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rfc:rfc1928

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: mleech@bnr.ca

Leech, et al Standards Track [Page 9]

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