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IETF RFC 6535
Last modified on Wednesday, February 29th, 2012
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Internet Engineering Task Force (IETF) B. Huang
Request for Comments: 6535 H. Deng
Obsoletes: 2767, 3338 China Mobile
Category: Standards Track T. Savolainen
ISSN: 2070-1721 Nokia
February 2012
Dual-Stack Hosts Using "Bump-in-the-Host" (BIH)
 Abstract
Bump-in-the-Host (BIH) is a host-based IPv4 to IPv6 protocol
translation mechanism that allows a class of IPv4-only applications
that work through NATs to communicate with IPv6-only peers. The host
on which applications are running may be connected to IPv6-only or
dual-stack access networks. BIH hides IPv6 and makes the IPv4-only
applications think they are talking with IPv4 peers by local
synthesis of IPv4 addresses. This document obsoletes RFC 2767 and
RFC 3338.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6535.
Huang, et al. Standards Track PAGE 1 
RFC 6535 BIH February 2012
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Huang, et al. Standards Track PAGE 2
RFC 6535 BIH February 2012
Table of Contents
1. Introduction ....................................................4
1.1. Terminology ................................................5
1.2. Acknowledgment of Previous Work ............................5
2. Components of the Bump-in-the-Host ..............................6
2.1. Function Mapper ............................................8
2.2. Protocol Translator ........................................8
2.3. Extension Name Resolver ....................................8
2.3.1. Special Exclusion Sets for A and AAAA Records .......9
2.3.2. DNSSEC Support .....................................10
2.3.3. Reverse DNS Lookup .................................10
2.3.4. DNS Caches and Synthetic IPv4 Addresses ............10
2.4. Address Mapper ............................................11
3. Behavior and Network Examples ..................................11
4. Considerations .................................................15
4.1. Socket API Conversion .....................................15
4.2. Socket Bindings ...........................................15
4.3. ICMP Message Handling .....................................15
4.4. IPv4 Address Pool and Mapping Table .......................15
4.5. Multi-Interface ...........................................17
4.6. Multicast .................................................17
5. Application-Level Gateway Requirements Considerations ..........17
6. Security Considerations ........................................17
6.1. Implications on End-to-End Security .......................18
6.2. Filtering .................................................18
6.3. Attacks on BIH ............................................18
6.4. DNS Considerations ........................................19
7. Changes since RFC 2767 and RFC 3338 ............................19
8. Acknowledgments ................................................20
9. References .....................................................21
9.1. Normative References ......................................21
9.2. Informative References ....................................21
Appendix A. API List Intercepted by BIH ...........................23
Huang, et al. Standards Track PAGE 3
RFC 6535 BIH February 2012
1. Introduction
This document describes Bump-in-the-Host (BIH), a successor and
combination of the Bump-in-the-Stack (BIS)[RFC 2767] and Bump-in-the-
API (BIA) [RFC 3338] technologies, which enable IPv4-only legacy
applications to communicate with IPv6-only servers by synthesizing
IPv4 addresses from AAAA records. Section 7 describes the reasons
for making RFC 2767 and RFC 3338 obsolete.
The supported class of applications includes those that use DNS for
IP address resolution and that do not embed IP address literals in
application-protocol payloads. This includes legacy client-server
applications using the DNS that are agnostic to the IP address family
used by the destination and that are able to do NAT traversal. The
synthetic IPv4 addresses shown to applications are taken from the
private address pool of [RFC 1918] in order to ensure that possible
NAT traversal techniques will be initiated.
The IETF recommends using solutions based on dual stack or tunneling
for IPv6 transition and specifically recommends against deployments
utilizing double protocol translation. Use of BIH together with a
NAT64 is NOT RECOMMENDED [RFC 6180].
BIH includes two major implementation alternatives: a protocol
translator between the IPv4 and the IPv6 stacks of a host or an API
translator between the IPv4 socket API module and the TCP/IP module.
Essentially, IPv4 is translated into IPv6 at the socket API layer or
at the IP layer, the former of which is the recommended
implementation alternative.
When BIH is implemented at the socket API layer, the translator
intercepts IPv4 socket API function calls and invokes corresponding
IPv6 socket API function calls to communicate with IPv6 hosts.
When BIH is implemented at the network layer, the IPv4 packets are
intercepted and converted to IPv6 using the IP conversion mechanism
defined in the Stateless IP/ICMP Translation Algorithm (SIIT)
[RFC 6145]. The protocol translation has the same benefits and
drawbacks as SIIT.
The location of the BIH refers to the location of the protocol
translation function. The location of the IPv4 address and DNS A
record synthesis function is orthogonal to the location of the
protocol translation and may or may not happen at the same location.
Huang, et al. Standards Track PAGE 4
RFC 6535 BIH February 2012
BIH can be used whenever an IPv4-only application needs to
communicate with an IPv6-only server, independently of the address
families supported by the access network. Hence, the access network
can be IPv6-only or dual-stack capable.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC 2119].
This document uses terms defined in [RFC 2460] and [RFC 4213].
1.1. Terminology
DNS synthesis
The process of creating an A record containing a synthetic IPv4
address.
Real IPv4 address
An IPv4 address of a remote node a host has learned, for example,
from DNS response to an A query.
Real IPv6 address
An IPv6 address of a remote node a host has learned, for example,
from DNS response to a AAAA query.
Synthetic IPv4 address
An IPv4 address that has meaning only inside a host and that is
used to provide IPv4 representation of remote node's real IPv6
address.
1.2. Acknowledgment of Previous Work
This document is a direct derivative of [RFC 2767], "Dual Stack Hosts
using the "Bump-In-the-Stack" Technique (BIS)" by Kazuaki TSHUCHIYA,
Hidemitsu HIGUCHI, and Yoshifumi ATARASHI and of [RFC 3338], "Dual
Stack Hosts Using "Bump-in-the-API" (BIA)" by Seungyun Lee, Myung-Ki
Shin, Yong-Jin Kim, Alain Durand, and Erik Nordmark, which similarly
provides IPv4-only applications on dual-stack hosts the means to
operate over IPv6. Section 7 covers the changes since those
documents.
Huang, et al. Standards Track PAGE 5
RFC 6535 BIH February 2012
2. Components of the Bump-in-the-Host
Figure 1 shows the architecture of a host in which BIH is implemented
as a socket API-layer translator, i.e., as a "Bump-in-the-API".
+----------------------------------------------+
| +------------------------------------------+ |
| | | |
| | IPv4 applications | |
| | | |
| +------------------------------------------+ |
| +------------------------------------------+ |
| | Socket API (IPv4, IPv6) | |
| +------------------------------------------+ |
| +-[ API translator]------------------------+ |
| | +-----------+ +---------+ +------------+ | |
| | | Ext. Name | | Address | | Function | | |
| | | Resolver | | Mapper | | Mapper | | |
| | +-----------+ +---------+ +------------+ | |
| +------------------------------------------+ |
| +--------------------+ +-------------------+ |
| | | | | |
| | TCP(UDP)/IPv4 | | TCP(UDP)/IPv6 | |
| | | | | |
| +--------------------+ +-------------------+ |
+----------------------------------------------+
Figure 1: Architecture of a dual-stack host using protocol
translation at the socket layer
Figure 2 shows the architecture of a host in which BIH is implemented
as a network-layer translator, i.e., a "Bump-in-the-Stack".
Huang, et al. Standards Track PAGE 6
RFC 6535 BIH February 2012
+------------------------------------------------------------+
| +------------------------------------------+ |
| | IPv4 applications | |
| | Host's main DNS resolver | |
| +------------------------------------------+ |
| +------------------------------------------+ |
| | TCP/UDP | |
| +------------------------------------------+ |
| +------------------------------------------+ +---------+ |
| | IPv4 | | | |
| +------------------------------------------+ | Address | |
| +------------------+ +---------------------+ | Mapper | |
| | Protocol | | Extension Name | | | |
| | Translator | | Resolver | | | |
| +------------------+ +---------------------+ | | |
| +------------------------------------------+ | | |
| | IPv4 / IPv6 | | | |
| +------------------------------------------+ +---------+ |
+------------------------------------------------------------+
Figure 2: Architecture of a dual-stack host using protocol
translation at the network layer
Dual-stack hosts, defined in [RFC 4213], need applications, TCP/IP
modules, and addresses for both IPv4 and IPv6. The proposed hosts in
this document have an API or network-layer translator to allow legacy
IPv4 applications to communicate with IPv6-only peers. The BIH
architecture consists of an Extension Name Resolver, an address
mapper, and depending on implementation either a function mapper or a
protocol translator. It is worth noting that the Extension Name
Resolver's placement is orthogonal to the placement of protocol
translation. For example, the Extension Name Resolver may reside in
the socket API while protocol translation takes place at the network
layer.
The choice between the socket API- and network-layer architectures
varies case by case. While the socket API architecture alternative
is the recommended one, it may not always be possible to choose.
This may be the case, for example, when the used operating system
does not allow modifications to be done for API implementations, but
does allow the addition of virtual network interfaces and related
software modules. On the other hand, sometimes it may not be
possible to introduce protocol translators inside the operating
system, but it may be easy to modify implementations behind the API
provided for applications. The choice of architecture also depends
on who is creating implementation of BIH. For example, an
Huang, et al. Standards Track PAGE 7
RFC 6535 BIH February 2012
application framework provider, an operating system provider, and a
device vendor may all choose different approaches due their different
positions.
2.1. Function Mapper
The function mapper translates an IPv4 socket API function into an
IPv6 socket API function.
When detecting IPv4 socket API function calls from IPv4 applications,
the function mapper MUST intercept the function calls and invoke IPv6
socket API functions that correspond to the IPv4 socket API
functions.
The function mapper MUST NOT perform function mapping when the
application is initiating communications to the address range used by
local synthesis and the address mapping table does not have an entry
matching the address.
See Appendix A for an informational list of functions that would be
appropriate to intercept by the function mapper.
2.2. Protocol Translator
The protocol translator translates IPv4 into IPv6, and vice versa,
using the IP conversion mechanism defined in SIIT [RFC 6145]. To
avoid unnecessary fragmentation, the host's IPv4 module SHOULD be
configured with a small enough MTU (MTU of the IPv6 enabled link - 20
bytes).
Protocol translation cannot be performed for IPv4 packets sent to the
IPv4 address range used by local synthesis and for which a mapping
table entry does not exist. The implementation SHOULD attempt to
route such packets via IPv4 interfaces instead.
2.3. Extension Name Resolver
The Extension Name Resolver (ENR) returns an answer in response to
the IPv4 application's name resolution request.
In the case of the socket API-layer implementation alternative, when
an IPv4 application tries to do a forward lookup to resolve names via
the resolver library (e.g., gethostbyname()), BIH intercepts the
function call and instead calls the IPv6 equivalent functions (e.g.,
getaddrinfo()) that will resolve both A and AAAA records. This
implementation alternative is name resolution protocol agnostic;
hence, it supports techniques such as "hosts-file", NetBIOS, mDNS,
and anything else the underlying operating system uses.
Huang, et al. Standards Track PAGE 8
RFC 6535 BIH February 2012
In the case of the network-layer implementation alternative, the ENR
intercepts the A query and creates an additional AAAA query with
similar content. The ENR will then collect replies to both A and
AAAA queries and, depending on results, either return an A reply
unmodified or synthesize a new A reply. If no reply for the A query
is received after ENR-implementation-specific timeout, after
reception of positive AAAA response, the ENR MAY choose to proceed as
if there were only a AAAA record available for the destination.
The network-layer implementation alternative will only be able to
catch applications' name resolution requests that result in actual
DNS queries; hence, it is more limited when compared to the socket
API-layer implementation alternative. Hence, the socket API-layer
alternative is RECOMMENDED.
In either implementation alternative, if a DNS A record reply
contains non-excluded real IPv4 addresses, the ENR MUST NOT
synthesize IPv4 addresses.
The ENR asks the address mapper to assign a synthetic IPv4 address
corresponding to each received IPv6 address if the A record query
resulted in a negative response, all received real IPv4 addresses
were excluded, or the A query timed out. The timeout value is
implementation specific and may be short in order to provide a good
user experience.
In the case of the API-layer implementation alternative, the ENR will
simply make the API (e.g., gethostbyname) return the synthetic IPv4
address. In the case of the network-layer implementation
alternative, the ENR synthesizes an A record for the assigned
synthetic IPv4 address and delivers it up the stack. If the response
contains a CNAME or a DNAME record, then the CNAME or DNAME chain is
followed until the first terminating A or AAAA record is reached.
Application | Network | ENR behavior
query | response |
---------------+-----------------------+----------------------------
IPv4 address(es) | IPv4 address(es) | return real IPv4 address(es)
IPv4 address(es) | IPv6 address(es) | synthesize IPv4 address(es)
IPv4 address(es) | IPv4/IPv6 address(es) | return real IPv4 address(es)
Figure 3: ENR Behavior Illustration
2.3.1. Special Exclusion Sets for A and AAAA Records
An ENR implementation SHOULD, by default, exclude certain real IPv4
and IPv6 addresses seen on received A and AAAA records. The
addresses to be excluded by default MAY include addresses such as
Huang, et al. Standards Track PAGE 9
RFC 6535 BIH February 2012
those that should not appear in the DNS or on the wire (see Section
5.1.4 of [RFC 6147] and [RFC 5735]). Additional addresses MAY be
excluded based on possibly configurable local policies.
2.3.2. DNSSEC Support
When the ENR is implemented at the network layer, the A record
synthesis can cause similar issues as are described in [RFC 6147]
section 3. While running BIH, the main resolver of the host SHOULD
NOT perform validation of A records, as synthetic A records created
by ENR would fail in validation. While not running BIH, a host's
resolver can use DNS Security (DNSSEC) in the same way that any other
resolver can. The ENR MAY support DNSSEC, in which case the (stub)
resolver on a host can be configured to trust validations done by the
ENR located at the network layer. In some cases, the host's
validating stub resolver can implement the ENR by itself.
When the ENR is implemented at the socket API level, there are no
issues with DNSSEC use, as the ENR itself uses socket APIs for DNS
resolution. This approach is RECOMMENDED.
2.3.3. Reverse DNS Lookup
When an application requests a reverse lookup (PTR query) for an IPv4
address, the ENR MUST check whether the queried IPv4 address can be
found in the address mapper's mapping table and if it is a synthetic
IPv4 address. If an entry is found and the queried IPv4 address is
synthetic, the ENR MUST initiate a corresponding reverse lookup for
the real IPv6 address. In the case where the application requested a
reverse lookup for an address not part of the synthetic IPv4 address
pool, e.g., a global address, the request MUST be passed on
unmodified.
For example, when an application requests a reverse lookup for a
synthetic IPv4 address, the ENR needs to intercept that query. The
ENR asks the address mapper for the real IPv6 address that
corresponds to the synthetic IPv4 address. The ENR shall perform a
reverse lookup procedure for the destination's IPv6 address and
return the name received as a response to the application that
initiated the IPv4 query.
2.3.4. DNS Caches and Synthetic IPv4 Addresses
When BIH shuts down or address mapping table entries are cleared for
any reason, DNS cache entries for synthetic IPv4 addresses MUST be
flushed. There may be a DNS cache in the network-layer ENR itself
and at the host's stub resolver.
Huang, et al. Standards Track PAGE 10
RFC 6535 BIH February 2012
2.4. Address Mapper
The address mapper maintains an IPv4 address pool that can be used
for IPv4 address synthesis. The pool consists of the IPv4 addresses
of [RFC 1918] as per Section 4.4. Also, the address mapper maintains
a table consisting of pairs of synthetic IPv4 addresses and
destinations' real IPv6 addresses.
When the ENR, translator, or the function mapper requests the address
mapper to assign a synthetic IPv4 address corresponding to an IPv6
address, the address mapper selects and returns an IPv4 address out
of the local pool and registers a new entry into the table. The
registration occurs in the following three cases:
1. When the ENR gets only IPv6 addresses for the target host name
and there is no existing mapping entry for the IPv6 addresses.
One or more synthetic IPv4 addresses will be returned to the
application and mappings for synthetic IPv4 addresses to real
IPv6 addresses are created.
2. When the ENR gets both real IPv4 and IPv6 addresses, but the real
IPv4 addresses contain only excluded IPv4 addresses
(e.g., 127.0.0.1). The behavior will follow case (1).
3. When the function mapper is triggered by a received IPv6 packet
and there is no existing mapping entry for the IPv6 source
address (for example, the client sent a UDP request to an anycast
address, but a response was received from a unicast address).
Other possible combinations are outside of BIH.
3. Behavior and Network Examples
Figure 4 illustrates a very basic network scenario. An IPv4-only
application is running on a host attached to the IPv6-only Internet
and is talking to an IPv6-only server. Communication is made
possible by Bump-in-the-Host.
+----+ +-------------+
| H1 |----------- IPv6 Internet -------- | IPv6 server |
+----+ +-------------+
v4 only
application
Figure 4: Network Scenario #1
Huang, et al. Standards Track PAGE 11
RFC 6535 BIH February 2012
Figure 5 illustrates a possible network scenario where an IPv4-only
application is running on a host attached to a dual-stack network,
but the destination server is running on a private site that is
numbered with public IPv6 addresses and not globally reachable IPv4
addresses, such as the addresses of [RFC 1918], without port
forwarding set up on the NAT44. The only means to contact the server
is to use IPv6.
+----------------------+ +------------------------------+
| Dual-Stack Internet | | IPv4 Private site (Net 10) |
| | | IPv6 routed site |
| +---------+ +----------+ |
| +-| NAT44 |-------------+ | |
| +----+ | +---------+ | | |
| | H1 |---------+ | | | Server | |
| +----+ | +-----------+ | | |
| v4-only +-|IPv6 Router|-----------+ | |
| application +-----------+ +----------+ |
| | | Dual Stack |
| | | 10.1.1.1 |
| | | 2001:DB8::1 |
+----------------------+ +------------------------------+
Figure 5: Network Scenario #2
Illustrations of host behavior in both implementation alternatives
are given here. Figure 6 illustrates a setup where BIH (including
the ENR) is implemented at the socket API layer, and Figure 7
illustrates a setup where BIH (including the ENR) is implemented at
the network layer.
"dual stack" "host6"
IPv4 Socket | [ API Translator ] | TCP(UDP)/IP Name
appli- API | ENR Address Function| (v6/v4) Server
cation | Mapper Mapper |
| | | | | | | |
<<Resolve IPv4 addresses for "host6".>> | | |
| | | | | | | |
|------->|------->| Query IPv4 addresses for host6. | |
| | | | | | | |
| | |------------------------------------------------->|
| | | Query 'A' and 'AAAA' records for host6 |
| | | | | | | |
| | |<-------------------------------------------------|
| | | Reply with the 'AAAA' record. | |
| | | | | | |
| | |<<The 'AAAA' record is resolved.>> |
| | | | | | |
Huang, et al. Standards Track PAGE 12
RFC 6535 BIH February 2012
| | |+++++++>| Request synthetic IPv4 address |
| | | | corresponding to the IPv6 address.
| | | | | | |
| | | |<<Assign one synthetic IPv4 address.>>
| | | | | | |
| | |<+++++++| Reply with the synthetic IPv4 address.
| | | | | | |
|<-------|<-------| Reply with the IPv4 address |
| | | | | | |
| | | | | | |
<<Call IPv4 Socket API function >> | | |
| | | | | | |
|=======>|=========================>|An IPv4 Socket API action
| | | | | | |
| | | |<+++++++| Request IPv6 addresses|
| | | | | corresponding to the |
| | | | | synthetic IPv4 addresses.
| | | | | | |
| | | |+++++++>| Reply with the IPv6 addresses.
| | | | | | |
| | | | |<<Translate IPv4 into IPv6.>>
| | | | | | |
| An IPv6 Socket API action |=======================>|
| | | | | | |
| | | | |<<IPv6 data received |
| | | | | from network.>> |
| | | | | | |
| An IPv6 Socket API action |<=======================|
| | | | | | |
| | | | |<<Translate IPv6 into IPv4.>>
| | | | | | |
| | | |<+++++++| Request synthetic IPv4 addresses
| | | | | corresponding to the |
| | | | | IPv6 addresses. |
| | | | | | |
| | | |+++++++>| Reply with the IPv4 addresses.
| | | | | | |
|<=======|<=========================| An IPv4 Socket API action
| | | | | | |
Figure 6: Example of BIH as API Addition
Huang, et al. Standards Track PAGE 13
RFC 6535 BIH February 2012
"dual stack" "host6"
IPv4 stub TCP/ ENR address translator IPv6
app res. IPv4 mapper
| | | | | | | |
<<Resolve an IPv4 address for "host6".>> | |
|-->| | | | | | |
| |----------->| Query 'A' records for "host6". | Name
| | | | | | | | Server
| | | |------------------------------------------->|
| | | | Query 'A' and 'AAAA' records for "host6"
| | | | | | | | |
| | | |<-------------------------------------------|
| | | | Reply only with 'AAAA' record. |
| | | | | | | |
| | | |<<Only 'AAAA' record is resolved.>> |
| | | | | | | |
| | | |-------->| Request synthetic IPv4 address
| | | | | corresponding to each IPv6 address.
| | | | | | | |
| | | | |<<Assign synthetic IPv4 addresses.>>
| | | | | | | |
| | | |<--------| Reply with the synthetic IPv4 address.
| | | | | | | |
| | | |<<Create 'A' record for the IPv4 address.>>
| | | | | | | |
| |<-----------| Reply with the 'A' record. | |
| | | | | | | |
|<--|<<Reply with the IPv4 address | | |
| | | | | | | |
<<Send an IPv4 packet to "host6".>>| | |
| | | | | | | |
|=======>|========================>| An IPv4 packet. |
| | | | | | | |
| | | | |<++++++| Request IPv6 addresses
| | | | | | corresponding to the
| | | | | | synthetic IPv4 addresses.
| | | | | | | |
| | | | |++++++>| Reply with the IPv6|
| | | | | | addresses. |
| | | | | | | |
| | | | | |<<Translate IPv4 into IPv6.>>
| | | | | | | |
| | | |An IPv6 packet. |==========>|========>|
| | | | | | | |
| | | | | <<Reply with an IPv6 packet.>>
| | | | | | | |
| | | |An IPv6 packet. |<==========|<========|
| | | | | | | |
Huang, et al. Standards Track PAGE 14
RFC 6535 BIH February 2012
| | | | | |<<Translate IPv6 into IPv4.>>
| | | | | | | |
| | | | |<++++++| Request synthetic IPv4
| | | | | | addresses corresponding
| | | | | | to the IPv6 addresses.
| | | | | | | |
| | | | |++++++>| Reply with the IPv4 addresses.
| | | | | | | |
|<=======|=========================| An IPv4 packet. |
| | | | | | | |
Figure 7: Example of BIH at the Network Layer
4. Considerations
4.1. Socket API Conversion
IPv4 socket API functions are translated into IPv6 socket API
functions that are semantically as identical as possible, and vice
versa. See Appendix A for the API list intercepted by BIH. However,
some IPv4 socket API functions are not fully compatible with IPv6
since IPv4 supports features that are not present in IPv6, such as
SO_BROADCAST.
4.2. Socket Bindings
BIH SHOULD select a source address for a socket from the recommended
source address pool if a socket used for communications has not been
explicitly bound to any IPv4 address.
The binding of an explicitly bound socket MUST NOT be changed by the
BIH.
4.3. ICMP Message Handling
ICMPv4 and ICMPv6 messages MUST be translated as defined by SIIT
[RFC 6145]. In the network-layer implementation alternative, the
protocol translator MUST translate ICMPv6 packets to ICMPv4 and vice
versa, and in the socket API implementation alternative, the socket
API MUST handle conversions in similar fashion.
4.4. IPv4 Address Pool and Mapping Table
The address pool consists of the private IPv4 addresses of [RFC 1918].
This pool can be implemented at different granularities in the node,
e.g., a single pool per node, or at some finer granularity such as
per-user or per-process. In the case of a large number of IPv4
applications communicating with a large number of IPv6 servers, the
Huang, et al. Standards Track PAGE 15
RFC 6535 BIH February 2012
available address space may be exhausted if the granularity is not
fine enough. This should be a rare event and chances will decrease
as IPv6 support increases. The applications may use IPv4 addresses
they learn for a much longer period than DNS time to live indicates.
Therefore, the mapping table entries should be kept active for a long
period of time. For example, a web browser may initiate one DNS
query and then create multiple TCP sessions over time to the address
it learns. When address mapping table clean-up is required, the BIH
may utilize techniques used by network address translators, such as
described in [RFC 2663], [RFC 5382], and [RFC 5508].
The address space of RFC 1918 was chosen because legacy applications
generally understand it as a private address space. A new dedicated
address space would run the risk of not being understood by
applications as private. 127/8 and 169.254/16 are rejected due to
possible assumptions applications may make when seeing them.
The addresses of RFC 1918 used by the BIH have a risk of conflicting
with addresses used in the host's possible IPv4 interfaces and
corresponding local networks. The conflicts can be mitigated, but
not fully avoided, by using less commonly used portions of the
address space of RFC 1918. Addresses from 172.16/12 are thought to
be less likely to be in conflict than addresses from 10/8 or
192.168/16 spaces. A source address can usually be selected in a
non-conflicting manner, but a small possibility exists for
synthesized destination addresses being in conflict with real
addresses used in attached IPv4 networks.
The RECOMMENDED IPv4 addresses are following:
Primary source addresses: 172.21.112.0/20.
Source addresses have to be allocated because applications use
getsockname() calls and, in the network-layer mode, an IP
address of the IPv4 interface has to be shown (e.g., by
'ifconfig'). More than one address is allocated to allow
implementation flexibility, e.g., for cases where a host has
multiple IPv6 interfaces. The source addresses are from
different subnets than destination addresses to ensure
applications would not make on-link assumptions and would
instead enable NAT traversal functions.
Secondary source addresses: 10.170.224.0/20.
These addresses are recommended if a host has a conflict with
primary source addresses.
Huang, et al. Standards Track PAGE 16
RFC 6535 BIH February 2012
Primary destination addresses: 10.170.160.0/20.
The address mapper will select destination addresses primarily
out of this pool.
Secondary destination addresses: 172.21.80.0/20.
The address mapper will select destination addresses out of
this pool if the node has a dual-stack connection conflicting
with primary destination addresses.
4.5. Multi-Interface
In the case of dual-stack destinations, BIH MUST NOT do protocol
translation from IPv4 to IPv6 when the host has any IPv4 interfaces,
native or tunneled, available for use.
It is possible that an IPv4 interface is activated during BIH
operation, for example, if a node moves to a coverage area of an
IPv4-enabled network. In such an event, BIH MUST stop initiating
protocol translation sessions for new connections, and BIH MAY
disconnect active sessions. The choice of disconnection is left for
implementations, and it may depend on whether IPv4 address conflict
occurs between addresses used by BIH and addresses used by the new
IPv4 interface.
4.6. Multicast
Protocol translation for multicast is not supported.
5. Application-Level Gateway Requirements Considerations
No Application-Level Gateway (ALG) functionality is specified herein
as ALG design is generally not encouraged for host-based translation
and as BIH is intended for applications that do not include IP
addresses in protocol payloads.
6. Security Considerations
The security considerations of BIH follows closely, but not
completely, those of NAT64 [RFC 6146] and DNS64 [RFC 6147]. The
following sections are copied from RFC 6146 and RFC 6147 and modified
for BIH.
Huang, et al. Standards Track PAGE 17
RFC 6535 BIH February 2012
6.1. Implications on End-to-End Security
Any protocols that protect IP header information are essentially
incompatible with BIH. This implies that end-to-end IPsec
verification will fail when the Authentication Header (AH) is used
(both transport and tunnel mode) and when ESP is used in transport
mode. This is inherent in any network-layer translation mechanism.
End-to-end IPsec protection can be restored, using UDP encapsulation
as described in [RFC 3948]. The actual extensions to support IPsec
are out of the scope of this document.
6.2. Filtering
BIH creates binding state using packets flowing from the IPv4 side to
the IPv6 side. In accordance with the procedures defined in this
document, following the guidelines defined in [RFC 4787], a BIH
implementation MUST offer "Endpoint-Independent Mapping".
Implementations MAY also provide support for "Address-Dependent
Mapping" following the guidelines defined in [RFC 4787].
The security properties, however, are determined by which packets the
BIH allows in and which it does not. The security properties are
determined by the filtering behavior and by the possible filtering
configuration in the filtering portions of the BIH, not by the
address mapping behavior.
6.3. Attacks on BIH
The BIH implementation itself is a potential victim of different
types of attacks. In particular, the BIH can be a victim of Denial-
of-Service (DoS) attacks. The BIH implementation has a limited
number of resources that can be consumed by attackers creating a DoS
attack. The BIH has a limited number of IPv4 addresses that it uses
to create the bindings. Even though the BIH performs address
translation, it is possible for an attacker to consume the synthetic
IPv4 address pool by triggering a host to issue DNS queries for names
that cause ENR to synthesize A records. DoS attacks can also affect
other limited resources available in the host running BIH such as
memory or link capacity. For instance, it is possible for an
attacker to launch a DoS attack on the memory of the BIH running
device by sending fragments that the BIH will store for a given
period. If the number of fragments is large enough, the memory of
the host could be exhausted. BIH implementations MUST implement
proper protection against such attacks, for instance, allocating a
limited amount of memory for fragmented packet storage.
Huang, et al. Standards Track PAGE 18
RFC 6535 BIH February 2012
Another consideration related to BIH resource depletion is the
preservation of binding state. Attackers may try to keep a binding
state alive forever by sending periodic packets that refresh the
state. In order to allow the BIH to defend against such attacks, the
BIH implementation MAY choose not to extend the session entry
lifetime for a specific entry upon the reception of packets for that
entry through the external interface. However, such an action would
not allow one-way communication sessions to stay alive.
6.4. DNS Considerations
BIH operates in combination with the DNS, and it is therefore subject
to whatever security considerations are appropriate to the DNS mode
in which the BIH is operating (i.e., recursive or stub-resolver
mode).
BIH has the potential to interfere with the functioning of DNSSEC,
because BIH modifies DNS answers, and DNSSEC is designed to detect
such modifications and to treat modified answers as bogus.
7. Changes since RFC 2767 and RFC 3338
This document combines and obsoletes both [RFC 2767] and [RFC 3338].
The changes in this document mainly reflect the following:
1. Addresses of RFC 1918 used for synthesis
RFC 3338 used unassigned IPv4 addresses (e.g., 0.0.0.1 -
0.0.0.255) for synthetic IPv4 addresses. Those addresses should
not have been used and that may cause problems with applications.
It is preferable to use addresses defined in RFC 1918 instead, as
described in Section 4.4.
2. Support for reverse (PTR) DNS queries
Neither RFC 2767 nor RFC 3338 included support for reverse (PTR)
DNS queries. This document adds the support in Section 2.3.3.
3. DNSSEC support
RFC 2767 did not include DNSSEC considerations, which are now
included in Section 2.3.2
Huang, et al. Standards Track PAGE 19
RFC 6535 BIH February 2012
4. Architectural recommendation
This document recommends the socket API-layer implementation
option over network layer translation, i.e., it recommends the
approach introduced in RFC 2767 over the approach of RFC 3338.
5. Standards-Track document
RFC 2767 is classified as an Informational RFC and RFC 3338 as an
Experimental RFC. It was discussed and decided in the IETF that
this technology should be on the Standards Track.
6. Set of other extensions and improvements
A set of lesser extensions, improvements, and clarifications have
been introduced. These include but are not limited to IPv4 and
IPv6 address exclusion sets at Section 2.3.1, host's DNS cache
considerations, ENR behavior updates, updated security
considerations, example updates, and deployment scenario updates.
8. Acknowledgments
The authors are grateful for discussion from Gang Chen, Dapeng Liu,
Bo Zhou, Hong Liu, Tao Sun, Zhen Cao, and Feng Cao et al. in the
development of this document.
The efforts of Mohamed Boucadair, Dean Cheng, Lorenzo Colitti, Paco
Cortes, Ralph Droms, Stephen Farrell, Fernando Gont, Marnix Goossens,
Wassim Haddad, Ala Hamarsheh, Dave Harrington, Ed Jankiewizh, Suresh
Krishnan, Julien Laganier, Yiu L. Lee, Jan M. Melen, Qibo Niu,
Pierrick Seite, Christian Vogt, Magnus Westerlund, Dan Wing, and
James Woodyatt in reviewing this document are gratefully
acknowledged.
Special acknowledgments go to Dave Thaler for his extensive review
and support.
The authors of RFC 2767 acknowledged WIDE Project, Kazuhiko YAMAMOTO,
Jun MURAI, Munechika SUMIKAWA, Ken WATANABE, and Takahisa MIYAMOTO.
The authors of RFC 3338 acknowledged implementation contributions by
Wanjik Lee (wjlee@arang.miryang.ac.kr) and i2soft Corporation
(www.i2soft.net).
The authors of "Bump-in-the-Wire IPv4/IPv6 Translator" (a draft
document submitted to the v6ops WG in October 2006), P. Moster, L.
Chin, and D. Green, are acknowledged. Some ideas and clarifications
from BIW have been adapted to this document.
Huang, et al. Standards Track PAGE 20
RFC 6535 BIH February 2012
9. References
9.1. Normative References
[RFC 1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC 4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC 6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, April 2011.
[RFC 6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
[RFC 6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
April 2011.
9.2. Informative References
[RFC 2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC 2767] Tsuchiya, K., HIGUCHI, H., and Y. Atarashi, "Dual Stack
Hosts using the "Bump-In-the-Stack" Technique (BIS)",
RFC 2767, February 2000.
[RFC 3338] Lee, S., Shin, M-K., Kim, Y-J., Nordmark, E., and A.
Durand, "Dual Stack Hosts Using "Bump-in-the-API" (BIA)",
RFC 3338, October 2002.
Huang, et al. Standards Track PAGE 21
RFC 6535 BIH February 2012
[RFC 3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, February 2003.
[RFC 3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.
[RFC 5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
[RFC 5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508,
April 2009.
[RFC 5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[RFC 6180] Arkko, J. and F. Baker, "Guidelines for Using IPv6
Transition Mechanisms during IPv6 Deployment", RFC 6180,
May 2011.
Huang, et al. Standards Track PAGE 22
RFC 6535 BIH February 2012
Appendix A. API List Intercepted by BIH
The following informational list includes some of the API functions
that would be appropriate to intercept by BIH module when implemented
at the socket API layer. Please note that this list is not fully
exhaustive, as the function names and services that are available on
different APIs vary significantly.
The functions that the application uses to pass addresses into the
system are as follows:
bind()
connect()
sendmsg()
sendto()
gethostbyaddr()
getnameinfo()
The functions that return an address from the system to an
application are as follows:
accept()
recvfrom()
recvmsg()
getpeername()
getsockname()
gethostbyname()
getaddrinfo()
The functions that are related to socket options are as follows:
getsocketopt()
setsocketopt()
As well, raw sockets for IPv4 and IPv6 may be intercepted.
Huang, et al. Standards Track PAGE 23
RFC 6535 BIH February 2012
Most of the socket functions require a pointer to the socket address
structure as an argument. Each IPv4 argument is mapped into
corresponding an IPv6 argument, and vice versa.
According to [RFC 3493], the following new IPv6 basic APIs and
structures are required.
IPv4 new IPv6
------------------------------------------------
AF_INET AF_INET6
sockaddr_in sockaddr_in6
gethostbyname() getaddrinfo()
gethostbyaddr() getnameinfo()
inet_ntoa()/inet_addr() inet_pton()/inet_ntop()
INADDR_ANY in6addr_any
Figure 8
BIH may intercept inet_ntoa() and inet_addr() and use the address
mapper for those. Doing that enables BIH to support literal IP
addresses. However, IPv4 address literals can only be used after a
mapping entry between the IPv4 address and corresponding IPv6 address
has been created.
The gethostbyname() and getaddrinfo() calls return a list of
addresses. When the name resolver function invokes getaddrinfo(),
and getaddrinfo() returns multiple IP addresses, whether IPv4 or
IPv6, they should all be represented in the addresses returned by
gethostbyname(). Thus, if getaddrinfo() returns multiple IPv6
addresses, this implies that multiple address mappings will be
created: one for each IPv6 address.
Huang, et al. Standards Track PAGE 24
RFC 6535 BIH February 2012
Authors' Addresses
Bill Huang
China Mobile
No.32 Xuanwumen West Street
Xicheng District
Beijing 100053
China
EMail: bill.huang@chinamobile.com
Hui Deng
China Mobile
No.32 Xuanwumen West Street
Xicheng District
Beijing 100053
China
EMail: denghui@chinamobile.com
Teemu Savolainen
Nokia
Hermiankatu 12 D
FI-33720 TAMPERE
Finland
EMail: teemu.savolainen@nokia.com
Huang, et al. Standards Track PAGE 25
RFC TOTAL SIZE: 54452 bytes
PUBLICATION DATE: Wednesday, February 29th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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