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Versions: 00 01 02 rfc1961                                              
    Internet-Draft                 GSS-API Authentication for SOCKS V5
    Expires: 05FEB96                                           05JUL95
    <draft-ietf-aft-gssapi-02.txt>                    P V McMahon, ICL
                GSS-API Authentication Method for SOCKS Version 5
    Status of this Memo
       This document is an Internet-Draft. Internet-Drafts are working
       documents of the Internet Engineering Task Force (IETF),
       its areas, and its working groups. Note that other groups may
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       Comments on this document are welcome and should be sent to
       aft@unify.com, the mailing list of the Authenticated Firewall
       Traversal Working Group of the IETF.
    Contents List
         1. Purpose
         2. Introduction
         3. GSS-API Security Context Establishment
         4. GSS-API Protection-level Options
         5. GSS-API Per-message Protection
         6. GSS-API Security Context Termination
         7. References
         8. Acknowledgments
         9. Security Considerations
         10. Author's Address
    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.
    McMahon                                                   [Page 1]

    Internet-Draft                 GSS-API Authentication for SOCKS V5
    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
    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 options.
    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
    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.
    McMahon                                                   [Page 2]

    Internet-Draft                 GSS-API Authentication for SOCKS V5
    3.2 Client Context Establishment
    The client should then call gss_init_sec_context, typically
         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
         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_MUTUAL_FLAG | GSS_C_REPLAY_FLAG |
    GSS_C_DELEG_FLAG | 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
    McMahon                                                   [Page 3]

    Internet-Draft                 GSS-API Authentication for SOCKS V5
    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 server.
    + ver  | mtyp | len  |       token           |
    + 0x01 | 0x01 | 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 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 server.
    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
    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.
    McMahon                                                   [Page 4]

    Internet-Draft                 GSS-API Authentication for SOCKS V5
    If gss_accept_sec_context returns GSS_S_COMPLETE, then, if an
    output_token is returned, the server should return it to the
    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.
    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
    McMahon                                                   [Page 5]

    Internet-Draft                 GSS-API Authentication for SOCKS V5
    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.
    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 5.1.
    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.
    McMahon                                                   [Page 6]

    Internet-Draft                 GSS-API Authentication for SOCKS V5
    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
    + 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
    - "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
    McMahon                                                   [Page 7]

    Internet-Draft                 GSS-API Authentication for SOCKS V5
    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()/
    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
    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.
    McMahon                                                   [Page 8]

    Internet-Draft                 GSS-API Authentication for SOCKS V5
    7. References
    [RFC 1508] Generic Security Service API, J Linn,
               September 1993
    [RFC 1509] Generic Security Service API : C-bindings, J Wray,
               September 1993
    [SOCKS V5] SOCKS Protocol V5, draft-ietf-aft-socks-proto-v5-03.txt
               M Leech, June 1995
    8. Acknowledgment
    This document builds from a previous draft 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.
    10. Author's Address
    P V McMahon
    post:  ICL Enterprises, Kings House, 33 Kings Road, Reading,
           RG1 3PX, UK
    email: p.v.mcmahon@rea0803.wins.icl.co.uk
    phone: +44 1734 634882
    fax:   +44 1734 855106
    McMahon                                                   [Page 9]