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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.


   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.

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

   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


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