rfc3089.txt

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Network Working Group                                        H. Kitamura
Request for Comments: 3089                               NEC Corporation
Category: Informational                                       April 2001


               A SOCKS-based IPv6/IPv4 Gateway Mechanism

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This document describes a SOCKS-based IPv6/IPv4 gateway mechanism
   that enables smooth heterogeneous communications between the IPv6
   nodes and IPv4 nodes.

   It is based on the SOCKS protocol [SOCKSv5].  By applying the SOCKS
   mechanism to the heterogeneous communications and relaying two
   "terminated" IPv4 and IPv6 connections at the "application layer"
   (the SOCKS server), the SOCKS-based IPv6/IPv4 gateway mechanism is
   accomplished.

   Since it is accomplished without introducing new protocols, it
   provides the same communication environment that is provided by the
   SOCKS mechanism.  The same appearance is provided to the
   heterogeneous communications.  No conveniences or functionalities of
   current communications are sacrificed.

1. Introduction

   The SOCKS-based IPv6/IPv4 gateway mechanism is based on a mechanism
   that relays two "terminated" IPv4 and IPv6 connections at the
   "application layer" (the SOCKS server); its characteristics are
   inherited from those of the connection relay mechanism at the
   application layer and those of the native SOCKS mechanism.









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RFC 3089        SOCKS-based IPv6/IPv4 Gateway Mechanism       April 2001


2. Basic SOCKS-based Gateway Mechanism

   Figure 1 shows the basic SOCKS-based gateway mechanism.

                  Client C       Gateway G     Destination D
               +-----------+     (Server)
               |Application|
           +-->+===========+  +-------------+  +-----------+
      same-+   |*SOCKS Lib*|  |  *Gateway*  |  |Application|
       API +-->+===========+  +=====---=====+  +-----------+
               | Socket DNS|  | Socket  DNS |  | Socket DNS|
               +-----------+  +-------------+  +-----------+
               | [ IPv X ] |  |[IPvX]|(IPvY)|  | ( IPv Y ) |
               +-----------+  +-------------+  +-----------+
               |Network I/F|  | Network I/F |  |Network I/F|
               +-----+-----+  +---+-----+---+  +-----+-----+
                     |            |     |            |
                     +============+     +------------+
                       socksified           normal
                       connection         connection
                      (ctrl)+data          data only

                Fig. 1 Basic SOCKS-based Gateway Mechanism

   In this figure, the Client C initiates the communication to the
   Destination D.  Two new functional blocks are introduced and they
   compose the mechanism.

   One, *Socks Lib*, is introduced into the client side (Client C) (this
   procedure is called "socksifying").  The *Socks Lib* is located
   between the application layer and the socket layer, and can replace
   applications' socket APIs and DNS name resolving APIs (e.g.,
   gethostbyname(), getaddrinfo() etc.).  There is a mapping table in it
   for a "DNS name resolving delegation" feature (described below).
   Each socksified application has its own *Socks Lib*.

   The other, *Gateway*, is installed on the IPv6 and IPv4 dual stack
   node (Gateway G).  It is an enhanced SOCKS server that enables any
   types of protocol combination relays between Client C (IPvX) and
   Destination D (IPvY).  When the *Socks Lib* invokes a relay, one
   corresponding *Gateway* process (thread) is spawned from the parent
   *Gateway* to take charge of the relay connection.

   The following four types of combinations of IPvX and IPvY are
   possible in the mechanism.






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RFC 3089        SOCKS-based IPv6/IPv4 Gateway Mechanism       April 2001


    type C ------ G ------ D
           [IPvX]   (IPvY)
     A      IPv4     IPv4       homogeneous (normal SOCKS)
     B      IPv4     IPv6     * heterogeneous *
     C      IPv6     IPv4     * heterogeneous *
     D      IPv6     IPv6       homogeneous


   Type A is supported by the normal SOCKS mechanism.  Type B and C are
   the main targets for the SOCKS-based IPv6/IPv4 gateway mechanism.
   They provide heterogeneous communications.  Type D can be supported
   by the natural extension of the SOCKS mechanism, because it is a
   homogeneous communication.

   Since the *Socks Lib* communicates with the *Gateway* by using SOCKS
   protocol [SOCKSv5], the connection between them (the Client C and the
   Gateway G) is a special connection and is called a "socksified
   connection".  It can transfer not only data but also control
   information (e.g., the location information of Destination D).

   The connection between the Gateway G and the Destination D is a
   normal connection.  It is not modified (socksified).  A server
   application that runs on Destination D does not notice the existence
   of the Client C.  It recognizes that the peer node of the connection
   is the Gateway G (not Client C).

   No new protocols are introduced to the SOCKS protocol [SOCKSv5] to
   accomplish the mechanism.

   * Packet Size Adjustment

     Since the length of the IPv6 header is different from that of the
     IPv4 header, it is necessary to consider the packet size adjustment
     in heterogeneous communications.  If this is not taken into
     consideration, the packet size may exceed the MTU of the network.

     In the SOCKS-based IPv6/IPv4 gateway mechanism, it never exceeds
     the MTU, because the mechanism is based on relaying two
     "terminated" connections at the "application layer".  The relayed
     data is a simple data stream for the application, and the packet
     size is naturally adjusted at each relayed connection side.

   * Authenticated Relay

     Since the SOCKS is originally designed for firewall systems and it
     has various authentication methods, the relayed connections can be
     authenticated by the native SOCKS authentication methods.




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RFC 3089        SOCKS-based IPv6/IPv4 Gateway Mechanism       April 2001


3. DNS Name Resolving Procedure

   In all communication applications, it is a necessary to obtain
   destination IP address information to start a communication.  It is,
   however, theoretically impossible for the heterogeneous
   communications to obtain correct information, because an existing
   IPv4 application can not deal with an IPv6 address.  It prepares only
   a 4-byte address space to store an IP address information, and it can
   not store an IPv6 address information into there.  This is a critical
   problem caused by differences in address length.

   In order to solve the problem, a feature called "DNS name resolving
   delegation" is used in the SOCKS-based IPv6/IPv4 gateway mechanism.
   The feature involves the delegating of DNS name resolving actions at
   the source node (Client C) to the relay server (Gateway G).  Since
   the relay server is an IPv4 and IPv6 dual stack node, DNS name
   resolving queries for any address family types of destinations can be
   made without causing any problems.  Therefore, it is not necessary to
   modify the existing DNS mechanism at all.

   The feature supports not only the case in which a destination logical
   host name (FQDN) information is given but also the case in which a
   destination literal (numerical) IP address is given.  The latter case
   is supported in almost the same way as the former case.  Since the
   literal IPv6 address expression includes colons (":"), it is
   identified as an FQDN (not a literal IPv4 address) for the IPv4
   application.

   The SOCKS protocol specification [SOCKSv5] defines that IPv4 address,
   IPv6 address, and DOMAINNAME (FQDN) information can be used in the
   ATYP (address type) field of the SOCKS protocol format.  In the "DNS
   name resolving delegation" feature, the DOMAINNAME (FQDN) information
   is used in the ATYP (address type) field.  The FQDN information is
   transferred from the Client C to the Gateway G to indicate the
   Destination D.

   In order to solve the formerly explained critical problem, an
   appropriate "fake IP" address is introduced in the feature, and it is
   used as a virtual destination IP address for a socksified
   application.  A mapping table is also introduced in the *Socks Lib*
   (at the Client C) to manage mappings between "fake IP" and "FQDN".  A
   "fake IP" address is used as a key to look up the corresponding
   "FQDN" information.  The mapping table is local and independent of
   other applications or their *Socks Lib*s.







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RFC 3089        SOCKS-based IPv6/IPv4 Gateway Mechanism       April 2001


   The transparentness to applications is maintained in the feature.
   Nothing special is required to execute it except socksifying the
   applications.  Since DNS name resolving APIs are replaced by the
   *Socks Lib*, the "DNS name resolving delegation" is executed
   internally merely by calling the DNS name resolving APIs in ordinal
   methods.

   The "DNS name resolving delegation" is accomplished only when FQDN
   information is used in the ATYP (address type) field of the SOCKS
   command.  Therefore, it is mandatory to do so for heterogeneous
   communications.  The method of using FQDN information in the ATYP
   field depends on the configuration setting and implementation of the
   SOCKS protocol.  In order to simplify the discussion, only the case
   in which the FQDN information is used in the ATYP field is discussed
   here.

   The detailed internal procedure of the "DNS name resolving
   delegation" and address mapping management related issues are
   described as follows.

   1. An application on the source node (Client C) tries to get the
      IP address information of the destination node (Destination D) by
      calling the DNS name resolving function (e.g., gethostbyname()).
      At this time, the logical host name ("FQDN") information of the
      Destination D is passed to the application's *Socks Lib* as an
      argument of called APIs.

   2. Since the *Socks Lib* has replaced such DNS name resolving APIs,
      the real DNS name resolving APIs is not called here.  The argued
      "FQDN" information is merely registered into a mapping table in
      *Socks Lib*, and a "fake IP" address is selected as information
      that is replied to the application from a reserved special IP
      address space that is never used in real communications (e.g.,
      0.0.0.x).  The address family type of the "fake IP" address must be
      suitable for requests called by the applications.  Namely, it must
      belong to the same address family of the Client C, even if the
      address family of the Destination D is different from it.  After
      the selected "fake IP" address is registered into the mapping
      table as a pair with the "FQDN", it is replied to the application.

   3. The application receives the "fake IP" address, and prepares a
      "socket".  The "fake IP" address information is used as an element
      of the "socket".  The application calls socket APIs (e.g.,
      connect()) to start a communication.  The "socket" is used as an
      argument of the APIs.






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RFC 3089        SOCKS-based IPv6/IPv4 Gateway Mechanism       April 2001


   4. Since the *Socks Lib* has replaced such socket APIs, the real
      socket function is not called.  The IP address information of the
      argued socket is checked.  If the address belongs to the special
      address space for the fake address, the matched registered "FQDN"
      information of the "fake IP" address is obtained from the mapping
      table.

   5. The "FQDN" information is transferred to the *Gateway* on the
      relay server (Gateway G) by using the SOCKS command that is
      matched to the called socket APIs.  (e.g., for connect(), the
      CONNECT command is used.)

   6. Finally, the real DNS name resolving API (e.g., getaddrinfo()) is
      called at the *Gateway*.  At this time, the received "FQDN"
      information via the SOCKS protocol is used as an argument of the
      called APIs.

   7. The *Gateway* obtains the "real IP" address from a DNS server,
      and creates a "socket".  The "real IP" address information is used
      as an element of the "socket".

   8. The *Gateway* calls socket APIs (e.g., connect()) to communicate
      with the Destination D.  The "socket" is used as an argument of the
      APIs.

   The problem with the feature is that failures of the DNS name
   resolving process are detected incorrectly at the source node (Client
   C).  They are detected as connection-establishment failures.

   (Restrictions on applicability of "fake IP" address, etc., are
   described in Section 5.)

   * Operations for Address Management (reservation, mapping etc.)

   The SOCKS-based gateway mechanism does not require the reserving of a
   wide global address space for the address mapping, and complex
   address allocation and garbage-collection mechanisms are not
   necessary.

   Such address management operations are done at the *Socks Lib* by
   using the fake IP address and the mapping table for the DNS name
   resolving delegation.  Since the mapping table is prepared in each
   application, it is locally closed and independent of other
   applications.  Therefore, it is easy to manage the table, and it is
   not necessary to reserve a wide global address space.






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