Network Working Group                                           M. Leech
Request for Comments: 1928                    Bell-Northern Research Ltd
Category: Standards Track                                       M. Ganis
                                        International Business Machines
                                                                 Y. Lee
                                                 NEC Systems Laboratory
                                                               R. Kuris
                                                      Unify Corporation
                                                              D. Koblas
                                                 Independent Consultant
                                                               L. Jones
                                                Hewlett-Packard Company
                                                             March 1996


                       SOCKS Protocol 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.

Acknowledgments

  This memo describes a protocol that is an evolution of the previous
  version of the protocol, version 4 [1]. This new protocol stems from
  active discussions and prototype implementations.  The key
  contributors are: Marcus Leech: Bell-Northern Research, David Koblas:
  Independent Consultant, Ying-Da Lee: NEC Systems Laboratory, LaMont
  Jones: Hewlett-Packard Company, Ron Kuris: Unify Corporation, Matt
  Ganis: International Business Machines.

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.





Leech, et al                Standards Track                     [Page 1]

RFC 1928                SOCKS Protocol Version 5              March 1996


  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

  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



Leech, et al                Standards Track                     [Page 2]

RFC 1928                SOCKS Protocol Version 5              March 1996


  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.

  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.

  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' 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.



Leech, et al                Standards Track                     [Page 3]

RFC 1928                SOCKS Protocol Version 5              March 1996


  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 GSSAPI and SHOULD support
  USERNAME/PASSWORD authentication methods.

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 |  RSV  | ATYP | DST.ADDR | DST.PORT |
       +----+-----+-------+------+----------+----------+
       | 1  |  1  | X'00' |  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  RSV    RESERVED
         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
            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.





Leech, et al                Standards Track                     [Page 4]

RFC 1928                SOCKS Protocol Version 5              March 1996


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 |  RSV  | ATYP | BND.ADDR | BND.PORT |
       +----+-----+-------+------+----------+----------+
       | 1  |  1  | X'00' |  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
            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' to X'FF' unassigned
         o  RSV    RESERVED
         o  ATYP   address type of following address



Leech, et al                Standards Track                     [Page 5]

RFC 1928                SOCKS Protocol Version 5              March 1996


            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

  Fields marked RESERVED (RSV) must be set to X'00'.

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

CONNECT

  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.

BIND

  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.  In 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.





Leech, et al                Standards Track                     [Page 6]

RFC 1928                SOCKS Protocol Version 5              March 1996


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

UDP ASSOCIATE

  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 a port number and address of all
  zeros.

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

  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.

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.

7.  Procedure for UDP-based clients

  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:





Leech, et al                Standards Track                     [Page 7]

RFC 1928                SOCKS Protocol Version 5              March 1996


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

    The fields in the UDP request header are:

         o  RSV  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

  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 FRAG field indicates whether or not this datagram is one of a
  number of fragments.  If implemented, the high-order bit indicates
  end-of-fragment sequence, while a value of X'00' indicates that this
  datagram is standalone.  Values between 1 and 127 indicate the
  fragment position within a fragment sequence.  Each receiver will
  have a REASSEMBLY QUEUE and a REASSEMBLY TIMER associated with these
  fragments.  The reassembly queue must be reinitialized and the
  associated fragments abandoned whenever the REASSEMBLY TIMER expires,
  or a new datagram arrives carrying a FRAG field whose value is less
  than the highest FRAG value processed for this fragment sequence.
  The reassembly timer MUST be no less than 5 seconds.  It is
  recommended that fragmentation be avoided by applications wherever
  possible.

  Implementation of fragmentation is optional; an implementation that
  does not support fragmentation MUST drop any datagram whose FRAG
  field is other than X'00'.



Leech, et al                Standards Track                     [Page 8]

RFC 1928                SOCKS Protocol Version 5              March 1996


  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

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

Author's Address

      Marcus Leech
      Bell-Northern Research Ltd
      P.O. Box 3511, Stn. C,
      Ottawa, ON
      CANADA K1Y 4H7

      Phone: (613) 763-9145
      EMail: [email protected]


















Leech, et al                Standards Track                     [Page 9]