Rfc1928-SOCKS Protocol version 5

Rfc1928-SOCKS Protocol version 5

Network Working Group                                           M. LeechRequest for Comments: 1928                    Bell-Northern Research LtdCategory: 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 5Status 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 smaller8.  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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