  Stack options . . . . . . . . . . . . . . . . . . . . . .  22
       8.1.1.  IP TOS options  . . . . . . . . . . . . . . . . . . .  24
       8.1.2.  Happy Eyeballs options  . . . . . . . . . . . . . . .  24
       8.1.3.  TTL options . . . . . . . . . . . . . . . . . . . . .  25
       8.1.4.  No Fragmentaion options . . . . . . . . . . . . . . .  25
       8.1.5.  TFO options . . . . . . . . . . . . . . . . . . . . .  25
       8.1.6.  Multipath options . . . . . . . . . . . . . . . . . .  26
       8.1.7.  Listen Backlog options  . . . . . . . . . . . . . . .  27
       8.1.8.  UDP Error options . . . . . . . . . . . . . . . . . .  28
       8.1.9.  Port Parity options . . . . . . . . . . . . . . . . .  28
     8.2.  Authentication Method options . . . . . . . . . . . . . .  29
     8.3.  Authentication Data options . . . . . . . . . . . . . . .  31
     8.4.  Session options . . . . . . . . . . . . . . . . . . . . .  31
       8.4.1.  Session initiation  . . . . . . . . . . . . . . . . .  32
       8.4.2.  Further SOCKS Requests  . . . . . . . . . . . . . . .  33
       8.4.3.  Tearing down the session  . . . . . . . . . . . . . .  33
     8.5.  Idempotence options . . . . . . . . . . . . . . . . . . .  34
       8.5.1.  Requesting a token window . . . . . . . . . . . . . .  34
       8.5.2.  Spending a token  . . . . . . . . . . . . . . . . . .  35
       8.5.3.  Shifting windows  . . . . . . . . . . . . . . . . . .  37
       8.5.4.  Out-of-order Window Advertisements  . . . . . . . . .  37
   9.  Username/Password Authentication  . . . . . . . . . . . . . .  37
   10. TCP Fast Open on the Client-Proxy Leg . . . . . . . . . . . .  38
   11. False Starts  . . . . . . . . . . . . . . . . . . . . . . . .  38
   12. DNS provided by SOCKS . . . . . . . . . . . . . . . . . . . .  39
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  39



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     13.1.  Large requests . . . . . . . . . . . . . . . . . . . . .  39
     13.2.  Replay attacks . . . . . . . . . . . . . . . . . . . . .  40
     13.3.  Resource exhaustion  . . . . . . . . . . . . . . . . . .  40
   14. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  40
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  40
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  41
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  42
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  42
     17.2.  Informative References . . . . . . . . . . . . . . . . .  42
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

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.



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   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-10

   o  Removed untrusted sessions

   o  IP DF

   o  UDP relay:

      *  Support ICMPv6 Too Big

      *  Shifted some fields in the error messages

      *  RTP support

   draft-09

   o  Revamped UDP relay

      *  Support for ICMP errors: host/net unreachable, TTL exceeded

      *  Datagrams can be sent over TCP

      *  Timeout for the receipt of the initial datagram

   o  TTL stack option (intended use: traceroute)

   o  Added the "Privacy Considerations" section

   o  SOCKS-provided DNS: the proxy may provide a valid bind address and
      port

   draft-08




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   o  Removed Address Resolution options

   o  Happy Eyeballs options

   o  DNS provided by SOCKS

   draft-07

   o  All fields are now alignned.

   o  Eliminated version minors

   o  Lots of changes to options

      *  2-byte option kinds

      *  Flattened option kinds/types/reply codes; also renamed some
         options

      *  Socket options

         +  Proxies MUST always answer them (Clients can probe for
            support)

         +  MPTCP Options: expanded functionality ("please do/don't do
            MPTCP on my behalf")

         +  MPTCP Scheduler options removed

         +  Listen Backlog options: code changed to 0x03

      *  Revamped Idempotence options

      *  Auth data options limited to one per method

   o  Authentication Reply: all authentication-related information is
      now in the options

      *  Authentication replies no longer have a field indicating the
         chosen auth. method

      *  Method that must proceed (or whereby authentication succeeded)
         indicated in options

      *  Username/password authentication: proxy now sends reply in
         option





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   o  Removed requirements w.r.t. caching authentication methods by
      multihomed clients

   o  UDP: 8-byte association IDs

   o  Sessions

      *  The proxy is now free to terminate ongoing connections along
         with the session.

      *  The session-terminating request is not part of the session that
         it terminated.

   o  Address Resolution options

   draft-06

   o  Session options

   o  Options now have a 2-byte length field.

   o  Stack options

      *  Stack options can no longer contain duplicate information.

      *  TFO: Better payload size semantics

      *  TOS: Added missing code field.

      *  MPTCP Scheduler options:

         +  Removed support for round-robin

         +  "Default" renamed to "Lowest latency first"

      *  Listen Backlog options: now tied to sessions, instead of an
         authenticated user

   o  Idempotence options

      *  Now used in the context of a session (no longer tied to an
         authenticated user)

      *  Idempotence options have a different codepoint: 0x05.  (Was
         0x04.)

      *  Clarified that implementations that support Idempotence Options
         must support all Idempotence Option Types.



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      *  Shifted Idempotence Option Types by 1.  (Makes implementation
         easier.)

   o  Shrunk vendor-specific option range to 32 (down from 64).

   o  Removed reference to dropping initial data.  (It could no longer
      be done as of -05.)

   o  Initial data size capped at 16KB.

   o  Application data is never encrypted by SOCKS 6.  (It can still be
      encrypted by the TLS layer under SOCKS.)

   o  Messages now carry the total length of the options, rather than
      the number of options.  Limited options length to 16KB.

   o  Security Considerations

      *  Updated the section to reflect the smaller maximum message
         size.

      *  Added a subsection on resource exhaustion.

   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




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   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.

   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.




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      *  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

         +  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.




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   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.

   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










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    CLIENT                                                        PROXY

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

            +------------------------+
    --------> Initial data           +------------------------------>
            +------------------------+

                                        +-----------------------+
                   Authentication Reply | Type                  |
     <----------------------------------+ Options               <-----
                                        +-----------------------+

     <-------------------(Authentication protocol)------------------>


                                        +-----------------------+
                   Authentication Reply | Type = Success        |
     <----------------------------------+ Options               <-----
                                        +-----------------------+

                          +-----------------------+
        Operation Reply   | Reply code            |
     <--------------------+ Bind address          <------------------
                          | Bind port             |
                          | Options               |
                          +-----------------------+


          Figure 1: The SOCKS version 6 protocol message exchange

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

   Next, the server sends an authentication reply.  If the request did
   not contain the necessary authentication information, the proxy
   indicates an authentication method that must proceed.  This may
   trigger a longer authentication sequence that could include tokens



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   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.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |  Version = 6  | Command Code  |        Options Length         |
   +---------------+---------------+---------------+---------------+
   |             Port              |  Padding = 0  | Address Type  |
   +-------------------------------+---------------+---------------+
   |                                                             ...
   ...                 Address (variable length)                 ...
   ...                                                             |
   +---------------------------------------------------------------+
   |                                                             ...
   ...                 Options (variable length)                 ...
   ...                                                             |
   +---------------------------------------------------------------+


                         Figure 2: SOCKS 6 Request

   o  Version: 6

   o  Command Code:

      *  0x00 NOOP: does nothing.

      *  0x01 CONNECT: requests the establishment of a TCP connection.
         TFO MUST NOT be used unless explicitly requested.

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



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      *  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,
         but padded by NUL characters, if needed.

      *  IPv6: a 16-byte IPv6 address

   o  Port: the port in network byte order.

   o  Padding: set to 0

   o  Options Length: the total size of the SOCKS options that appear in
      the Options field.  MUST NOT exceed 16KB.

   o  Options: see Section 8.

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

   o  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.

   o  CONNECT: The fields signify the address and port to which the
      client wishes to connect.

   o  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 Advertisement option (see
   Section 8.2).



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5.  Version Mismatch Replies

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


    0 1 2 3 4 5 6 7
   +---------------+
   |  Version = 6  |
   +---------------+


                 Figure 3: SOCKS 6 Version Mismatch Reply

   o  Version: 6

   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:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |  Version = 6  |     Type      |        Options Length         |
   +---------------+---------------+-------------------------------+
   |                                                             ...
   ...                 Options (variable length)                 ...
   ...                                                             |
   +---------------------------------------------------------------+


                  Figure 4: SOCKS 6 Authentication Reply

   o  Version: 6

   o  Type:

      *  0x00: authentication successful.

      *  0x01: authentication failed.

   o  Options Length: the total size of the SOCKS options that appear in
      the Options field.  MUST NOT exceed 16KB.

   o  Options: see Section 8.



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   If the server signals that the authentication has failed and does not
   signal that any authentication negotiation can continue (via an
   Authentication Method Selection option), the client MUST close the
   connection.

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

   When the negotiation is complete (either successfully or
   unsuccessfully), the proxy sends a second Authentication Reply.  The
   second Authentication Reply MUST NOT allow for further negotiations.

7.  Operation Replies

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

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |  Version = 6  |  Reply Code   |        Options Length         |
   +---------------+---------------+---------------+---------------+
   |           Bind Port           |  Padding = 0  | Address Type  |
   +-------------------------------+---------------+---------------+
   |                                                             ...
   ...              Bind Address (variable length)               ...
   ...                                                             |
   +---------------------------------------------------------------+
   |                                                             ...
   ...                 Options (variable length)                 ...
   ...                                                             |
   +---------------------------------------------------------------+


                     Figure 5: SOCKS 6 Operation Reply

   o  Version: 6

   o  Reply Code:

      *  0x00: Succes

      *  0x01: General SOCKS server failure




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

   o  Padding: set to 0

   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,
         but padded by NUL characters, if needed.

      *  IPv6: a 16-byte IPv6 address

   o  Options Length: the total size of the SOCKS options that appear in
      the Options field.  MUST NOT exceed 16KB.

   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.



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

7.1.  Handling CONNECT

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

7.2.  Handling BIND

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

7.3.  Handling UDP ASSOCIATE

   Proxies offering UDP functionality may 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 client and the proxy
   begin exchanging messages with the following header:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |  Version = 6  | Message Type  |        Message Length         |
   +---------------+---------------+-------------------------------+
   |                        Association ID                         |
   |                           (8 bytes)                           |
   +---------------------------------------------------------------+


                     Figure 6: UDP Association Header

   o  Message Type:

      *  0x01: Association Initialization

      *  0x02: Association Confirmation

      *  0x03: Datagram

      *  0x04: Error

   o  Message Length: the total length of the message




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   o  Association ID: the identifier of the UDP association

   First, the proxy picks an Association ID sends a an Association
   Initialization message:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |  Version = 6  |     0x01      |      Message Length = 12      |
   +---------------+---------------+-------------------------------+
   |                        Association ID                         |
   |                           (8 bytes)                           |
   +---------------------------------------------------------------+


                 Figure 7: UDP Association Initialization

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

   Clients may start sending datagrams to the proxy either:

   o  over the TCP connection,

   o  in plaintext, using the proxy's configured UDP port(s), or

   o  over an established DTLS session.

   A client's datagrams are prefixed by a Datagram Header, indicating
   the remote host's address and port:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |  Version = 6  |     0x03      |        Message Length         |
   +---------------+---------------+-------------------------------+
   |                        Association ID                         |
   |                           (8 bytes)                           |
   +---------------+---------------+-------------------------------+
   | Address Type  |  Padding = 0  |             Port              |
   +---------------+---------------+-------------------------------+
   |                                                             ...
   ...                 Address (variable length)                 ...
   ...                                                             |
   +---------------------------------------------------------------+


                         Figure 8: Datagram Header



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   o  Version: 0x06

   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.

   Datagrams sent over UDP MAY be padded with arbitrary data (i. e., the
   Message Length MAY be smaller than the actual UDP/DTLS payload).
   Client and proxy implementations MUST ignore the padding.  If the
   Message Length is larger than the size of the UDP or DTLS payload,
   the message MUST be silently ignored.

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

   o  the TCP connection, if it was received over TCP, or

   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].

   The proxy SHOULD close the TCP connection if the initial datagram is
   not received after a timeout.

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





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

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |  Version = 6  |     0x02      |      Message Length = 12      |
   +---------------+---------------+-------------------------------+
   |                        Association ID                         |
   |                           (8 bytes)                           |
   +---------------------------------------------------------------+


                  Figure 9: UDP Association Confirmation

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

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

7.3.1.  Proxying UDP servers

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

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

7.3.2.  Proxying multicast traffic

   The use of multicast addessses is permitted for UDP traffic only.

7.3.3.  Reporting ICMP Errors

   If a client has opted in (see Section 8.1.8), the proxy MAY relay
   information contained in some ICMP Error packets.  The message format
   is as follows:








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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |  Version = 6  |     0x04      |        Message Length         |
   +---------------+---------------+-------------------------------+
   |                        Association ID                         |
   |                           (8 bytes)                           |
   +---------------+---------------+-------------------------------+
   | Address Type  |  Padding = 0  |             Port              |
   +---------------+---------------+-------------------------------+
   |                                                             ...
   ...                 Address (variable length)                 ...
   ...                                                             |
   +---------------+---------------+-------------------------------+
   | Reporter ATYP |  Error Code   |          Padding = 0          |
   +---------------+---------------+-------------------------------+
   |                                                             ...
   ...           Reporter Address (variable length)              ...
   ...                                                             |
   +---------------------------------------------------------------+


                     Figure 10: Datagram Error Message

   o  Address: The destination address of the IP header contained in the
      ICMP payload

   o  Address Type: Either 0x01 (IPv4) or 0x04 (IPv6)

   o  Port: The destination port of the UDP header contained in the ICMP
      payload

   o  Reporter Address: The IP address of the host that issued the ICMP
      error

   o  Reporter Address Type (ATYP): Either 0x01 (IPv4) or 0x04 (IPv6)

   o  Error code:

      *  0x01: Network unreachable

      *  0x02: Host unreachable

      *  0x03: TTL expired

      *  0x04: Datagram too big (IPv6 only)





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   It is possible for ICMP Error packets to be spurious, and not be
   related to any UDP packet that was sent out.  The proxy is not
   required to check the validity of ICMP Error packets before reporting
   them to the client.

   Clients MUST NOT send Datagram Error messages to the proxy.  Proxies
   MUST NOT send Error messages unless the clients have opted in.

8.  SOCKS Options

   SOCKS options have the following format:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |             Kind              |            Length             |
   +-------------------------------+-------------------------------+
   |                                                             ...
   ...               Option Data (variable length)               ...
   ...                                                             |
   +---------------------------------------------------------------+


                         Figure 11: SOCKS 6 Option

   o  Kind: Allocated by IANA.  (See Section 15.)

   o  Length: The total length of the option.  MUST be a multiple of 4.

   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,
   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 choose not to honor any Stack options sent by the client.





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   Proxies include Stack options in their Operation Replies to signal
   their behavior, and MUST do so for every supported Stack option sent
   by the client.  Said options MAY also be unsolicited, i. e. the proxy
   MAY send them to signal behaviour that was not explicitly requested
   by the client.

   If a particular Stack option is unsupported, the proxy MUST silently
   ignore it.

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

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

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 1            |            Length             |
   +---+-----------+---------------+-------------------------------+
   |Leg|   Level   |     Code      |                             ...
   +---+-----------+---------------+                             ...
   ...                                                           ...
   ...               Option Data (variable length)               ...
   ...                                                             |
   +---------------------------------------------------------------+


                          Figure 12: Stack Option

   o  Leg:

      *  1: Client-Proxy Leg

      *  2: Proxy-Remote Leg

      *  3: Both Legs

   o  Level:

      *  1: IP: options that apply to either IPv4 or IPv6

      *  2: IPv4

      *  3: IPv6

      *  4: TCP




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      *  5: UDP

   o  Code: Option code

   o  Option Data: Option-specific data

8.1.1.  IP TOS options

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 1            |          Length = 8           |
   +---+-----------+---------------+---------------+---------------+
   |Leg| Level = 1 |   Code = 1    |      TOS      |  Padding = 0  |
   +---+-----------+---------------+---------------+---------------+


                         Figure 13: IP TOS Option

   o  TOS: The IP TOS code

   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.  Happy Eyeballs options

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 1            |          Length = 8           |
   +---+-----------+---------------+-------------------------------+
   | 2 | Level = 1 |   Code = 2    |          Padding = 0          |
   +---+-----------+---------------+-------------------------------+


                     Figure 14: Happy Eyeballs Option

   This memo provides enough features for clients to implement a
   mechanism analogous to Happy Eyeballs [RFC8305] over SOCKS.  However,
   when the delay between the client and the proxy, or the proxy's
   vantage point, is high, doing so can become impractical or
   inefficient.

   In such cases, the client can instruct the proxy to employ the Happy
   Eyeballs technique on its behalf when connecting to a remote host.





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   The client MUST supply a Domain Name as part of its Request.
   Otherwise, the proxy MUST silently ignore the option.

   TODO: Figure out which knobs to include.

8.1.3.  TTL options

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 1            |          Length = 8           |
   +---+-----------+---------------+---------------+---------------+
   |Leg| Level = 1 |   Code = 3    |      TTL      |  Padding = 0  |
   +---+-----------+---------------+---------------+---------------+


                         Figure 15: IP TTL Option

   o  TTL: The IP TTL or Hop Limit

8.1.4.  No Fragmentaion options

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 1            |          Length = 8           |
   +---+-----------+---------------+-------------------------------+
   |Leg| Level = 1 |   Code = 4    |          Padding = 0          |
   +---+-----------+---------------+-------------------------------+


                     Figure 16: No Fragmentaion Option

   A No Fragmentation option instructs the proxy to avoid IP
   fragmentation.  In the case of IPv4, this also entails setting the DF
   bit on outgoing packets.

8.1.5.  TFO options













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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 1            |          Length = 8           |
   +---+-----------+---------------+-------------------------------+
   | 2 | Level = 4 |   Code = 1    |         Payload Size          |
   +---+-----------+---------------+-------------------------------+


                           Figure 17: TFO Option

   o  Payload Size: The desired payload size of the TFO SYN.  Ignored 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.  If the field is 0, the proxy can use an
   arbitrary payload size.  If the field is non-zero, the proxy MUST NOT
   use a payload size larger than the one indicated.  The proxy MAY use
   a smaller payload size than the one indicated.

8.1.6.  Multipath options

   In case of a CONNECT or BIND command, the client can inform the proxy
   whether MPTCP is desired on the proxy-remote leg by sending a
   Multipath option.

   Conversely, the proxy can use a Multipath option to convey the
   following information:

   o  whether or not the connection uses MPTCP or not, when replying to
      a CONNECT command, or in the second Operation reply to a BIND
      command, or

   o  whether an MPTCP connection will be accepted, when first replying
      to a BIND command.











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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 1            |          Length = 8           |
   +---+-----------+---------------+---------------+---------------+
   | 2 | Level = 4 |   Code = 2    | Availability  |  Padding = 0  |
   +---+-----------+---------------+---------------+---------------+


                        Figure 18: Multipath Option

   o  Availability:

      *  0x01: MPTCP is not desired or available

      *  0x02: MPTCP is desired or available

   In the absence of such an option, the proxy SHOULD NOT enable MPTCP.

8.1.7.  Listen Backlog options

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 1            |          Length = 8           |
   +---+-----------+---------------+-------------------------------+
   | 2 | Level = 4 |   Code = 3    |            Backlog            |
   +---+-----------+---------------+-------------------------------+


                     Figure 19: Listen Backlog Option

   o  Backlog: The length of the listen backlog.

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

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

   Following the successful negotiation of a backlog, the proxy listens
   for incoming connections for as long as the initial connection stays




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   open.  The initial connection is not used to relay data between the
   client and a remote host.

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

   If no backlog is issued, the proxy signals a backlog length of 0, and
   BIND's behavior remains unaffected.

8.1.8.  UDP Error options

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 1            |          Length = 8           |
   +---+-----------+---------------+-------------------------------+
   | 2 | Level = 5 |   Code = 1    |          Padding = 0          |
   +---+-----------+---------------+-------------------------------+


                        Figure 20: UDP Error Option

   Clients can use this option to turn on error reporting for a
   particular UDP association.  See Section 7.3.3.

8.1.9.  Port Parity options

   The RTP specification [RFC3550] recommends runnng the protocol on
   consecutive UDP ports, where the even port is the lower of the two.

   SOCKS clients can specify the desired port parity when issuing a UDP
   ASSOCIATE command, and request that the port's counterpart be
   reserved.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 1            |          Length = 8           |
   +---+-----------+---------------+---------------+---------------+
   | 2 | Level = 5 |   Code = 2    |    Parity     |    Reserve    |
   +---+-----------+---------------+---------------+---------------+


                       Figure 21: Port Parity Option

   o  Parity:



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      *  0x00: Even

      *  Ox01: Odd

   o  Reserve: whether or not to reserve the port's counterpart

      *  0x00: Don't reserve

      *  0x01: Reserve

   If the UDP ASSOCIATE request does not have the Port field set to 0
   (indicating that an arbitrary port can be chosen), the proxy MUST
   silently ignore this option.

   A port's counterpart is determined as follows:

   o  for even ports, it is the next higher port and

   o  for odd ports, it is the next lower port.

   If the proxy can not or will not comply with the requested parity, it
   does so silently, and also does not reserve the allocated port's
   counterpart.  If it can not or will not comply with the reservation
   request, the reply MUST have its Reserve field set to 0.

   Port reservations are in place until either:

   o  the original association ends, or

   o  an association involving the reserved port is made.

   An association involving a reserved port can only be made if a client
   explicitly requests said port.  Further, if the original association
   is part of a session (see Section 8.4), the reserved port can only be
   claimed from within the same session.

8.2.  Authentication Method options

   A client that is willing to go through the authentication phase MUST
   include an Authentication Method Advertisement option in its Request.
   In case of a CONNECT Request, the option is also used to specify the
   amount of initial data supplied before any method-specific
   authentication negotiations take place.








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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 2            |            Length             |
   +-------------------------------+-------------------------------+
   |      Initial Data Length      |                             ...
   +-------------------------------+                             ...
   ...                                                           ...
   ...                 Methods (variable length)                 ...
   ...                                                           ...
   ...             +-----------------------------------------------+
   ...             |   Padding = 0 (variable length, 0-3 bytes)    |
   +---------------+-----------------------------------------------+


           Figure 22: Authentication Method Advertisement 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 2^14.

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

   o  Padding: A minimally-sized sequence of zeroes, such that the
      option length is a multiple of 4.  Note that 0 coincides with the
      value for "No Authentication Required".

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

   The proxy indicates which authentication method must proceed by
   sending an Authentication Method Selection option in the
   corresponding Authentication Reply:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 3            |          Length = 8           |
   +---------------+---------------+-------------------------------+
   |    Method     |                  Padding = 0                  |
   +---------------+-----------------------------------------------+


             Figure 23: Authentication Method Selection Option




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   o  Method: The selected method.

   If the proxy selects "No Acceptable Methods", the client MUST close
   the connection.

   If authentication is successful via some other means, or not required
   at all, the proxy silently ignores the Authentication Method
   Advertisement 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:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 4            |            Length             |
   +---------------+---------------+-------------------------------+
   |    Method     |                                             ...
   +---------------+                                             ...
   ...                                                           ...
   ...           Authentication Data (variable length)           ...
   ...                                                             |
   +---------------------------------------------------------------+


                   Figure 24: Authentication Data 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 MUST only place one Authentication Data option per
   authentication method.

8.4.  Session options

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

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



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8.4.1.  Session initiation

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

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 5            |          Length = 4           |
   +-------------------------------+-------------------------------+


                     Figure 25: Session Request Option

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

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 6            |            Length             |
   +-------------------------------+-------------------------------+
   |                                                             ...
   ...               Session ID (variable length)                ...
   ...                                                             |
   +---------------------------------------------------------------+


                       Figure 26: Session ID Option

   o  Session ID: An opaque sequence of bytes specific to the session.
      The size MUST be a multiple of 4.  MUST NOT be empty.

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

   The credentials, or lack thereof, used to initiate the session are
   tied to the session.

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

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







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8.4.2.  Further SOCKS Requests

   Any further SOCKS Requests that are part of the session MUST include
   a Session ID Option (as seen in Figure 26).  The proxy MUST silently
   ignore any authentication attempt in the Request, and MUST NOT
   require any authentication.

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

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 8            |          Length = 4           |
   +-------------------------------+-------------------------------+


                       Figure 27: Session OK Option

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

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 9            |          Length = 4           |
   +-------------------------------+-------------------------------+


                     Figure 28: Session Invalid Option

8.4.3.  Tearing down the session

   Proxies can, at their discretion, tear down a session and free all
   associated state.  Proxy implementations SHOULD feature a timeout
   mechanism that destroys sessions after a period of inactivity.  When
   a session is terminated, the proxy MAY close all connections
   associated with said session.

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







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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 10           |          Length = 4           |
   +-------------------------------+-------------------------------+


                    Figure 29: Session Teardown Option

   After sending such an option, the client MUST assume that the session
   is no longer valid.  The proxy MUST treat the session-terminating
   request as if it were not part of any session.

8.5.  Idempotence options

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

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

   Proxies grant contiguous ranges of tokens called token windows.
   Token windows are defined by their base (the first token in the
   range) and size.

   All token-related operations are done via Idempotence options.

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

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

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

8.5.1.  Requesting a token window

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






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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 11           |          Length = 8           |
   +-------------------------------+-------------------------------+
   |                          Window Size                          |
   +---------------------------------------------------------------+


                         Figure 30: Token Request

   o  Window Size: The requested window size.

   Once a token window is issued, the proxy MUST include an Idempotence
   Window option in all subsequent successful Authentication Replies:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 12           |          Length = 12          |
   +-------------------------------+-------------------------------+
   |                          Window Base                          |
   +---------------------------------------------------------------+
   |                          Window Size                          |
   +---------------------------------------------------------------+


                       Figure 31: Idempotence Window

   o  Window Base: The first token in the window.

   o  Window Size: The window size.  This value MAY differ from 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.  If there is already a token window associated with
   the session, the proxy MUST NOT issue a new window.

8.5.2.  Spending a token

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








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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 13           |          Length = 4           |
   +-------------------------------+-------------------------------+
   |                             Token                             |
   +---------------------------------------------------------------+


                    Figure 32: Idempotence Expenditure

   o  Kind: 13 (Idempotence Expenditure option)

   o  Length: 8

   o  Token: The token being spent.

   Clients SHOULD prioritize spending the smaller tokens.

   The proxy responds by sending either an Idempotence Accepted or
   Rejected option as part of the Authentication Reply:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 14           |          Length = 4           |
   +-------------------------------+-------------------------------+


                      Figure 33: Idempotence Accepted

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 15           |          Length = 4           |
   +-------------------------------+-------------------------------+


                      Figure 34: Idempotence Rejected

   If eligible, the token is spent before attempting to honor the
   Request.  If the token is not eligible for spending, the
   Authentication Reply MUST indicate failure.








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8.5.3.  Shifting windows

   Windows can be shifted (i. e. have their base increased, while
   retaining their size) unilaterally by the proxy.

   Proxy implementations SHOULD shift the window: * as soon as the
   lowest-order token in the window is spent and * when a sufficiently
   high-order token is spent.

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

8.5.4.  Out-of-order 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
   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.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 4            |            Length             |
   +---------------+---------------+---------------+---------------+
   |  Method = 2   |                                             ...
   +---------------+                                             ...
   ...                                                           ...
   ...                 Username/Password Request                 ...
   ...                                                           ...
   ...             +-----------------------------------------------+
   ...             |   Padding = 0 (variable length, 0-3 bytes)    |
   +---------------+-----------------------------------------------+


           Figure 35: Password authentication via a SOCKS Option

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





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   Proxies reply by including a Authentication Data Option in the next
   Authentication Reply which contains the Username/Password reply:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-------------------------------+-------------------------------+
   |           Kind = 4            |          Length = 8           |
   +---------------+---------------+---------------+---------------+
   |  Method = 2   |    Username/Password Reply    |  Padding = 0  |
   +---------------+-------------------------------+---------------+


      Figure 36: Reply to password authentication via a SOCKS Option

   o  Username/Password Reply: The Username/Password Reply, 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:

   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 very unlikely,
      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:




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   o  If the Request does not contain an Authentication Method
      Advertisement 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.  DNS provided by SOCKS

   Clients may require information typically obtained from DNS servers,
   albeit from the proxy's vantage point.

   While the CONNECT command can work with domain names, some clients'
   workflows require that addresses be resolved as a separate step prior
   to connecting.  Moreover, the SOCKS Datagram Header, as described in
   Section 7.3, can be reduced in size by providing the resolved
   destination IP address, rather than the FQDN.

   Emerging techniques may also make use of DNS to deliver server-
   specific information to clients.  For example, Encrypted SNI
   [I-D.ietf-tls-esni] relies on DNS to publish encryption keys.

   Proxy implementations MAY provide a default plaintext DNS service.  A
   client looking to make use of it issues a CONNECT Request to IP
   address 0.0.0.0 or 0:0:0:0:0:0:0:0 on port 53.  Following successful
   authentication, the Operation Reply MAY indicate an unspecified bind
   address (0.0.0.0 or ::) and port (0).  The client and proxy then
   behave as per [RFC7766].

   The service itself can be provided directly by the proxy daemon, or
   by proxying the client's request to a pre-configured DNS server.

   If the proxy does not implement such functionality, it MAY return an
   error code signalling "Connection refused".

13.  Security Considerations

13.1.  Large requests

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




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   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 or their size exceeds an imposed hard cap.

13.2.  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, anything prior to the initial Token Request is still
   vulnerable.  As such, client implementations SHOULD NOT make use of
   TLS early data unless the Request attempts to spend a token.

13.3.  Resource exhaustion

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

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

14.  Privacy Considerations

   The timing of Operation Replies can reveal some information about a
   proxy's recent usage:

   o  The DNS resolver used by the proxy may cache the answer to recent
      queries.  As such, subsequent connection attempts to the same
      hostname are likely to be slightly faster, even if requested by
      different clients.

   o  Likewise, the proxy's OS typically caches TFO cookies.  Repeated
      TFO connection attempts tend to be sped up, regardless of the
      client.

15.  IANA Considerations

   This document requests that IANA allocate 2-byte option kinds for
   SOCKS 6 options.  Further, this document requests the following
   option kinds:

   o  Unassigned: 0



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   o  Stack: 1

   o  Authentication Method Advertisement: 2

   o  Authentication Method Selection: 3

   o  Authentication Data: 4

   o  Session Request: 5

   o  Session ID: 6

   o  Session OK: 8

   o  Session Invalid: 9

   o  Session Teardown: 10

   o  Idempotence Request: 11

   o  Idempotence Window: 12

   o  Idempotence Expenditure: 13

   o  Idempotence Accepted: 14

   o  Idempotence Rejected: 15

   o  Resolution Request: 16

   o  IPv4 Resolution: 17

   o  IPv6 Resolution: 18

   o  Vendor-specific: 64512-0xFFFF

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

16.  Acknowledgements

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








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17.  References

17.1.  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>.

   [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
              <https://www.rfc-editor.org/info/rfc7766>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

17.2.  Informative References

   [I-D.ietf-tls-dtls-connection-id]
              Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
              Identifiers for DTLS 1.2", draft-ietf-tls-dtls-connection-
              id-07 (work in progress), October 2019.

   [I-D.ietf-tls-esni]
              Rescorla, E., Oku, K., Sullivan, N., and C. Wood, "TLS
              Encrypted Client Hello", draft-ietf-tls-esni-07 (work in
              progress), June 2020.

   [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>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.







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   [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
   313 Splaiul Independentei, Sector 6
   Bucharest
   Romania

   Email: vladimir.olteanu@cs.pub.ro


   Dragos Niculescu
   University Politehnica of Bucharest
   313 Splaiul Independentei, Sector 6
   Bucharest
   Romania

   Email: dragos.niculescu@cs.pub.ro




















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