[tor-dev] Proposal 229: Further SOCKS5 extensions

Nick Mathewson nickm at alum.mit.edu
Fri Feb 28 17:08:28 UTC 2014

On Fri, Feb 28, 2014 at 3:53 AM, Yawning Angel <yawning at schwanenlied.me> wrote:
> Hello all,
> Based on feedback from nickm/arma, I have extended the proposal to also
> cover Tor's SOCKS port, including response codes related to hidden
> service status.
> Additionally, instead of having a username/password field pre-defined,
> the extended authentication method has been generalized to contain
> key/value pairs for extra flexibility in the future.
> Questions, comments, feedback appreciated as always,

This is now proposal 229!  (I've wrapped the lines a little, but made
no additional changes.)

After adding it to Git, I made some redundancy tweaks and asked a
bunch of annoying questions that we'll probably want to figure out
before implementing anything like this.

Filename: 229-further-socks5-extensions.txt
Title: Further SOCKS5 extensions
Author: Yawning Angel
Created: 25-Feb-2014
Status: Draft

0. Abstract

   We propose extending the SOCKS5 protocol to allow passing more
   per-session metadata, and to allow returning more meaningful
   response failure codes back to the client.

1. Introduction

   The SOCKS5 protocol is used by Tor both as the primary interface
   for applications to transfer data, and as the interface by which
   Tor communicates with pluggable transport implementations.

   While the current specifications allow for passing a limited
   amount of per-session metadata via hijacking the
   Username/Password authentication method fields, this solution is
   limited in that the amount of payload that can be conveyed is
   restricted to 510 bytes, does not allow the SOCKS server to
   return a response, and precludes using authentication on the
   SOCKS port.

   The first part of this proposal defines a new authentication
   method to overcome both of these limitations.

   The second part of this proposal defines a range of SOCKS5
   response codes that can be used to signal Tor specific error
   conditions when processing SOCKS requests.

2. Proposal

2.1. Tor Extended SOCKS5 Authentication

   We introduce a new authentication method to the SOCKS5 protocol.

   The METHOD number to be returned to indicate support for or
   select this method is X'97', which belongs to the "RESERVED FOR
   PRIVATE METHODS" range in RFC 1928.

   After the authentication method has been negotiated following the
   standard SOCKS5 protocol, the actual authentication phase begins.

   If any requirement labeled with a "MUST" below in this protocol
   is violated, the party receiving the violation MUST close the

   All multibyte numeric values in this protocol MUST be transmitted
   in network (big-endian) byte order.

   The initiator will send an Extended Authentication request:

    |VER | NR PAIRS | KLEN1 |    KEY1     | VLEN1 |   VALUE1    | ...
    | 1  |    2     |   2   | KLEN1 bytes |   2   | VLEN1 bytes | ...

    VER: 8 bits (unsigned integer)

       This field specifies the version of the authentication
       method.  It MUST be set to X'01'.

    NR PAIRS: 16 bits (unsigned integer)

       This field specifies the number of key/value pairs to follow.

    KLEN: 16 bits (unsigned integer)

       This field specifies the length of the key in bytes.  It MUST
       be greater than 0.

    KEY: variable length

       This field contains the key associated with the subsequent
       VALUE field as an ASCII string, without a NUL terminator.

    VLEN: 16 bits (unsigned integer)

       This field specifies the length of the value in bytes. It MAY
       be X'0000', in which case the corresponding VALUE field is

    VALUE: variable length, optional

       The value corresponding to the KEY.

   The responder will verify the contents of the Extended
   Authentication request and send the following response:

    |VER | STATUS | NR PAIRS | KLEN1 |    KEY1     | VLEN1 |   VALUE1    | ...
    | 1  |   1    |    2     |   2   | KLEN1 bytes |   2   | VLEN1 bytes | ...

    VER: 8 bits (unsigned integer)

       This field specifies the version of the authentication
       method.  It MUST be set to X'01'.

    STATUS: 8 bits (unsigned integer)

       The status of the Extended Authentication request where:

        * X'00' SUCCESS

       If a server sends a response indicating failure (STATUS value
       other than X'00') it MUST close the connection.

       [XXXX What should a client if it gets a value here it does
       not recognize?]


       These fields have the same format as they do in Extended
       Authentication requests.

   The currently defined KEYs are:

    * "USERNAME" The username for authentication.
    * "PASSWD" The password for authentication.

    [XXXX What do these do?  What is their behavior?  Are they
      client-only? Right now, Tor uses SOCKS5 usernames and
      passwords in two ways:

            1) as a way to control isolation, when receiving them
               from a SOCKS client.
            2) as a way to encode arbitrary data, when sending data
               to a PT.

      Neither of these seem necessary any more.  We can turn 1 into
       a new KEY, and we can turn 2 into a new set of keys.  -NM]

    [XXX - Add some more here, Stream isolation? -YA]

    [XXXX What should a client if it gets a value here it does
     not recognize? -NM]

    [XXXX Should we recommend any namespace conventions for these? -NM]

2.2. Tor Extended SOCKS5 Reply Codes

   We introduce the following additional SOCKS5 reply codes to be
   sent in the REP field of a SOCKS5 message.  Implementations MUST
   NOT send any of the extended codes unless the initiator has
   indicated that it understands the "Tor Extended SOCKS5
   Authentication" as part of the version identifier/method
   selection SOCKS5 message.

    [Actually, should this perhaps be controlled by additional KEY?
       (I'm not sure.) -NM]


    * X'E0' Hidden Service Not Found

      The requested Tor Hidden Service was not reachable.

    * X'E1' Hidden Service Not Reachable

      The requested Tor Hidden Service was not found.

    * X'F0' Temporary Pluggable Transport failure, retry immediately

      Pluggable transports SHOULD return this status code if the
      connection attempt failed, but the pluggable transport
      believes that subsequent connections with the same parameters
      are likely to succeed.


         The ScrambleSuit Session Ticket handshake failed, but
         reconnecting is likely to succeed as it will use the
         UniformDH handshake.

    * X'F1' Pluggable transport protocol failure, invalid bridge

      Pluggable transports MUST return this status code if the
      connection attempt failed in a manner that indicates that the
      remote peer is not likely to accept connections at a later


         The obfs3 handshake failed.

    * X'F2' Pluggable transport internal error

      Pluggable transports SHOULD return this status code if the
      connection attempt failed due to an internal error in the
      pluggable transport implementation.

      Tor might wish to restart the pluggable transport executable,
      or retry after a delay.

3. Compatibility

   SOCKS5 negotiates authentication methods so backward and forward
   compatibility is obtained for free, assuming a non-broken SOCKS5
   implementation on the responder side that ignores unrecognised
   authentication methods in the negotiation phase.

4. Security Considerations

   Identical security considerations to RFC 1929 Username/Password
   authentication applies when doing Username/Password
   authentication using the keys reserved for such.  As SOCKS5 is
   sent in cleartext, this extension (like the rest of the SOCKS5
   protocol) MUST NOT be used in scenarios where sniffing is possible.

   The authors of this proposal note that binding any of the Tor
   (and associated) SOCKS5 servers to non-loopback interfaces is
   strongly discouraged currently, so in the current model this is
   believed to be acceptable.

5. References

   Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., Jones L., "SOCKS
   Protocol Version 5", RFC 1928, March 1996.

   Tor Project, "Tor's extensions to the SOCKS protocol"

   Leech, M. "Username/Password Authentication for SOCKS V5", RFC 1929,
   March 1996.

   Appelbaum, J., Mathewson, N., "Pluggable Transport Specification",
   June 2012.

[XXX -  Changelog (Remove when accepted) -YA]

   2014-02-28 (Thanks to nickm/arma)
    * Generalize to also support tor
      * Add status codes for bug #6031
    * Switch from always having a username/password field to making them just
      predefined keys.
    * Change the auth method number to 0x97

   2014-02-28 (nickm's fault)
    * check it into git
    * clean text a little, fix redundancy
    * ask some questions

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