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RFC 1928 - SOCKS Protocol Version 5


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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.

   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

   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.

   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.

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

             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.

   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:

      +----+------+------+----------+----------+----------+
      |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'.

   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: mleech@bnr.ca

 

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