Network Working Group                                         P. McMahon
Request for Comments: 1961                                           ICL
Category: Standards Track                                      June 1996

           GSS-API Authentication Method for SOCKS Version 5

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Table of Contents

         1. Purpose ............................................ 1
         2. Introduction ....................................... 1
         3. GSS-API Security Context Establishment ............. 2
         4. GSS-API Protection-level Options ................... 5
         5. GSS-API Per-message Protection ..................... 7
         6. GSS-API Security Context Termination ............... 8
         7. References ......................................... 8
         8. Acknowledgments .................................... 8
         9. Security Considerations ............................ 8
         10. Author's Address .................................. 9

1. Purpose

   The protocol specification for SOCKS Version 5 specifies a
   generalized framework for the use of arbitrary authentication
   protocols in the initial SOCKS connection setup.  This document
   provides the specification for the SOCKS V5 GSS-API authentication
   protocol, and defines a GSS-API-based encapsulation for provision of
   integrity, authentication and optional confidentiality.

2. Introduction

   GSS-API provides an abstract interface which provides security
   services for use in distributed applications, but isolates callers
   from specific security mechanisms and implementations.

   GSS-API peers achieve interoperability by establishing a common
   security mechanism for security context establishment - either
   through administrative action, or through negotiation.  GSS-API is
   specified in [RFC 1508], and [RFC 1509].  This specification is
   intended for use with implementations of GSS-API, and the emerging

McMahon                     Standards Track                     [Page 1]

RFC 1961          GSS-API Authentication for SOCKS V5          June 1996

   GSS-API V2 specification.

   The approach for use of GSS-API in SOCKS V5 is to authenticate the
   client and server by successfully establishing a GSS-API security
   context - such that the GSS-API encapsulates any negotiation protocol
   for mechanism selection, and the agreement of security service

   The GSS-API enables the context initiator to know what security
   services the target supports for the chosen mechanism.  The required
   level of protection is then agreed by negotiation.

   The GSS-API per-message protection calls are subsequently used to
   encapsulate any further TCP and UDP traffic between client and

3. GSS-API Security Context Establishment

3.1 Preparation

   Prior to use of GSS-API primitives, the client and server should be
   locally authenticated, and have established default GSS-API

   The client should call gss_import_name to obtain an internal
   representation of the server name.  For maximal portability the
   default name_type GSS_C_NULL_OID should be used to specify the
   default name space, and the input name_string should treated by the
   client's code as an opaque name-space specific input.

   For example, when using Kerberos V5 naming, the imported name may be
   of the form "SERVICE:socks@socks_server_hostname" where
   "socks_server_hostname" is the fully qualified host name of the
   server with all letters in lower case. Other mechanisms may, however,
   have different name forms, so the client should not make assumptions
   about the name syntax.

3.2 Client Context Establishment

   The client should then call gss_init_sec_context, typically passing:

         GSS_C_NO_CREDENTIAL into cred_handle to specify the default
         credential (for initiator usage),

         GSS_C_NULL_OID into mech_type to specify the default

McMahon                     Standards Track                     [Page 2]

RFC 1961          GSS-API Authentication for SOCKS V5          June 1996

         GSS_C_NO_CONTEXT into context_handle to specify a NULL
         context (initially), and,

         the previously imported server name into target_name.

   The client must also specify its requirements for replay protection,
   delegation, and sequence protection via the gss_init_sec_context
   req_flags parameter.  It is required by this specification that the
   client always requests these service options (i.e. passes
   GSS_C_SEQUENCE_FLAG into req_flags).

   However, GSS_C_SEQUENCE_FLAG should only be passed in for TCP-based
   clients, not for UDP-based clients.

3.3 Client Context Establishment Major Status codes

   The gss_init_sec_context returned status code can take two different
   success values:

    - If gss_init_sec_context returns GSS_S_CONTINUE_NEEDED, then the
      client should expect the server to issue a token in the
      subsequent subnegotiation response.  The client must pass the
      token to another call to gss_init_sec_context, and repeat this
      procedure until "continue" operations are complete.

    - If gss_init_sec_context returns GSS_S_COMPLETE, then the client
      should respond to the server with any resulting output_token.

      If there is no output_token, the client should proceed to send
      the protected request details, including any required message
      protection subnegotiation as specified in sections 4 and 5

3.4 Client initial token

   The client's GSS-API implementation then typically responds with the
   resulting output_token which the client sends in a message to the

    + ver  | mtyp | len  |       token           |
    + 0x01 | 0x01 | 0x02 | up to 2^16 - 1 octets |

McMahon                     Standards Track                     [Page 3]

RFC 1961          GSS-API Authentication for SOCKS V5          June 1996


    - "ver" is the protocol version number, here 1 to represent the
      first version of the SOCKS/GSS-API protocol

    - "mtyp" is the message type, here 1 to represent an
      authentication message

    - "len" is the length of the "token" field in octets

    - "token" is the opaque authentication token emitted by GSS-API

3.5 Client GSS-API Initialisation Failure

   If, however, the client's GSS-API implementation failed during
   gss_init_sec_context, the client must close its connection to the

3.6 Server Context Establishment

   For the case where a client successfully sends a token emitted by
   gss_init_sec_context() to the server, the server must pass the
   client-supplied token to gss_accept_sec_context as input_token.

   When calling gss_accept_sec_context() for the first time, the
   context_handle argument is initially set to GSS_C_NO_CONTEXT.

   For portability, verifier_cred_handle is set to GSS_C_NO_CREDENTIAL
   to specify default credentials (for acceptor usage).

   If gss_accept_sec_context returns GSS_CONTINUE_NEEDED, the server
   should return the generated output_token to the client, and
   subsequently pass the resulting client supplied token to another call
   to gss_accept_sec_context.

   If gss_accept_sec_context returns GSS_S_COMPLETE, then, if an
   output_token is returned, the server should return it to the client.

   If no token is returned, a zero length token should be sent by the
   server to signal to the client that it is ready to receive the
   client's request.

McMahon                     Standards Track                     [Page 4]

RFC 1961          GSS-API Authentication for SOCKS V5          June 1996

3.7 Server Reply

   In all continue/confirmation cases, the server uses the same message
   type as for the client -> server interaction.

    + ver  | mtyp | len  |       token           |
    + 0x01 | 0x01 | 0x02 | up to 2^16 - 1 octets |

3.8 Security Context Failure

   If the server refuses the client's connection for any reason (GSS-API
   authentication failure or otherwise), it will return:

    + ver  | mtyp |
    + 0x01 | 0xff |


    - "ver" is the protocol version number, here 1 to represent the
      first version of the SOCKS/GSS-API protocol

    - "mtyp" is the message type, here 0xff to represent an abort

4. GSS-API Protection-level Options

4.1 Message protection

   Establishment of a GSS-API security context enables comunicating
   peers to determine which per-message protection services are
   available to them through the gss_init_sec_context() and
   gss_accept_sec_context() ret_flags GSS_C_INTEG_FLAG and
   GSS_C_CONF_FLAG which respectively indicate message integrity and
   confidentiality services.

   It is necessary to ensure that the message protection applied to the
   traffic is appropriate to the sensitivity of the data, and the
   severity of the threats.

McMahon                     Standards Track                     [Page 5]

RFC 1961          GSS-API Authentication for SOCKS V5          June 1996

4.2 Message Protection Subnegotiation

   For TCP and UDP clients and servers, different levels of protection
   are possible in the SOCKS V5 protocol, so an additional
   subnegotiation stage is needed to agree the message protection level.
   After successful completion of this subnegotiation, TCP and UDP
   clients and servers use GSS-API encapsulation as defined in section

   After successful establishment of a GSS-API security context, the
   client's GSS-API implementation sends its required security context
   protection level to the server.  The server then returns the security
   context protection level which it agrees to - which may or may not
   take the the client's request into account.

   The security context protection level sent by client and server must
   be one of the following values:

         1 required per-message integrity
         2 required per-message integrity and confidentiality
         3 selective per-message integrity or confidentiality based on
           local client and server configurations

   It is anticipated that most implementations will agree on level 1 or
   2 due to the practical difficulties in applying selective controls to
   messages passed through a socks library.

4.3 Message Protection Subnegotiation Message Format

   The security context protection level is sent from client to server
   and vice versa using the following protected message format:

    + ver  | mtyp | len  |   token               |
    + 0x01 | 0x02 | 0x02 | up to 2^16 - 1 octets |


    - "ver" is the protocol version number, here 1 to represent the
      first version of the SOCKS/GSS-API protocol

    - "mtyp" is the message type, here 2 to represent a protection
      -level negotiation message

    - "len" is the length of the "token" field in octets

McMahon                     Standards Track                     [Page 6]

RFC 1961          GSS-API Authentication for SOCKS V5          June 1996

    - "token" is the GSS-API encapsulated protection level

4.4 Message Protection Subnegotiation Message Generation

   The token is produced by encapsulating an octet containing the
   required protection level using gss_seal()/gss_wrap() with conf_req
   set to FALSE.  The token is verified using gss_unseal()/

   If the server's choice of protection level is unacceptable to the
   client, then the client must close its connection to the server

5. GSS-API Per-message Protection

   For TCP and UDP clients and servers, the GSS-API functions for
   encapsulation and de-encapsulation shall be used by implementations -
   i.e. gss_seal()/gss_wrap(), and gss_unseal()/ gss_unwrap().

   The default value of quality of protection shall be specified, and
   the use of conf_req_flag shall be as determined by the previous
   subnegotiation step.  If protection level 1 is agreed then
   conf_req_flag MUST always be FALSE; if protection level 2 is agreed
   then conf_req_flag MUST always be TRUE; and if protection level 3 is
   agreed then conf_req is determined on a per-message basis by client
   and server using local configuration.

   All encapsulated messages are prefixed by the following framing:

    + ver  | mtyp | len  |       token           |
    + 0x01 | 0x03 | 0x02 | up to 2^16 - 1 octets |


    - "ver" is the protocol version number, here 1 to represent the
      first version of the SOCKS/GSS-API protocol

    - "mtyp" is the message type, here 3 to represent encapulated user

    - "len" is the length of the "token" field in octets

    - "token" is the user data encapsulated by GSS-API

McMahon                     Standards Track                     [Page 7]

RFC 1961          GSS-API Authentication for SOCKS V5          June 1996

6. GSS-API Security Context Termination

   The GSS-API context termination message (emitted by
   gss_delete_sec_context) is not used by this protocol.

   When the connection is closed, each peer invokes
   gss_delete_sec_context() passing GSS_C_NO_BUFFER into the
   output_token argument.

7. References

    [RFC 1508] Linn, J., "Generic Security Service API",
               September 1993.

    [RFC 1509] Wray, J., "Generic Security Service API : C-bindings",
               September 1993.

    [SOCKS V5] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D.,
               and L. Jones, "SOCKS Protocol V5", RFC 1928, April

8. Acknowledgment

   This document builds from a previous memo produced by Marcus Leech
   (BNR) - whose comments are gratefully acknowleged.  It also reflects
   input from the AFT WG, and comments arising from implementation
   experience by Xavier Gosselin (IUT Lyons).

9. Security Considerations

   The security services provided through the GSS-API are entirely
   dependent on the effectiveness of the underlying security mechanisms,
   and the correctness of the implementation of the underlying
   algorithms and protocols.

   The user of a GSS-API service must ensure that the quality of
   protection provided by the mechanism implementation is consistent
   with their security policy.

   In addition, where negotiation is supported under the GSS-API,
   constraints on acceptable mechanisms may be imposed to ensure
   suitability for application to authenticated firewall traversal.

McMahon                     Standards Track                     [Page 8]

RFC 1961          GSS-API Authentication for SOCKS V5          June 1996

10. Author's Address

   P. V. McMahon
   ICL Enterprises
   Kings House
   33 Kings Road
   Reading, RG1 3PX

   Phone: +44 1734 634882
   Fax:   +44 1734 855106

McMahon                     Standards Track                     [Page 9]