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AFT Working Group                                     Marc VanHeyningen
draft-ietf-aft-socks-pro-v5-04                           Aventail Corp.
Expires August 22, 1999                                22 February 1999

                        SOCKS Protocol Version 5

   Status of this Memo

      This document is an Internet-Draft and is in full conformance with
      all provisions of Section 10 of RFC2026.

      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.

      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."

      The list of current Internet-Drafts can be accessed at
      http://www.ietf.org/ietf/1id-abstracts.txt

      The list of Internet-Draft Shadow Directories can be accessed at
      http://www.ietf.org/shadow.html.

      This document is a submission to the IETF Authenticated Firewall
      Traversal (AFT) Working Group. Comments are solicited and should
      be addressed to the working group mailing list (aft@socks.nec.com)
      or to the editor.

   Abstract

      This document is a revision of RFC 1928, the SOCKS version 5
      protocol.  SOCKS is a generic proxying protocol for traversing
      firewalls and other trust boundaries; version 5 of the protocol
      adds new features such as authentication and UDP support.  Changes
      from the RFC in this draft include formatting cleanups,
      authentication clarification, and fixing UDP-related problems
      found during implementation.

1.  Introduction

   The use of network firewalls, systems that effectively isolate an
   organizations internal network structure from an exterior network,
   such as the INTERNET is becoming increasingly popular.  These



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   firewall systems typically act as application-layer gateways between
   networks, usually offering controlled TELNET, FTP, and SMTP access.
   With the emergence of more sophisticated application layer protocols
   designed to facilitate global information discovery, there exists a
   need to provide a general framework for these protocols to
   transparently and securely traverse a firewall.

   There exists, also, a need for strong authentication of such
   traversal in as fine-grained a manner as is practical. This
   requirement stems from the realization that client-server
   relationships emerge between the networks of various organizations,
   and that such relationships need to be controlled and often strongly
   authenticated.

   The protocol described here is designed to provide a framework for
   client-server applications in both the TCP and UDP domains to
   conveniently and securely use the services of a network firewall.
   The protocol is conceptually a "shim-layer" between the application
   layer and the transport layer, and as such does not provide network-
   layer gateway services, such as forwarding of ICMP messages.

2.  Existing practice

   There currently exists a protocol, SOCKS Version 4, that provides for
   unsecured firewall traversal for TCP-based client-server
   applications, including TELNET, FTP and the popular information-
   discovery protocols such as HTTP, WAIS and GOPHER.

   This new protocol extends the SOCKS Version 4 model to include UDP,
   and extends the framework to include provisions for generalized
   strong authentication schemes, and extends the addressing scheme to
   encompass domain-name and V6 IP addresses.

   The implementation of the SOCKS protocol typically involves the
   recompilation or relinking of TCP-based client applications to use
   the appropriate encapsulation routines in the SOCKS library.

 Note:

   Unless otherwise noted, the decimal numbers appearing in packet-
   format diagrams represent the length of the corresponding field, in
   octets.  Where a given octet must take on a specific value, the
   syntax X'hh' is used to denote the value of the single octet in that
   field. When the word 'Variable' is used, it indicates that the
   corresponding field has a variable length defined either by an
   associated (one or two octet) length field, or by a data type field.

3.  Procedure for TCP-based clients



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   When a TCP-based client wishes to establish a connection to an object
   that is reachable only via a firewall (such determination is left up
   to the implementation), it must open a TCP connection to the
   appropriate SOCKS port on the SOCKS server system.  The SOCKS service
   is conventionally located on TCP port 1080.  If the connection
   request succeeds, the client enters a negotiation for the
   authentication method to be used, authenticates with the chosen
   method, then sends a relay request.  The SOCKS server evaluates the
   request, and either establishes the appropriate connection or denies
   it.

   The client connects to the server, and sends a version
   identifier/method selection message:

                      +----+----------+----------+
                      |VER | NMETHODS | METHODS  |
                      +----+----------+----------+
                      | 1  |    1     | 1 to 255 |
                      +----+----------+----------+

   The VER field is set to X'05' for this version of the protocol.  The
   NMETHODS field contains the number of method identifier octets that
   appear in the METHODS field.

   The server selects from one of the methods given in METHODS, and
   sends a METHOD selection message:

                            +----+--------+
                            |VER | METHOD |
                            +----+--------+
                            | 1  |   1    |
                            +----+--------+

   If the selected METHOD is X'FF', none of the methods listed by the
   client are acceptable, and the client MUST close the connection.

   The values currently defined for METHOD are:

      o  X'00' NO AUTHENTICATION REQUIRED
      o  X'01' GSSAPI
      o  X'02' USERNAME/PASSWORD
      o  X'03' CHAP
      o  X'04' to X'7F' IANA ASSIGNED
      o  X'80' to X'FE' RESERVED FOR PRIVATE METHODS
      o  X'FF' NO ACCEPTABLE METHODS

   The client and server then enter a method-specific sub-negotiation.




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   Descriptions of the method-dependent sub-negotiations appear in
   separate memos.

   Developers of new METHOD support for this protocol should contact
   IANA for a METHOD number.  The ASSIGNED NUMBERS document should be
   referred to for a current list of METHOD numbers and their
   corresponding protocols.

   Compliant implementations MUST support NO AUTHENTICATION REQUIRED and
   GSSAPI, SHOULD support USERNAME/PASSWORD and MAY support CHAP.

   As with other TCP application data, out of band data is normally
   proxied to the SOCKS server as out of band data; note that
   implementations may be limited to handling a single byte of such data
   at a time.  Authentication methods which define some content
   encapsulation SHOULD define a method-specific mechanism for proxying
   out of band data.

4.  Requests

   Once the method-dependent subnegotiation has completed, the client
   sends the request details.  If the negotiated method includes
   encapsulation for purposes of integrity checking and/or
   confidentiality, these requests MUST be encapsulated in the method-
   dependent encapsulation.

   The SOCKS request is formed as follows:

           +----+-----+------+------+----------+----------+
           |VER | CMD | FLAG | ATYP | DST.ADDR | DST.PORT |
           +----+-----+------+------+----------+----------+
           | 1  |  1  |  1   |  1   | Variable |    2     |
           +----+-----+------+------+----------+----------+

   Where:

      o VER    protocol version: X'05'
      o CMD
         o CONNECT          X'01'
         o BIND             X'02'
         o UDP ASSOCIATE    X'03'
         o IANA Reserved    X'04' to X'7F'
         o Private methods  X'80' to X'FF'
      o FLAG              command dependent flag (defaults to X'00')
      o ATYP              address type of following address
        o IP V4 address     X'01'
        o DOMAINNAME        X'03'
        o IP V6 address     X'04'



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      o DST.ADDR          desired destination address
      o DST.PORT          desired destination port (network octet order)

      The SOCKS server will typically evaluate the request based on
      source and destination addresses, and return one or more reply
      messages, as appropriate for the request type.

5.  Addressing

   In an address field (DST.ADDR, BND.ADDR), the ATYP field specifies
   the type of address contained within the field:

       o  X'01'

   The address is a version-4 IP address, with a length of 4 octets.

       o  X'03'

   The address field contains a fully-qualified domain name.  The first
   octet of the address field contains the number of octets of name that
   follow, there is no terminating NUL octet.

       o  X'04'

   The address is a version-6 IP address, with a length of 16 octets.

6.  Replies

   The SOCKS request information is sent by the client as soon as it has
   established a connection to the SOCKS server, and completed the
   authentication negotiations.  The server evaluates the request, and
   returns a reply formed as follows:

           +----+-----+------+------+----------+----------+
           |VER | REP | FLAG | ATYP | BND.ADDR | BND.PORT |
           +----+-----+------+------+----------+----------+
           | 1  |  1  |  1   |  1   | Variable |    2     |
           +----+-----+------+------+----------+----------+

   Where:

          o  VER    protocol version: X'05'
          o  REP    Reply field:
             o  X'00' succeeded
             o  X'01' general SOCKS server failure
             o  X'02' connection not allowed by ruleset
             o  X'03' Network unreachable
             o  X'04' Host unreachable



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             o  X'05' Connection refused
             o  X'06' TTL expired
             o  X'07' Command not supported
             o  X'08' Address type not supported
             o  X'09' Invalid address
             o  X'0A' to X'FF' unassigned
          o  FLAG   command dependent flag
          o  ATYP   address type of following address
             o  IP V4 address: X'01'
             o  DOMAINNAME:    X'03'
             o  IP V6 address: X'04'
          o  BND.ADDR server bound address
          o  BND.PORT server bound port (network octet order)

   If the chosen method includes encapsulation for purposes of
   authentication, integrity and/or confidentiality, the replies are
   encapsulated in the method-dependent encapsulation.

 Reply Processing

   When a reply indicates a failure (REP value other than X'00',) the
   SOCKS server MUST terminate the TCP connection shortly after sending
   the reply.  This must be no more than 10 seconds after detecting the
   condition that caused a failure.

   If the reply code indicates a success, the client may now start
   passing data.  If the selected authentication method supports
   encapsulation for the purposes of integrity, authentication and/or
   confidentiality, the data are encapsulated using the method-dependent
   encapsulation.  Similarly, when data arrives at the SOCKS server for
   the client, the server MUST encapsulate the data as appropriate for
   the authentication method in use.

7. TCP Procedure

 CONNECT

   In the reply to a CONNECT, BND.PORT contains the port number that the
   server assigned to connect to the target host, and BND.ADDR contains
   the associated IP address.  The supplied BND.ADDR is often different
   from the IP address that the client uses to reach the SOCKS server,
   since such servers are often multi-homed.  It is expected that the
   SOCKS server will use DST.ADDR and DST.PORT, and the client-side
   source address and port in evaluating the CONNECT request.

 BIND

   The BIND request is used in protocols which require the client to



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   accept connections from the server.  FTP is a well-known example,
   which uses the primary client-to-server connection for commands and
   status reports, but may use a server-to-client connection for
   transferring data on demand (e.g. LS, GET, PUT).

   It is expected that the client side of an application protocol will
   use the BIND request only to establish secondary connections after a
   primary connection is established using CONNECT.  DST.ADDR must be
   the address of the primary connection's destination.  DST.PORT should
   be the requested port (or 0 for a random, unused port).  It is
   expected that a SOCKS server will use DST.ADDR and DST.PORT in
   evaluating the BIND request.

   Two replies are sent from the SOCKS server to the client during a
   BIND operation.  The first is sent after the server creates and binds
   a new socket.  The BND.PORT field contains the port number that the
   SOCKS server assigned to listen for an incoming connection.  The
   BND.ADDR field contains the associated IP address.  The client will
   typically use these pieces of information to notify (via the primary
   or control connection) the application server of the rendezvous
   address.  The second reply occurs only after the anticipated incoming
   connection succeeds or fails.

   In the second reply, the BND.PORT and BND.ADDR fields contain the
   address and port number of the connecting host.

8. UDP procedure

 UDP ASSOCIATE requests

   The UDP ASSOCIATE request is used to establish an association within
   the UDP relay process to handle UDP datagrams.  The DST.ADDR and
   DST.PORT fields contain the address and port that the client expects
   to use to send UDP datagrams on for the association.  The server MAY
   use this information to limit access to the association.  If the
   client is not in possesion of the information at the time of the UDP
   ASSOCIATE, the client MUST use address type X'01' with a port number
   and address of all zeros.

   A UDP association terminates when the TCP connection that the UDP
   ASSOCIATE request arrived on terminates.

   Flag bits in the request and reply are defined as follows:

         INTERFACE REQUEST X'01'
         USECLIENTSPORT    X'04'

   If the USECLIENTSPORT bit is set in the flag field of the request, the



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   server SHOULD interact with the application server using the same port
   the client used in the request, and set the USECLIENTSPORT bit in the
   flag field of the reply to acknowledge having done so.  If no port
   number was specified in the UDP ASSOCIATE request, this flag is
   meaningless and MUST not be used.

   If the INTERFACE REQUEST bit is set in the flag field of the request,
   the server may indicate its support for this extension by setting this
   bit in the reply.  If both client and server support this feature, the
   client SHOULD send INTERFACE DATA subcommands, described below, during
   the UDP association.

   In the reply to a UDP ASSOCIATE request, the BND.PORT and BND.ADDR
   fields indicate the port number/address where the client MUST send UDP
   request messages to be relayed.

 UDP Control Channel

   A UDP association terminates when the TCP connection that the UDP
   ASSOCIATE request arrived on terminates.  If the flag negotiation
   indicated mutual support for it, the client SHOULD send INTERFACE DATA
   subcommands to learn the external address information for the UDP
   assocaiation with respect to a particular destination.  The server, in
   turn, MAY use this information to limit access to the association to
   those destination addresses for which it has received INTERFACE DATA
   queries; multiple INTERFACE DATA commands are permitted, and have a
   cumulative effect.

   Such requests are formatted as follows:

       +----+-----+------+------+----------+------+
       |RSV | SUB | FLAG | ATYP | ADDR     | PORT |
       +----+-----+------+------+----------+------+
       | 1  |  1  |  1   |   1  | Variable |  2   |
       +----+-----+------+------+----------+------+

   The fields in the CONTROL CHANNEL packet are:

      o  RSV  Reserved X'00'
      o  SUB  Subcommand
            o  INTERFACE DATA: X'01'
      o  FLAG  subcommand dependent flag (normally X'00')
      o  ATYP  address type of following addresses:
            o  IP V4 address: X'01'
            o  DOMAINNAME:    X'03'
            o  IP V6 address: X'04'
      o  ADDR  destination address information
      o  PORT  destination port information (network octet order)



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   Replies to INTERFACE DATA commands are structured the same way as
   ordinary SOCKS replies, as per section 6.

 UDP packet structure

   A UDP-based client MUST send its datagrams to the UDP relay server at
   the UDP port indicated by BND.PORT in the reply to the UDP ASSOCIATE
   request.  If the selected authentication method provides
   encapsulation for the purposes of authenticity, integrity, and/or
   confidentiality, the datagram MUST be encapsulated using the
   appropriate encapsulation.  Each UDP datagram carries a UDP request
   header with it:

         +------+------+------+----------+----------+----------+
         | FLAG | FRAG | ATYP | DST.ADDR | DST.PORT |   DATA   |
         +------+------+------+----------+----------+----------+
         |  2   |  1   |  1   | Variable |    2     | Variable |
         +------+------+------+----------+----------+----------+

   The fields in the UDP request header are:

          o  FLAG     Reserved (currently X'0000')
          o  FRAG     Current fragment number
          o  ATYP     address type of following addresses:
             o  IP V4 address: X'01'
             o  DOMAINNAME:    X'03'
             o  IP V6 address: X'04'
          o  DST.ADDR desired destination address
          o  DST.PORT desired destination port (network octet order)
          o  DATA     user data

   FRAG is currently unused, and reserved for future work to deal with
   fragmentation; it must be set to X'00'.

   When a UDP relay server decides to relay a UDP datagram, it does so
   silently, without any notification to the requesting client.
   Similarly, it will drop datagrams it cannot or will not relay.  When
   a UDP relay server receives a reply datagram from a remote host, it
   MUST encapsulate that datagram using the above UDP request header,
   and any authentication-method-dependent encapsulation.

   The UDP relay server MUST acquire from the SOCKS server the expected
   IP address of the client that will send datagrams to the BND.PORT
   given in the reply to UDP ASSOCIATE.  It MUST drop any datagrams
   arriving from any source IP address other than the one recorded for
   the particular association.

   The programming interface for a SOCKS-aware UDP MUST report an



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   available buffer space for UDP datagrams that is smaller than the
   actual space provided by the operating system:

          o  if ATYP is X'01' - 10+method_dependent octets smaller
          o  if ATYP is X'03' - 262+method_dependent octets smaller
          o  if ATYP is X'04' - 20+method_dependent octets smaller

9.  Security Considerations

   This document describes a protocol for the application-layer
   traversal of IP network firewalls.  The security of such traversal is
   highly dependent on the particular authentication and encapsulation
   methods provided in a particular implementation, and selected during
   negotiation between SOCKS client and SOCKS server.

   Careful consideration should be given by the administrator to the
   selection of authentication methods.

Acknowledgments

   This memo describes a protocol that is an evolution of the previous
   version of the protocol, version 4[SOCKS]. This new protocol stems
   from active discussions and prototype implementations.  The key
   contributors are:

   o Marcus Leech: Bell-Northern Research
   o David Koblas: Independent Consultant
   o Ying-Da Lee: NEC Systems Laboratory
   o LaMont Jones: Hewlett-Packard Company
   o Ron Kuris: Unify Corporation
   o Matt Ganis: International Business Machines
   o David Blob: NEC USA
   o Wei Lu: NEC USA.
   o William Perry: Aventail
   o Dave Chouinard: Intel

10.  References


   [CHAP]     VanHeyningen, M., "Challenge-Handshake Authentication
              Protocol for SOCKS V5," work in progress.

   [GSSAPI]   Margrave, D., "GSS-API Authentication Method for SOCKS
              Version 5," work in progress.

   [RFC 1928] Leech, M., Ganis, M., Lee, Y., Kuris, R. Koblas, D., &
              Jones, L., "SOCKS Protocol V5," April 1996.




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

   [RFC 1961] McMahon, P., "GSS-API Authentication Method for SOCKS
              Version 5," June 1996.

   [SOCKS]    Koblas, D., "SOCKS", Proceedings: 1992 Usenix Security
              Symposium.

Author's Address

   Marc VanHeyningen
   Aventail Corporation
   808 Howell Streeet; Suite 200
   Seattle, WA  98101

   Phone: +1 (206) 215-1111
   Email: marcvh@aventail.com

































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--
Marc VanHeyningen                 marcvh@aventail.com
Internet Security Architect
Aventail                          http://www.aventail.com/


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