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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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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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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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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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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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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4. Multiple Chained Relay Mechanism (Advanced usage)
The SOCKS-based gateway mechanism has the flexibility to support
multiple chained relay topologies. With the mechanism, IPv4 and IPv6
mixed various communication topologies are accomplished.
Figure 2 shows the structure of the multiple chained relay mechanism.
Client C Gateway G1 Gateway G2 Destination D
+-----------+ (Server 1) (Server 2)
|Application|
+===========+ +-------------+ +-------------+ +-----------+
|*SOCKS Lib*| | *Gateway1* | | *Gateway2* | |Application|
+===========+ +=====---=====+ +=====---=====+ +-----------+
| Socket DNS| | Socket DNS | | Socket DNS | | Socket DNS|
+-----------+ +-------------+ +-------------+ +-----------+
| [ IPv X ] | |[IPvX]|(IPvY)| |(IPvY)|{IPvZ}| | { IPv Z } |
+-----------+ +-------------+ +-------------+ +-----------+
|Network I/F| | Network I/F | | Network I/F | |Network I/F|
+-----+-----+ +---+-----+---+ +---+-----+---+ +-----+-----+
| | | | | |
+============+ +==========+ +------------+
socksified socksified normal
connection connection connection
(ctrl)+data (ctrl)+data data only
Fig. 2 Multiple Chained Relay Mechanism
In this figure, the source node (Client C) initiates the
communication with the destination (Destination D). Underneath, the
connection is replaced with three connections, and they are relayed
at the two relay servers (Gateway G1 and G2). The *Gateway* includes
the same type of functions of *Socks Lib*. By enabling the *Socks
Lib* functions at the *Gateway1* on the first relay server (Gateway
G1), the multiple chained relay topology is accomplished.
There is no limitation on the number of relay operations between the
source node and the final destination node. It is possible to have
more than two intermediate relay servers. To simplify the
explanation, a twice-relayed topology is shown here.
Since the multiple chained relay is more complex than one-time relay
and causes complexity, it is recommended that the multiple chained
relay communication should be used only when it is necessary for some
reason (e.g., usable protocols or topologies are limited by routers
etc.).
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5. Applicability statement
The SOCKS-based gateway mechanism requests socksification of
applications (install *Socks Lib*) to accomplish heterogeneous
communications. It is not necessary to modify (change source codes
and recompile them, etc.) the applications, because typical
socksification is done by changing the linking order of dynamic link
libraries (specifically, by linking the SOCKS dynamic link library
before the dynamic link libraries for normal socket and DNS name
resolving APIs).
The mechanism does not request modification of the DNS system,
because the DNS name resolving procedure at the Client C is delegated
to the dual stack node Gateway G.
Other than the socksification, the SOCKS-based gateway mechanism has
the following three types of constraints.
1. Essential constraints:
Constraints are caused by the address length difference between
IPv4 and IPv6.
Functions that request an IP address as one of the return values
(e.g., getpeername() and getsockname() etc.) can not provide the
correct IP address as a return value. However, a suitable port
value can be provided, because IPv4 and IPv6 use the same size
port space and an appropriate port information is transferred by
the SOCKS protocol.
2. Constraints of the SOCKS mechanism:
Since the current SOCKS system can not socksify all of the tricky
applications in which extraordinary manners are used to create
connections, the SOCKS-based gateway mechanism can not be applied
to them.
3. Constraints to deal with the fake address:
The fake address must be dealt with as a temporary value at the
application. It is used as a key value in the mapping table for
the "DNS name resolving delegation" feature. When the application
is finished and the mapping table disappears, the fake address
information must be also released.
Even if it is recorded permanently (e.g., recorded as a bookmark),
serious problems will not occur. The recorded fake address
information will merely become useless, because fake address
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information is taken from a reserved special IP address space that
is never used in real communications (e.g., 0.0.0.x) and such a
information is useless for the normal communication applications.
Furthermore, such cases will be rare because most applications
usually record FQDN information (not fake IP address information)
to the bookmark, etc.
5.1 Native SOCKS mechanism considerations
The characteristics of the SOCKS-based IPv6/IPv4 gateway mechanism
are inherited from those of the native SOCKS mechanism. Therefore,
consideration issues of the native SOCKS mechanism are discussed in
this section.
The SOCKSv5 protocol is composed of three commands (CONNECT, BIND and
UDP ASSOCIATE). All of three commands can be applied in the SOCKS-
based IPv6/IPv4 gateway mechanism.
This document is described with assuming the usage of the CONNECT
command mainly, because the CONNECT command is the main and most
frequently used command in the SOCKS mechanism. Since the CONNECT
command does not have clear week points, we can use it freely without
considerations.
The other (BIND and UDP ASSOCIATE) commands have the following weak
points. So, we have to consider these points when we use the BIND or
UDP ASSOCIATE commands in the mechanism.
The BIND command is basically designed to support reverse-channel
rendezvous of the FTP type applications. So, general usages of the
BIND command may cause problems.
The UDP ASSOCIATE command is basically designed for simple UDP
applications (e.g., archie). It is not general enough to support a
large class of applications that use both TCP and UDP.
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6. Security Considerations
Since the SOCKS-based IPv6/IPv4 gateway mechanism is based on SOCKSv5
protocol, the security feature of the mechanism matches that of
SOCKSv5. It is described in the Security Considerations section of
the SOCKS Protocol Version 5 [SOCKSv5].
The mechanism is based on relaying two "terminated" connections at
the "application layer". The end-to-end security is maintained at
each of the relayed connections (i.e., between Client C and Gateway
G, and between Gateway G and Destination D). The mechanism does not
provide total end-to-end security relay between the original source
(Client C) and the final destination (Destination D).
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Appendix A. Implementations
Currently, there are two independent implementations of the SOCKS-
based IPv6/IPv4 gateway mechanism. Both of them are open to the
public.
One is NEC's implementation. Its source codes are available at the
following URL.
http://www.socks.nec.com/
The other is Fujitsu Lab.'s implementation, which is called
"SOCKS64". Its source codes are available at the following URL.
ftp://ftp.kame.net/pub/kame/misc/socks64-...
References
[SOCKSv5] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and
L. Jones, "SOCKS Protocol V5", RFC 1928, April 1996.
[TRANSMECH] Gilligan, R. and E. Nordmark, "Transition Mechanisms for
IPv6 Hosts and Routers", RFC 2893, August 2000.
[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[INET99] H. Kitamura, "Entering the IPv6 communication world by
the SOCKS-based IPv6/IPv4 Translator", in Proceedings of
INET99, July 1999.
Author's Address
Hiroshi Kitamura
NEC Corporation
Development Laboratories
(Igarashi Building 4F) 11-5, Shibaura 2-Chome,
Minato-Ku, Tokyo 108-8557, JAPAN
Phone: +81 (3) 5476-1071
Fax: +81 (3) 5476-1005
EMail: kitamura@da.jp.nec.com
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Acknowledgement
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