GSS-API Authentication Method for SOCKS Version 5
draft-ietf-aft-gssapi-02

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
 also distribute working documents as Internet-Drafts.
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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.
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 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
 server.
 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
 credentials.
 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.
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 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
 mechanism,
 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
 below.
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 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 |
 +------+------+------+.......................+
 Where:
 - "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
 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.
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 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.
 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 |
 +------+------+
 Where:
 - "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
 message
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 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.
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 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 |
 +------+------+------+.......................+
 Where:
 - "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()/
 gss_unwrap().
 If the server's choice of protection level is unacceptable to the
 client, then the client must close its connection to the server
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 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 |
 +------+------+------+.......................+
 Where:
 - "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
 data
 - "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.
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 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
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