AFT Working Group                                         William Perry
draft-ietf-aft-socks-pro-v5-01                          Aventail,                                     Marc VanHeyningen
draft-ietf-aft-socks-pro-v5-02                           Aventail Corp.

                        SOCKS Protocol Version 5

Status of this Memo
   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 ( or to
   the editor.

   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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   and may be updated, replaced, or obsoleted by other documents at  any
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   To learn the current status of any Internet-Draft, please  check  the
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   Distribution of this memo is unlimited


   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

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
   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

   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.


   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

   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

   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'01' GSSAPI
      o  X'03' CHAP
      o  X'04' to X'7F' IANA ASSIGNED

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

   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   CHAP,   SHOULD   support  GSSAPI
   USERNAME/PASSWORD GSSAPI authentication methods.

   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     |


      o VER    protocol version: X'05'
      o CMD
         o CONNECT X'01'
         o BIND X'02'
         o UDP ASSOCIATE X'03'
         o  X'04' to X'7F' IANA ASSIGNED
      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'
      o DST.ADDR       desired destination address
      o DST.PORT desired destination port in network octet

      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 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     |


             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
                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 in 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.


   In the reply to a CONNECT, BND.PORT contains the port number that the
   server  assigned  to  connect  to  the  target  host,  while 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.


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

7. UDP procedure


   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:

         USECLIENTSPORT    X'04'

   If the USECLIENTSPORT bit is set in the flag field of the request, the
   server SHOULD use 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 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 MAY send interface-request 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 (unless the UDP relaying  is  done
   in the TCP channel as specified by the TCP RELAY flag).

   NOTE:  The  current  UDP ASSOCIATE command is not powerful enough for
   many newer protocols, and does not handle multicast traffic  at  all.
   A proposal to address these issues is available [Ch97]. relayed.

 Reply Processing

   When a reply (REP value other than X'00') indicates a failure, 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 (REP value of X'00') indicates a success, and the
   request was either a BIND or a CONNECT, 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.

 UDP Control Channel

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

7.  Procedure  If the flag negotiation
   indicated mutual support for UDP-based clients it, the client may send INTERFACE-REQUEST
   commands to learn the external address information for the UDP
   assocaiation with respect to a particular destination.

   Such requests are formatted as follows:

       |RSV | SUB | FLAG | ATYP | ADDR     | PORT | SIZE | DATA     |
       | 1  |  1  |  1   |   1  | Variable |  2   |  4   | Variable |

   The fields in the CONTROL CHANNEL packet are:

      o  RSV  Reserved X'00'
      o  SUB  Subcommand
            o  INTERFACE DATA: X'01'
      o  FLAG  A 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  any address information
      o  PORT  any port information
      o  SIZE  the size (in octets) of data in network order
      o  DATA  user data

   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 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
             o  DATA     user data

   FRAG is currently unused, and reserved for future work to  deal  with

   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
   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

8.  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.

9.  References

   [Ch97]     Chouinard, D., "SOCKS V5 UDP and Multicast Extensions",
               July 1997,

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

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

   [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

Author's Address

   William M. Perry
   Aventail, Corp.

   Marc VanHeyningen
   Aventail Corporation
   117 South Main Street, Suite 400
   Seattle, WA  98104

   Phone: +1 (206) 777-5615 215-1111