Internet Engineering Task Force Francis Dupont
INTERNET DRAFT ENST Bretagne
Expires in June 2003 January 2003
Address Management for IKE version 2
<draft-dupont-ikev2-addrmgmt-00.txt>
Status of this Memo
This document is an Internet Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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Distribution of this memo is unlimited.
Abstract
The current IKEv2 proposal [1] lacks an address management
feature. As it includes a NAT traversal capability, this document
extends it to a complete address management with support for NAT
traversal, multi-homing and mobility.
1. Introduction
In this document, the addresses used to transport IKE messages are
named the "peer addresses" (term introduced by [2]). These peer
addresses should no more be directly or indirectly included in
identities ([3] and [4]) as it is commonly done for IKEv1.
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NAT traversal, multi-homing and mobility should take benefit from
this flexibility but the current IKEv2 draft [1] takes only account
of NAT traversal in a flawed way [5]. This document describes the
goals of an address management for IKEv2, including the
requirements for NAT traversal, multi-homing and mobility support,
and finishes by a concrete proposal.
2. Goals
The goals of the address management proposed in the document can be
divided in some general goals and in requirements for the three
mechanisms which can change the peer addresses.
2.1 General goals
2.1.1 Simplicity, Performance and Security
The address management should be as simple as possible, i.e., it
should introduce minimal changes to the current IKEv2 draft [1]
and each changes should be justified.
The performance is an important criterion. For instance, rekeying
can update the peer addresses of an IKE SA or of an IPsec SA pair,
but rekeying is too expensive and a specific solution is needed.
As a security protocol, IKEv2 should get a high security
level. Unfortunately we already showed that the NAT traversal
feature comes with a security issue (the transient pseudo-NAT
attack [5]). Such problems introduced by the peer address
flexibility must be described in this document and at least be
mitigated by options in configurations. For instance, the NAT
traversal feature should never be enabled when one knows that
there can not be a NAT as in today IPv6.
2.1.4 Extensions of the IKEv2 draft
The first things to fix in the current IKEv2 draft [1] are the
notifications for NAT traversal (NAT-DETECTION-*-IP):
- They use a hash of the SPIs, address and port, following
a specification for IKEv1. This makes no sense for IKEv2.
- There is no specified way to request the inclusion of
these notifications in messages.
- There can be multiple source notifications. IMHO this is a good
idea but the rationale (the sender does not know what address
it uses) is weak. The API to get the address used for UDP
packets to a destination is very standard.
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The second missing thing in the current IKEv2 draft [1] is about
some misusage of the peer addresses:
- At the reception of a message, the lookup of the corresponding
IKE SA MUST be done using only the SPIs, never using the peer
addresses.
- An INITIAL-CONTACT notification deletes old IKE SAs associated
to the peer Identity, not to the peer address. Current wording
is misleading.
- The revised identity proposal [3] should be integrated in the
IKEv2 specifications. According to IAB recommendations [4],
addresses should not be used as or associated to identities.
Note that the last point stresses the issue of the lack of
protection of peer addresses.
The last thing to fix in the current IKEv2 draft [1] is the
support of the proxy case: the setup of transport mode IPsec SAs
on the behalf of another party.
2.2 NAT traversal requirements
The NAT traversal feature is the support of en-route peer address
modifications:
- NATs must be detected, including their position, i.e., the
receiver of an IKE message should be able to detect any peer
address alteration.
- The peer addresses are included in the pseudo IP header of
transport checksums when a transport mode IPsec SA is used. The
peers must know the original peer address of their correspondent.
- When there are several VPN clients behind a NAT, the ability to
request a three way handshake (a.k.a. a return routability
check) is needed [6].
2.3 Multi-homing requirements
In this document, the support of multi-homing is the support of
nodes with several global addresses. Some of the addresses can be
"better" than others, or "better" for some destinations. Some can,
from time to time, be unavailable.
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The main requirement for the support of multi-homing is the
management of a set of peer addresses for each peer. The set can
be partially ordered or some subset can be loosely associated with
some destinations (i.e., some subset of the other peer address
set).
For the communication between multi-addressed hosts, the support
of the proxy case can be useful because it provides an easy way to
setup transport mode IPsec SAs with different addresses from one
IKE SA. In such cases the other party is in fact the same host,
this dramatically simplifies the authorization issue.
2.4 Mobility requirements
In the context of Mobile IPv6 ([7] and for the special case of
Home Agents [8]), the interaction of Mobility and IPsec was
analyzed in another document [9]. This document assumes an IPv6
context as Mobile IPv6 is the most powerful mobility proposal
available today.
An IPv6 mobile node is another type of multi-addressed node with:
- a care-of address in a prefix of the visited link.
The care-of address is used to route packets.
- the home address in a prefix of the home link.
The home address is used to identify the mobile node.
The care-of address is transient and usually the mobile node can
not provide a proof that it is the node using it. So it must be
trusted and a return routability check (i.e., an enforced answer
from this address) should be used if it is not.
With a common correspondent, the mobility is transparent and
there is no reason to use another address than the home address.
With the home agent, there are three main cases (c.f. [8]):
- The mobility signaling which is mandatory protected and
raises a specific issue in its initial phase: the IKE SA
must be setup using the care-of address as the peer address
but this IKE SA is used to build an IPsec SA pair with
the home address as traffic selector. This IPsec SA will
protect the home registration which will make the home
address available. This can be considered as a special
proxy case.
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- Other genuine communications between the home agent and
the mobile node can be covered by the proxy case support
too. Note this is the only case at the exception of signaling
where mobility behaves in a different way than a mobile IPsec
VPN (so we proposed to relax the corresponding rule in a
future version of [7] and [8]).
- The traffic relayed by the home agent through a tunnel with the
mobile node can be partially or fully protected by IPsec SA
pair(s). Encapsulation should be performed only once, including
for degenerated (but not for free) encapsulation like the home
address option or the mobility routing header.
In all cases of interaction with the home agent, the mobile node
peer address should be a care-of address. When the mobile node
moves, another care-of address is used and some SAs, including the
IKE SA, must be updated to use the new address.
Usually the previous peer address is no more usable. In order to
avoid a trivial denial of services, a strong sequencing of updates
is required with a way to cancel possible pending updates when
fast multiple handoff happen.
The IPsec pair which protects the mobility signaling uses the home
address as its traffic selector for the mobile node. It must not
be updated at each handoff. The update mechanism must provide a
fine grain (i.e., per SA) update.
3. Proposal
The proposal for an address management in IKEv2 is mainly an
extension of the NAT traversal notifications with a new peer
address update notification. But there are some points that have
to be kept as they are in the current IKEv2 draft [1].
3.1 Kept points from draft 04
A peer address change has to be supported but not at any time:
the peer addresses MUST NOT change during an exchange, i.e.,
they are allowed to change only between two exchanges.
This address stability requirement applies in fact only to the
Initial exchange as it is the only exchange with more than two
messages specified today.
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The peer addresses are used to transport messages. The reply to a
request MUST be sent to the source of the request from the
destination request, i.e., addresses and ports are reversed
between the request and its reply.
For tunnel mode IPsec SAs, the endpoint addresses are the peer
addresses. We don't propose an alternate way to specify them. The
same requirement applies to transport mode IPsec SAs at the
exception of the proxy case.
3.2 Small points
In the proxy case, the initiator is acting as a client negotiator
on the behalf of another party. The address of this other party is
sent in the initiator traffic selector and will become the address
of this end of the transport mode IPsec SA pair. A proper
authorization in the local policy of the responder is
REQUIRED. Note that the IPsec SAs built in such cases are not
managed in the proposal of these document, and that the proxy case
is limited to the transport mode.
The INITIAL-CONTACT notification uses only identities. All the
references to peer addresses must be removed from or fixed in the
current wording.
3.3 Peer address notifications
The peer address notifications replace the current
NAT-DETECTION-SOURCE-IP and NAT-DETECTION-DESTINATION-IP
notifications. They includes the peer source or destination
address and the source or destination port. They SHOULD be used
only in protected messages (i.e., not in the first two messages)
and SHOULD be ignored when they are not protected, i.e., outside
an encrypted payload.
For NAT traversal, they are used to detect NATs and their
position, and they provide the original peer addresses for
transport mode. For multi-homing and mobility, they provide a
cryptographically proof of no alteration en-route of the peer
addresses and, when multiple peer address notifications for the
same peer are included, they encode its whole peer address set.
Note: IMHO a whole/partial set flag is needed.
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A new notification has to be defined for the peer address
notification request by the responder. Typically a responder
asking for either NAT traversal or a peer address protection (i.e.,
the opposite) will put this notification in the second message of
the initial exchange. All following messages MUST include the
requested peer address notifications.
Note: is a way to request source or destination protection needed?
For the initiator a simple way to request peer address
notifications is to include then in requests: when peer address
notifications are required, peer address notifications MUST be
included in the encrypted payload of requests. When a peer address
notification is included in a request, peer address notifications
are required, all following messages MUST include the peer address
protection notification(s), beginning by the reply message.
If multiple peer address notifications for the same peer are
included in a message, the first one SHOULD for the source and
MUST for the destination be the used peer address. The extra
addresses describe the other possible peer addresses, i.e., there
is at least one peer address notification per address in the peer
address set.
In order to associate some possible peer source addresses to
possible peer destination addresses, the source and destination
peer addresses notifications MAY be mixed (i.e., not in the common
order source(s) first, destination(s) after). For instance S1, S2,
D1, S3, D2, D3 is a hint: S1 or S2 should be used in conjunction
with D1, S3 with D2 or D3.
When a peer can not find its peer addresses in the peer address
notifications for its side, if NAT traversal is disabled then it
MUST reject the compromised message and send an error notification
to its peer, if NAT traversal is enabled it SHOULD take proper
actions when a NAT is detected, for instance switch to the port
4500. This applies only to protected peer address notifications.
3.4 Implicit address update
For address update, there is a choice between implicit and
explicit mechanisms for IPsec SAs and IKE SAs.
We claim that the implicit mechanism for IPsec SAs is far too
unsecure: this mechanism is very (too?) simple. When a packet is
received from another address, the peer addresses of the IPsec SA
pair are updated. This opens the door to easy denial of service
attacks and assumes extra-processing by the receiver device.
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For the implicit mechanism for IKE SAs the things are even
simpler. The implicit mechanism is mainly activated by keepalive
exchanges: to switch from the implicit mechanism to the explicit
one, only an update notification has to be included. The real
difference is in the explicit case an observed peer address change
is only a trigger.
Note: should we specify a mechanism to advertise the other peer?
It seems that NAT traversal needs it or should use an update all
SAs in all IKE SA keepalive messages.
3.5 Explicit address update
The explicit mechanism MUST be used. A new notification has to
be defined. We propose to copy it from the delete notification.
The new peer address notification has strong sequencing
requirements. It MUST be processed in order and when a pending
exchange with a peer address update has to be overrided by a more
recent one, the peer address update notification MUST be
canceled.
Note: IKEv2 messages have a sequence number so the only sequencing
issues are the window of processing and pending exchanges. But
the sequence number (a.k.a. the message ID) must be protected,
for instance by repetion inside the notification(s).
Note: there are two obvious ways to cancel it (remove it from
the message or define a cancellation flag) but both modify the
message.
When the receiver of an update request has to check the validity
of the new peer address, it MAY use a return routability check
sending an informational request at the new address and waiting
for an answer. As informational exchanges are protected no more is
needed.
Example of a return routability check:
I --- address update request --> R
I <-- informational RR request - R
I --- informational RR reply --> R
now the responder knows the initiator should be where it said.
I <--- address update reply ---- R
As for the delete notification, the peer address update
notification specifies the SPIs of the IPsec and IKE SAs it
applies to. But a simple way to specify all SAs (i.e., the IKE SA
and all the IPsec SAs it negotiated) is needed.
Note: the "all SAs" MUST NOT be used when there is a proxy case
SA pair.
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4. Security Considerations
Great care was used to avoid to introduce security threats.
The NAT traversal feature comes with a security flaw (the transient
pseudo-NAT attack [5]) which can not be easily avoid. IMHO the NAT
traversal feature should be enabled only when the presence of NATs
is likely/possible.
When the NAT traversal feature is disabled, the other peer address
can not be changed en-route by an attacker but the proofs the peer
is really at its address are:
- the trust in the peer
- the proof that the peer can receive messages sent to its address
The second (a.k.a. the return routability check) works only with at
least three messages, i.e., for the initial exchange (with the
address stability requirement) and for the explicit optional
checks. IMHO these checks SHOULD be required by default.
5. Acknowledgments
The need for an address management for IKEv2 was explained at the
ipsec session of the Yokohama IETF meeting. It seems most people
agree but do not propose concrete solutions.
The rare people in the Mobility world with IPsec interests, or in
the IPsec world with Mobility interest, should receive all thanks
because without them we (me and all the future co-authors) have
given up for a long time.
6. Normative References
None?
7. Informative References
[1] C. Kaufman, ed., "Proposal for the IKEv2 Protocol",
draft-ietf-ipsec-ikev2-04.txt, January 2003.
[2] B. Korver, E. Rescorla, "The Internet IP Security PKI
Profile of ISAKMP and PKIX",
draft-ietf-ipsec-pki-profile-01.txt, October 2002.
[3] P. Hoffman, "Adding revised identities to IKEv2",
http://www.vpnc.org/ietf-ipsec/,
Message-Id: <p05200f06b9edf48ac57b@[165.227.249.18]>,
November 2002.
[4] M. Kaat, "Overview of 1999 IAB Network Layer Workshop",
RFC 2956, October 2000.
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[5] F. Dupont, J.-J. Bernard, "Transient pseudo-NAT attacks
or how NATs are even more evil than you believed",
draft-dupont-transient-pseudonat-01.txt, December 2002.
[6] Jayant Shukla, "RE: peer address protection and NAT Traversal",
http://www.vpnc.org/ietf-ipsec/,
Message-ID: <000201c2bb27$e7021ff0$5803a8c0@trlhpc1>,
January 2003.
[7] D. Johnson, C. Perkins, J. Arkko, "Mobility Support in IPv6",
draft-ietf-mobileip-ipv6-20.txt, January 2003.
[8] J. Arkko, V. Devarapalli, F. Dupont, "Using IPsec to Protect
Mobile IPv6 Signaling between Mobile Nodes and Home Agents",
draft-ietf-mobileip-mipv6-ha-ipsec-02.txt, January 2003.
[9] F. Dupont, "How to make IPsec more mobile IPv6 friendly",
draft-dupont-ipsec-mipv6-02.txt, January 2003.
9. Author's Address
Francis Dupont
ENST Bretagne
Campus de Rennes
2, rue de la Chataigneraie
BP 78
35512 Cesson-Sevigne Cedex
FRANCE
Fax: +33 2 99 12 70 30
EMail: Francis.Dupont@enst-bretagne.fr
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