IPv6 Operations WG R. Graveman
Internet-Draft RFG Security, LLC
Intended status: Informational M. Parthasarathy
Expires: April 12, 2007 Nokia
P. Savola
CSC/FUNET
H. Tschofenig
Siemens
October 9, 2006
Using IPsec to Secure IPv6-in-IPv4 Tunnels
draft-ietf-v6ops-ipsec-tunnels-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document gives guidance on securing manually configured IPv6-in-
IPv4 tunnels using IPsec. No additional protocol extensions are
described beyond those available with the IPsec framework.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Threats and the Use of IPsec . . . . . . . . . . . . . . . . . 3
2.1. IPsec in Transport Mode . . . . . . . . . . . . . . . . . 4
2.2. IPsec in Tunnel Mode . . . . . . . . . . . . . . . . . . . 5
3. Scenarios and Overview . . . . . . . . . . . . . . . . . . . . 5
3.1. Router-to-Router Tunnels . . . . . . . . . . . . . . . . . 5
3.2. Site-to-Router/Router-to-Site Tunnels . . . . . . . . . . 6
3.3. Host-to-Host Tunnels . . . . . . . . . . . . . . . . . . . 8
4. IKE and IPsec Versions . . . . . . . . . . . . . . . . . . . . 8
5. IPsec Configuration Details . . . . . . . . . . . . . . . . . 9
5.1. IPsec Transport Mode . . . . . . . . . . . . . . . . . . . 10
5.2. IPsec Tunnel Mode . . . . . . . . . . . . . . . . . . . . 10
5.3. Peer Authorization Database . . . . . . . . . . . . . . . 12
6. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 13
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.1. Normative References . . . . . . . . . . . . . . . . . . . 14
11.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Using Tunnel Mode . . . . . . . . . . . . . . . . . . 15
A.1. Tunnel Mode Implementation Methods . . . . . . . . . . . . 15
A.2. Specific SPD for Host-to-Host Scenario . . . . . . . . . . 16
A.3. Specific SPD for Host-to-Router scenario . . . . . . . . . 17
Appendix B. Optional Features . . . . . . . . . . . . . . . . . . 18
B.1. Dynamic Address Configuration . . . . . . . . . . . . . . 18
B.2. NAT Traversal and Mobility . . . . . . . . . . . . . . . . 18
B.3. Tunnel Endpoint Discovery . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 22
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1. Introduction
The IPv6 operations (v6ops) working group has selected (manually
configured) IPv6-in-IPv4 tunneling [RFC4213] as one of the IPv6
transition mechanisms for IPv6 deployment.
[RFC4213] identified a number of threats that had not been adequately
analyzed or addressed in its predecessor [RFC2893]. The most
complete solution is to use IPsec to protect IPv6-in-IPv4 tunneling.
The document was intentionally not expanded to include the details on
how to set up an IPsec-protected tunnel in an interoperable manner,
but instead the details were deferred to this memo.
First four sections of this document analyse the threats and
scenarios that can be addressed by IPsec and assumptions made by this
document for successful IPsec Security Association (SA)
establishment. Section 5 gives the details of Internet Key Exchange
(IKE) and IP security (IPsec) exchange with packet formats and
Security Policy Database (SPD) entries. Section 6 gives
recommendations. Appendices further discuss Tunnel mode usage and
optional extensions.
This document does not address the use of IPsec for tunnels that are
not manually configured (e.g., 6to4 tunnels [RFC3056]). Presumably,
some form of opportunistic encryption or "better-than-nothing
security" might or might not be applicable. Similarly, propagating
quality of service attributes (apart from Explicit Congestion
Notification (ECN) bits [RFC4213]) from the encapsulated packets to
the tunnel path is out of scope.
2. Threats and the Use of IPsec
[RFC4213] is mostly concerned about address spoofing threats:
1. IPv4 address of the encapsulating ("outer") packet can be
spoofed.
2. IPv6 address of the encapsulated ("inner") packet can be spoofed.
IPsec can obviously also provide payload integrity and
confidentiality as well for the part of the end-to-end path that is
tunneled.
The reason for threat (1) is the lack of widespread deployment of
IPv4 ingress filtering [RFC3704]. The reason for threat (2) is that
the IPv6 packet is encapsulated in IPv4 and hence may escape IPv6
ingress filtering. [RFC4213] specifies the following strict address
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checks as mitigating measures:
o To mitigate threat (1), the decapsulator verifies that the IPv4
source address of the packet is the same as the address of the
configured tunnel endpoint. The decapsulator may also implement
IPv4 ingress filtering, i.e., check whether the packet is received
on a legitimate interface.
o To mitigate threat (2), the decapsulator verifies whether the
inner IPv6 address is a valid IPv6 address and also applies IPv6
ingress filtering before accepting the IPv6 packet.
This memo proposes using IPsec for providing stronger security in
preventing these threats and additionally providing integrity and
confidentiality.
IPsec can be used in two ways, in transport and tunnel mode; detailed
discussion about applicability in this context is described in
Section 5.
2.1. IPsec in Transport Mode
In transport mode, the IPsec security association (SA) is established
to protect the traffic defined by (IPv4-source, IPv4-dest, protocol =
41). On receiving such an IPsec packet, the receiver first applies
the IPsec transform (e.g., ESP) and then matches the packet against
the Security Parameter Index (SPI) and the inbound selectors
associated with the SA to verify that the packet is appropriate for
the SA via which it was received. A successful verification implies
that the packet came from the right IPv4 endpoint as the SA is bound
to the IPv4 source address.
This prevents threat (1) but not the threat (2). IPsec in transport
mode does not verify the contents of the payload itself where the
IPv6 addresses are carried, that is, two nodes that are using IPsec
transport mode to secure the tunnel can spoof the inner payload. The
packet will be decapsulated successfully and accepted.
The shortcoming can be mitigated by IPv6 ingress filtering i.e.,
check that the packet is arriving from the interface in the direction
of the route towards the tunnel end-point, similar to a Strict
Reverse Path Forwarding (RPF) check [RFC3704].
In most implementations, a transport mode SA is applied to a normal
IPv6-in-IPv4 tunnel. Therefore, ingress filtering can be applied in
the tunnel interface. (Transport mode is often also used in other
kind of tunnels such as GRE and L2TP.)
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2.2. IPsec in Tunnel Mode
In tunnel mode, the IPsec SA is established to protect the traffic
defined by (IPv6-source, IPv6-destination). On receiving such an
IPsec packet, the receiver first applies the IPsec transform (e.g.,
ESP) and then matches the packet against the SPI and the inbound
selectors associated with the SA to verify that the packet is
appropriate for the SA via which it was received. The successful
verification implies that the packet came from the right endpoint.
The outer IPv4 addresses may be spoofed and IPsec cannot detect it in
this mode; the packets will be demultiplexed based on the SPI and
possibly the IPv6 address bound to the SA. Thus, the outer address
spoofing is irrelevant as long as the decryption succeeds and the
inner IPv6 packet can be verified to come from the right tunnel
endpoint.
As described in Section 5.2, using tunnel mode is more difficult than
applying transport mode to a tunnel interface, and as a result this
document recommends transport mode.
3. Scenarios and Overview
There are roughly three kinds of scenarios:
1. (generic) router-to-router tunnels.
2. site-to-router/router-to-site tunnels. This refers to a tunnel
between a site's IPv6 (border) device to an IPv6 upstream
provider's router. A degenerate case of a site is a single host.
3. Host-to-host tunnels.
3.1. Router-to-Router Tunnels
IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of
IPv4 routing topology by encapsulating them within IPv4 packets.
Tunneling can be used in a variety of ways.
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.--------. _----_ .--------.
|v6-in-v4| _( IPv4 )_ |v6-in-v4|
| Router | <======( Internet )=====> | Router |
| A | (_ _) | B |
'--------' '----' '--------'
^ IPsec tunnel between ^
| Router A and Router B |
V V
Figure 1: Router-to-Router Scenario
IPv6/IPv4 routers interconnected by an IPv4 infrastructure can tunnel
IPv6 packets between themselves. In this case, the tunnel spans one
segment of the end-to-end path that the IPv6 packet takes.
The source and destination addresses of the IPv6 packets traversing
the tunnel could come from a wide range of IPv6 prefixes, so binding
IPv6 addresses to be used to the SA is not feasible. IPv6 ingress
filtering must be performed to mitigate the IPv6 address spoofing
threat.
A specific case of router-to-router tunnels, when one router resides
at an end site, is described in the next section.
3.2. Site-to-Router/Router-to-Site Tunnels
This is a generalization of host-to-router and router-to-host
tunneling, because the issues when connecting a whole site (using a
router), and connecting a single host are roughly equal.
_----_ .---------. IPsec _----_ IPsec .-------.
_( IPv6 )_ |v6-in-v4 | Tunnel _( IPv4 )_ Tunnel | V4/V6 |
( Internet )<--->| Router |<=======( Internet )=======>| Site B |
(_ _) | A | (_ _) '--------'
'----' '---------' '----'
^
|
V
.--------.
| Native |
| IPv6 |
| node |
'--------'
Figure 2: Router-to-Site Scenario
IPv6/IPv4 routers can tunnel IPv6 packets to their final destination
IPv6/IPv4 site. This tunnel spans only the last segment of the end-
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to-end path.
+---------------------+
| IPv6 Network |
| |
.--------. _----_ | .--------. |
| V6/V4 | _( IPv4 )_ | |v6-in-v4| |
| Site B |<====( Internet )==========>| Router | |
'--------' (_ _) | | A | |
'----' | '--------' |
IPsec tunnel between | ^ |
IPv6 Site and Router A | | |
| V |
| .-------. |
| | V6 | |
| | Hosts | |
| '--------' |
+---------------------+
Figure 3: Site-to-Router Scenario
Respectively, IPv6/IPv4 hosts can tunnel IPv6 packets to an
intermediary IPv6/IPv4 router that is reachable via an IPv4
infrastructure. This type of tunnel spans the first segment of the
packet's end-to-end path.
The hosts in the site originate the packets with source addresses
coming from a well known prefix whereas the destination address could
be any node on the Internet.
In this case, the IPsec tunnel mode SA could be bound to the prefix
that was allocated to the router at Site B and router A could verify
that the source address of the packet matches the prefix. Site B
will not be able to do a similar verification for the packets it
receives. This may be quite reasonable for most of the deployment
cases, for example, the Internet Service Provider (ISP) allocating a
/48 to a customer. The Customer Premises Equipment (CPE) where the
tunnel is terminated "trusts" (in a weak sense) the ISP's router and
the ISP's router can verify that the Site B is the only one that can
originate packets within the /48.
IPv6 spoofing must be prevented, and setting up ingress filtering may
require some amount of manual configuration; see more of these
options in Section 5.
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3.3. Host-to-Host Tunnels
.--------. _----_ .--------.
| V6/V4 | _( IPv4 )_ | V6/V4 |
| Host | <======( Internet )=====> | Host |
| A | (_ _) | B |
'--------' '----' '--------'
IPsec tunnel between
Host A and Host B
Figure 4: Host-to-Host Scenario
IPv6/IPv4 hosts that are interconnected by an IPv4 infrastructure can
tunnel IPv6 packets between themselves. In this case, the tunnel
spans the entire end-to-end path that the packet takes.
In this case, the source and the destination IPv6 address are known a
priori. A tunnel mode SA could be bound to the specific address.
The address verification prevents IPv6 address spoofing completely.
As noted in the Introduction, automatic host-to-host tunneling
methods (e.g., 6to4) are out of scope of this memo.
4. IKE and IPsec Versions
This section discusses the different versions of the IKE and IPsec
security architecture and their applicability to this document.
The IPsec security architecture was originally defined in [RFC2401]
and now superseded by [RFC4301]. IKE was originally defined in
[RFC2409] (which is referred to as IKEv1 in this document) and is now
superseded by [RFC4306] (referred to as IKEv2). There are several
differences between them. The differences relevant to this document
are discussed below.
1. [RFC2401] does not allow IP as the next layer protocol in traffic
selectors when an IPsec SA is negotiated. [RFC4301] also allows
IP as the next layer protocol like TCP or UDP in traffic
selectors.
2. [RFC4301] assumes IKEv2, as some of the new features cannot be
negotiated using IKEv1. It is valid to negotiate multiple
traffic selectors for a given IPsec SA in [RFC4301]. This is
possible only with [RFC4306]. If [RFC2409] is used, then
multiple SAs need to be set up for each traffic selector.
Note that the existing implementations based on [RFC2409] may already
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be able to support the [RFC4301] features described in (1) and (2).
If appropriate, the deployment may choose to use the two versions of
the security architecture.
IKEv2 supports features that are useful for configuring and securing
tunnels that are not present with IKEv1.
1. IKEv2 supports legacy authentication methods by carrying them in
EAP payloads. This can be used to authenticate the hosts/sites
to the ISP using EAP methods that support username and password.
2. IKEv2 supports dynamic address configuration which may be used to
configure the IPv6 address of the host.
NAT traversal works with both the old and revised IPsec
architectures, but the negotiation is integrated with IKEv2.
For the purposes of this document, where the confidentiality of ESP
is not required, Authentication Header (AH) [RFC4302] can be used
interchangeably. The main difference is that AH is able to provide
integrity-protection for certain fields in the outer IP header and IP
options. However, as the outer IP header will be discarded in any
case and those particular fields are not believed to be relevant in
this particular application, there is no particular reason to use AH.
5. IPsec Configuration Details
This section describes details about the establishment of an IPsec
tunnel for the protection of IPv4/IPv6 data traffic. However, first
we will take a look at the packet format on the wire.
The packet format is the same for both transport mode and tunnel mode
as shown in Table 1.
+----------------------------+------------------------------------+
| Components (first to last) | Contains |
+----------------------------+------------------------------------+
| IPv4 header | (src = IPV4-TEP1, dst = IPV4-TEP2) |
| ESP header | |
| IPv6 header | (src = IPV6-EP1, dst = IPV6-EP2) |
| (payload) | |
+----------------------------+------------------------------------+
Table 1
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5.1. IPsec Transport Mode
The transport mode has typically been applied to L2TP, GRE, and other
kind of tunneling methods, especially when the user wants to tunnel
non-IP traffic. [RFC3884], [RFC3193], and [RFC4023] provide examples
of applying transport mode to protect tunnel traffic that spans only
a part of an end-to-end path.
IPv6 ingress filtering must be applied on the tunnel interface on all
the packets that pass the inbound IPsec processing.
The following SPD entries assume that there are two routers Router1
and Router2, with tunnel endpoint IPv4 addresses denoted by IPV4-TEP1
and IPV4-TEP2 respectively. (In other scenarios, the SPDs are set up
in a similar fashion.)
Router1's SPD:
Next Layer
Rule Local Remote Protocol Action
---- ----- ------ ---------- --------
1 IPV4-TEP1 IPV4-TEP2 ESP BYPASS
2 IPV4-TEP1 IPV4-TEP2 IKE BYPASS
3 IPv4-TEP1 IPV4-TEP2 41 PROTECT(ESP,transport)
Router2's SPD:
Next Layer
Rule Local Remote Protocol Action
---- ----- ------ ---------- --------
1 IPV4-TEP2 IPV4-TEP1 ESP BYPASS
2 IPV4-TEP2 IPV4-TEP1 IKE BYPASS
3 IPv4-TEP2 IPV4-TEP1 41 PROTECT(ESP,transport)
In both SPD entries, "IKE" refers to UDP destination port 500
and possibly also port 4500 if NAT traversal is used.
The IDci and IDcr payloads of IKEv1 carry the IPv4-TEP1, IPV4-TEP2
and protocol value 41 as phase 2 identities. With IKEv2, the traffic
selectors are used to carry the same information.
5.2. IPsec Tunnel Mode
A tunnel mode SA is essentially an SA applied to an IP tunnel, with
the access controls applied to the headers of the traffic inside the
tunnel [RFC4301].
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Several requirements arise when IPsec is used to protect the IPv6
traffic (inner header) for the scenarios mentioned in Section 3.
1. All of IPv6 traffic should be protected including link-local
(e.g., Neighbor Discovery) and multicast traffic.
2. In Router-to-Router tunnels, the source and destination addresses
of the traffic could come from a wide range of prefixes that are
normally learnt through routing. As routing can always learn a
new prefix, there is no way to know all the prefixes a priori
[RFC3884].
3. Source address selection depends on the notion of routes and
interfaces. This affects scenarios (2) and (3).
The implementations may or may not model the IPsec tunnel mode SA as
an interface as described in Appendix A.1.
If IPsec tunnel mode SA is not modeled as an interface (e.g., as of
this writing, popular in many open source implementations), the SPD
entries for protecting all the traffic between the two endpoints must
be described. Evaluating against the requirements above, link-local
traffic cannot be sent because there is no interface and multicast
traffic would need to be identified, possibly resulting in a long
list of SPD entries. The second requirement is difficult to satisfy
because the traffic that needs to be protected is not necessarily
(e.g., router-to-router tunnel) known a priori [RFC3884]. The third
requirement is equally hard because almost all implementations assume
addresses are assigned on interfaces (rather than configured in SPDs)
for proper source address selection.
If IPsec tunnel mode SA is modeled as interface, the traffic that
needs protection can be modeled as routes pointing to the interface.
The second requirement is difficult to satisfy because the traffic
that needs to be protected is not necessarily known a priori. The
third requirement is easily solved because IPsec is modeled as an
interface.
In practice (2) has been solved by protecting all the traffic (::/0),
but there are no inter-operable implementations supporting this
feature. For a detailed list of issues pertaining to this, see
[I-D.duffy-ppvpn-ipsec-vlink].
Because applying transport mode to protect a tunnel is a much more
simpler solution and also easily protects link-local and multicast
traffic, we do not recommend using tunnel mode in this context.
Tunnel mode is still discussed in Appendix A.
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5.3. Peer Authorization Database
The Peer Authorization Database (PAD) provides the link between SPD
and the key management daemon [RFC4306]. This is defined in
[RFC4301] and hence relevant only when used with IKEv2.
As there is no way to currently discover the PAD related parameters
dynamically, it is assumed that these are manually configured:
o The Identity of the peer which will be asserted in the IKEv2
exchange. There are many different types of identities that can
be used. At least, IPv4 address of the peer should be supported.
o The IKEv2 can authenticate the peer using several methods. Pre-
shared key and X.509 certificate based authentication is required
by [RFC4301]. At least, pre-shared key should be supported as it
interoperates with a larger number of implementations.
o The child SA authorization data should contain the IPv4 address of
the peer.
6. Recommendations
In Section 5 we examined the differences of setting up an IPsec IPv6-
in-IPv4 using either transport or tunnel mode. We observe that
applying transport mode to a tunnel interface is the simplest and
therefore recommended solution.
In Appendix A, we also explore what it would take to use so-called
Specific SPD (SSPD) tunnel mode. Such usage is more complicated
because IPv6 prefixes need to be known a priori and multicast and
link-local traffic do not work over such a tunnel. Fragment handling
in tunnel mode is also more difficult. However, because MOBIKE
[RFC4555] supports only tunnel mode, when the IPv4 endpoints of a
tunnel are dynamic and the other constraints are not applicable,
using tunnel mode may be an acceptable solution.
Therefore our primary recommendation is to use transport mode applied
to a tunnel interface. Spoofing can be prevented by enabling ingress
filtering on the tunnel interface.
7. IANA Considerations
This memo makes no request to IANA. [[ RFC-editor: please remove this
section prior to publication. ]]
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8. Security Considerations
When you run IPv6-in-IPv4 tunnels (unsecured) over the Internet, it
is possible to "inject" packets into the tunnel by spoofing the
source address (data plane security), or if the tunnel is signalled
somehow (e.g., some messages where you authenticate to the server, so
that you would get a static v6 prefix), someone might be able to
spoof the signalling (control plane security).
The IPsec framework plays an important role in adding security to
both the protocol for tunnel setup and data traffic.
Either IKEv1 or IKEv2 provides a secure signaling protocol for
establishing, maintaining and deleting an IPsec tunnel.
IPsec, with the Encapsulating Security Payload (ESP), offers
integrity and data origin authentication, confidentiality, with
optional (at the discretion of the receiver) anti-replay features.
The usage of confidentity-only is discouraged. ESP furthermore
provides limited traffic flow confidentiality.
IPsec provides access control mechanisms through the distribution of
keys and also through the usage of policies dictated by the Security
Policy Database (SPD).
The NAT traversal mechanism provided by IKEv2 introduces some
weaknesses into IKE and IPsec. These issues are discussed in more
detail in [RFC4306].
Please note that the usage of IPsec for the scenarios described in
Figure 3, Figure 2 and Figure 1 does not aim to protect the end-to-
end communication. It protects just the tunnel part. It is still
possible for an IPv6 endpoint that is not attached to the IPsec
tunnel to spoof packets.
9. Contributors
The authors are listed in alphabetical order.
Suresh Satapati also participated in the initial discussions on the
topic.
10. Acknowledgments
The authors would like to thank Stephen Kent, Michael Richardson,
Florian Weimer, Elwyn Davies, Eric Vyncke, Merike Kaeo, and Alfred
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Hoenes for their substantive feedback.
We would like to thank Pasi Eronen for his text contributions and
suggestions for improvement.
11. References
11.1. Normative References
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
11.2. Informative References
[I-D.duffy-ppvpn-ipsec-vlink]
Duffy, M., "Framework for IPsec Protected Virtual Links
for PPVPNs", draft-duffy-ppvpn-ipsec-vlink-00 (work in
progress), October 2002.
[I-D.palet-v6ops-tun-auto-disc]
Palet, J. and M. Diaz, "Analysis of IPv6 Tunnel End-point
Discovery Mechanisms", draft-palet-v6ops-tun-auto-disc-03
(work in progress), January 2005.
[RFC2893] Gilligan, R. and E. Nordmark, "Transition Mechanisms for
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IPv6 Hosts and Routers", RFC 2893, August 2000.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3193] Patel, B., Aboba, B., Dixon, W., Zorn, G., and S. Booth,
"Securing L2TP using IPsec", RFC 3193, November 2001.
[RFC3715] Aboba, B. and W. Dixon, "IPsec-Network Address Translation
(NAT) Compatibility Requirements", RFC 3715, March 2004.
[RFC3884] Touch, J., Eggert, L., and Y. Wang, "Use of IPsec
Transport Mode for Dynamic Routing", RFC 3884,
September 2004.
[RFC4023] Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating
MPLS in IP or Generic Routing Encapsulation (GRE)",
RFC 4023, March 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, June 2006.
Appendix A. Using Tunnel Mode
First, we describe the different tunnel mode implementation methods.
We note that in this context, only so-called Specific SPD (SSPD)
model (without a tunnel interface) can be made to work, but has
reduced applicability and the use of a transport mode tunnel is
recommended instead. However, we will describe how the SSPD Tunnel
Mode might look like if one would like to use it in any case.
A.1. Tunnel Mode Implementation Methods
Tunnel mode could (in theory) be deployed in two very different ways
depending on the implementation:
1. "Generic SPDs": some implementations model the tunnel mode SA as
an IP interface. In this case, an IPsec tunnel interface is
created and used with "any" address ("::/0 <-> ::/0" ) as IPsec
traffic selectors while setting up the SA. Though this allows
all traffic between the two nodes to be protected by IPsec, the
routing table would decide what traffic gets sent over the
tunnel. Ingress filtering must be separately applied on the
tunnel interface as the IPsec policy checks do not check the IPv6
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addresses at all. Routing protocols, multicast, etc. will work
through this tunnel. This mode is very similar to the transport
mode. The SPDs must be interface-specific. However, because IKE
uses IPv4 but the tunnel is IPv6, there is no standard solution
to map the IPv4 interface to IPv6 interface
[I-D.duffy-ppvpn-ipsec-vlink] and this approach is not feasible.
2. "Specific SPDs": some implementations don't model the tunnel mode
SA as an IP interface. Traffic selection is done based on
specific SPD entries, e.g., "2001:db8:1::/48 <-> 2001:db8:
2::/48". As the IPsec session between two endpoints does not
have an interface (though an implementation may have a common
pseudo-interface for all IPsec traffic), there is no DAD, MLD, or
link-local traffic to protect; multicast is not possible over
such a tunnel. Ingress filtering is performed automatically by
the IPsec traffic selectors.
Ingress filtering is guaranteed by IPsec processing when option (2)
is chosen whereas the operator has to enable them explicitly when
transport mode or option (1) of tunnel mode SA is chosen.
In summary, there does not appear to be a standard solution in this
context for the first implementation approach.
The second approach can be made to work, but is only applicable in
host-to-host or site-to-router/router-to-site scenarios (i.e., when
the IPv6 prefixes can be known a priori), and offers only a limited
set of features (e.g., no multicast) compared to a transport mode
tunnel.
When tunnel mode is used, fragment handling [RFC4301] may also be
more difficult compared to transport mode and, depending on
implementation, may need to be reflected in SPDs.
A.2. Specific SPD for Host-to-Host Scenario
The following SPD entries assume that there are two hosts Host1 and
Host2, whose IPv6 addresses are denoted by IPV6-EP1 and IPV6-EP2
(global addresses) and IPV4 addresses of the tunnel endpoints are
denoted by IPV4-TEP1 and IPV4-TEP2 respectively.
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Host1's SPD:
Next Layer
Rule Local Remote Protocol Action
---- ----- ------ ---------- --------
1 IPV6-EP1 IPV6-EP2 ESP BYPASS
2 IPV6-EP1 IPV6-EP2 IKE BYPASS
3 IPv6-EP1 IPV6-EP2 41 PROTECT(ESP,
tunnel{IPV4-TEP1,IPV4-TEP2})
Host2's SPD:
Next Layer
Rule Local Remote Protocol Action
---- ----- ------ ---------- --------
1 IPV6-EP2 IPV6-EP1 ESP BYPASS
2 IPV6-EP2 IPV6-EP1 IKE BYPASS
3 IPv6-EP2 IPV6-EP1 41 PROTECT(ESP,
tunnel{IPV4-TEP2,IPV4-TEP1})
"IKE" refers to UDP destination port 500 and possibly also
port 4500 if NAT traversal is used.
The IDci and IDcr payloads of IKEv1 carry the IPV6-EP1 and IPV6-TEP2
as phase 2 identities. With IKEv2, the traffic selectors are used to
carry the same information.
A.3. Specific SPD for Host-to-Router scenario
The following SPD entries assume that the host has the IPv6 address
IPV6-EP1 and the tunnel end points of the host and router are IPV4-
TEP1 and IPV4-TEP2 respectively. If the tunnel is between a router
and a host where the router has allocated a IPV6-PREF/48 to the host,
the corresponding SPD entries can be derived by substituting IPV6-EP1
by IPV6-PREF/48.
Please note the bypass entry for host's SPD, absent in router's SPD.
While this might be an implementation matter for host-to-router
tunneling, having a similar entry, "Local=IPV6-PREF/48 & Remote=IPV6-
PREF/48" is critical for site-to-router tunneling.
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Host's SPD:
Next Layer
Rule Local Remote Protocol Action
---- ----- ------ ---------- --------
1 IPV6-EP1 IPV6-EP2 ESP BYPASS
2 IPV6-EP1 IPV6-EP2 IKE BYPASS
3 IPV6-EP1 IPV6-EP1 ANY BYPASS
4 IPV6-EP1 ANY ANY PROTECT(ESP,
tunnel{IPV4-TEP1,IPV4-TEP2})
Router's SPD:
Next Layer
Rule Local Remote Protocol Action
---- ----- ------ ---------- --------
1 IPV6-EP2 IPV6-EP1 ESP BYPASS
2 IPV6-EP2 IPV6-EP1 IKE BYPASS
3 ANY IPV6-EP1 ANY PROTECT(ESP,
tunnel{IPV4-TEP1,IPV4-TEP2})
The IDci and IDcr payloads of IKEv1 carry the IPV6-EP1 and
ID_IPV6_ADDR_RANGE or ID_IPV6_ADDR_SUBNET as its phase 2 identity.
The starting address is zero IP address and the end address is all
ones for ID_IPV6_ADDR_RANGE. The starting address is zero IP address
and the end address is all zeroes for ID_IPV6_ADDR_SUBNET. With
IKEv2, the traffic selectors are used to carry the same information.
Appendix B. Optional Features
B.1. Dynamic Address Configuration
With the exchange of protected configuration payloads, IKEv2 is able
to provide the IKEv2 peer with DHCP-like information payloads. These
configuration payloads are exchanged between the IKEv2 initiator and
the responder.
This could be used (for example) by the host in the host-to-router
scenario to obtain the IPv6 address from the ISP as part of setting
up the IPsec tunnel mode SA. The details of these procedures are out
of scope of this memo.
B.2. NAT Traversal and Mobility
Network address (and port) translation devices are commonly found in
today's networks. A detailed description of the problem of IPsec
protected data traffic traversing a NAT including requirements are
discussed in [RFC3715].
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IKEv2 can detect the presence of a NAT automatically by sending
NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP payloads in
the initial IKE_SA_INIT exchange. Once a NAT is detected and both
end points support IPsec NAT traversal extensions UDP encapsulation
can be enabled.
More details about UDP encapsulation of IPsec protected IP packets
can be found in [RFC3948].
For IPv6-in-IPv4 tunneling, NAT traversal is interesting for two
reasons:
1. One of the tunnel endpoints is often behind a NAT, and configured
tunneling, using protocol 41, is not guaranteed to traverse the
NAT. Hence, using IPsec tunnels would enable one to both set-up
a secure tunnel, and set-up a tunnel where it might not always be
possible without other tunneling mechanisms.
2. Using NAT traversal allows the outer address to change without
having to renegotiate the SAs. This could be very beneficial for
a crude form of mobility, and in scenarios where the NAT changes
the IP addresses frequently. However, as the outer address may
change, this might introduce new security issues, and using
tunnel mode would be most appropriate.
When NAT is not applied, the second benefit would still be desirable.
In particular, using manually configured tunneling is an operational
challenge with dynamic IP addresses as both ends need to be
reconfigured if an address changes. Therefore an easy and efficient
way to re-establish the IPsec tunnel if the IP address changes would
be desirable. MOBIKE [RFC4555] provides a solution when IKEv2 is
used but only supports tunnel mode.
B.3. Tunnel Endpoint Discovery
The IKEv2 initiator needs to know the address of the IKEv2 responder
to start IKEv2 signaling. A number of ways can be used to provide
the initiator with this information, for example:
o Using out-of-band mechanisms, e.g., from the ISP's web page.
o Using DNS to look up a service name by appending it to the DNS
search path provided by DHCPv4 (e.g. "tunnel-
service.example.com").
o Using a DHCP option.
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o Using a pre-configured or pre-determined IPv4 anycast address.
o Using other, unspecified or proprietary methods.
For the purpose of this document it is assumed that this address can
be obtained somehow. Once the address has been learned, it is
configured as the tunnel end-point for the configured IPv6-in-IPv4
tunnel.
This problem is also discussed at more length in
[I-D.palet-v6ops-tun-auto-disc].
However, simply discovering the tunnel endpoint is not sufficient for
establishing an IKE session with the peer. The PAD information (see
Section 5.3) also needs to be learnt dynamically. Hence, currently
automatic endpoint discovery provides benefit only if PAD information
is chosen in such a manner that it is not IP-address specific.
Authors' Addresses
Richard Graveman
RFG Security, LLC
15 Park Avenue
Morristown, New Jersey 07960
USA
Email: rfg@acm.org
Mohan Parthasarathy
Nokia
313 Fairchild Drive
Mountain View CA-94043
USA
Email: mohanp@sbcglobal.net
Pekka Savola
CSC/FUNET
Espoo
Finland
Email: psavola@funet.fi
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Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
Email: Hannes.Tschofenig@siemens.com
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Full Copyright Statement
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