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Internet Engineering Task Force (IETF) Y. Nir
Request for Comments: 6867 Check Point
Category: Experimental Q. Wu
ISSN: 2070-1721 Huawei
January 2013
An Internet Key Exchange Protocol Version 2 (IKEv2)
Extension to Support EAP Re-authentication Protocol (ERP)
Abstract
This document updates the Internet Key Exchange Protocol version 2
(IKEv2) described in RFC 5996. This extension allows an IKE Security
Association (SA) to be created and authenticated using the Extensible
Authentication Protocol (EAP) Re-authentication Protocol extension,
as described in RFC 6696.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. 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). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see 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/rfc6867.
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Copyright Notice
Copyright (c) 2013 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.
1. Introduction
IKEv2, as specified in [RFC5996], allows (Section 2.16)
authentication of the initiator using an EAP method. Using EAP
significantly increases the count of round trips required to
establish the IPsec SA and also may require user interaction. This
makes it inconvenient to allow a single remote access client to
create multiple IPsec tunnels with multiple IPsec gateways that
belong to the same domain.
The EAP Re-authentication Protocol (ERP), as described in [RFC6696],
allows an EAP peer to authenticate to multiple authenticators while
performing the full EAP method only once. Subsequent authentications
require fewer round trips and no user interaction.
Bringing these two technologies together allows a remote access IPsec
client to create multiple tunnels with different gateways that belong
to a single domain as well as using the keys from other contexts of
using EAP, such as network access within the same domain, to
transparently connect to VPN gateways within this domain.
Additionally, it allows for faster set up of new tunnels when
previous tunnels have been torn down due to things like network
outage, device suspension, or a temporary move out of range. This is
similar to the session resumption mechanism described in [RFC5723].
One exception being that instead of a ticket stored by the client,
the re-authentication Master Session Key (rMSK) (see Section 4.6 of
[RFC6696]) is used as the session key stored on both the client and
the Authentication, Authorization, and Accounting (AAA) server.
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1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Usage Scenarios
This work is motivated by the following scenarios:
o Multiple tunnels for a single remote access VPN client. Suppose a
company has offices in New York City, Paris, and Shanghai. For
historical reasons, the email server is located in the Paris
office, most of the servers hosting the company's intranet are
located in Shanghai, and the finance department servers are in New
York City. An employee using a remote access VPN may need to
connect to servers from all three locations. While it is possible
to connect to a single gateway, and have that gateway route the
requests to the other gateways (perhaps through site to site VPN),
this is not efficient; it is more desirable to have the client
initiate three different tunnels. It is, however, not desirable
to have the user type in a password three times.
o Roaming. In these days of mobile phones and tablets, users often
move from the wireless LAN in their office, where access may be
granted through 802.1x, to a cellular network, where a VPN is
necessary, and back again. Both the VPN server and the 802.1x
access point are authenticators that connect to the same AAA
servers. So it makes sense to make the transition smooth, without
requiring user interaction. The device still needs to detect
whether it is within the protected network, in which case it
should not use VPN. However, this process is beyond the scope of
this document. [SECBEAC] is a now-abandoned attempt at this.
o Resumption. If a device gets disconnected from an IKE peer, ERP
can be used to reconnect to the same gateway without user
intervention.
3. Protocol Outline
Supporting EAP Re-authentication Extension (ERX) requires an EAP
payload in the first IKE_AUTH request. This is a deviation from the
rules in [RFC5996], so support needs to be indicated through a Notify
payload in the IKE_SA_INIT response. This Notify serves the same
purpose as the EAP-Initiate/Re-auth-Start message of ERX, as
specified in Section 5.3.1 of [RFC6696]. The "Domain Name" field
contains the content of the Domain-Name TLV as specified in Section
5.3.1.1 of the same document.
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A supporting initiator that has unexpired keys for this domain will
send the EAP-Initiate/Re-auth message in an EAP payload in the first
IKE_AUTH request.
The responder sends the EAP payload content to a backend AAA server.
If that server has a valid rMSK for that session, it sends those
along with an EAP-Finish/Re-auth message. The responder then
forwards the EAP-Finish/Re-auth message to the initiator in an EAP
payload within the first IKE_AUTH response.
The initiator then sends an additional IKE_AUTH request that includes
the AUTH payload, which has been calculated using the rMSK in the
role of the MSK as described in Sections 2.15 and 2.16 of [RFC5996].
The responder replies similarly, and the IKE_AUTH exchange is
finished.
If the backend AAA server does not have valid keys for the Re-auth-
Start message, it sends back a normal EAP request, and no rMSK key.
EAP flow continues as in [RFC5996].
The following figure is adapted from Appendixes C.1 and C.3 of
[RFC5996], with most of the optional payloads removed. Note that the
EAP-Initiate/Re-auth message is added.
IKE_SA_INIT Exchange:
| init request --> SA, KE, Ni,
|
| init response <-- SA, KE, Nr,
| N[ERX_SUPPORTED]
IKE_AUTH Exchanges:
| first request --> EAP(EAP-Initiate/Re-auth),
| IDi,
| SA, TSi, TSr
|
| first response <-- IDr, [CERT+], AUTH,
| EAP(EAP-Finish/Re-auth)
|
| last request --> AUTH
|
| last response <-- AUTH,
| SA, TSi, TSr
The IDi payload MUST have ID Type ID_RFC822_ADDR, and the data field
MUST contain the same value as the KeyName-NAI TLV in the EAP-
Initiate/Re-auth message. See Section 3.2 for details.
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3.1. Clarification about EAP Codes
Section 3.16 of [RFC5996] enumerates the EAP codes in EAP messages
that are carried in EAP payloads. The enumeration goes only to 4.
It is not clear whether or not that list is supposed to be
exhaustive.
To clarify, an implementation conforming to this specification MUST
accept and transmit EAP messages with at least the codes for Initiate
and Finish (5 and 6) from Section 5.3 of [RFC6696], in addition to
the four codes enumerated in [RFC5996]. This document is
intentionally silent about other EAP codes that are not enumerated in
those documents.
3.2. Username in the Protocol
The authors, as well as participants of the HOKEY and IPsecME working
groups, believe that all use cases for this extension to IKE have a
single backend AAA server doing both the authentication and the re-
authentication. The reasoning behind this is that IKE runs over the
Internet and would naturally connect to the user's home network.
This section addresses instances where this is not the case.
Section 5.3.2 of [RFC6696] describes the EAP-Initiate/Re-auth packet,
which, in the case of IKEv2, is carried in the first IKE_AUTH
request. This packet contains the KeyName-NAI TLV. This TLV
contains the username used in authentication. It is relayed to the
AAA server in the AccessRequest message and is returned from the AAA
server in the AccessAccept message.
The username part of the Network Access Identifier (NAI) within the
TLV is the EMSKName [RFC5295] encoded in hexadecimal digits. The
domain part is the domain name of the home domain of the user. The
username part is ephemeral in the sense that a new one is generated
for each full authentication. This ephemeral value is not a good
basis for making policy decisions, and it is also a poor source of
user identification for the purposes of logging.
Instead, it is up to the implementation in the IPsec gateway to make
policy decisions based on other factors. The following list is by no
means exhaustive:
o In some cases, the home domain name may be enough to make policy
decisions. If all users with a particular home domain get the
same authorization, then policy does not depend on the real
username. Meaningful logs can still be issued by correlating VPN
gateway IKE events with AAA servers access records.
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o Sometimes users receive different authorizations based on groups
to which they belong. The AAA server can communicate such
information to the VPN gateway, for example, using the CLASS
attribute [RFC2865] in RADIUS and Diameter [RFC6733]. Logging
again depends on correlation with AAA servers.
o AAA servers may support extensions that allow them to communicate
with their clients (in our case -- the VPN gateway) to push user
information. For example, a certain product integrates a RADIUS
server with the Lightweight Directory Access Protocol (LDAP)
[RFC4511], so a client could query the server using LDAP and
receive the real record for this user. Others may provide this
data through vendor-specific extensions to RADIUS or Diameter.
In any case, authorization is a major issue in deployments, if the
backend AAA server supporting the re-authentication is different from
the AAA server that had supported the original authentication. It is
up to the re-authenticating AAA server to provide the necessary
information for authorization. A conforming implementation of this
protocol MAY reject initiators for which it is unable to make policy
decisions because of these reasons.
4. ERX_SUPPORTED Notification
The Notify payload is as described in [RFC5996]:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Protocol ID ! SPI Size ! ERX Notify Message Type !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Domain Name !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Protocol ID (1 octet) - MUST be zero, as this message is related
to an IKE SA.
o Security Parameter Index (SPI) Size (1 octet) - MUST be zero, in
conformance with Section 3.10 of [RFC5996].
o ERX Notify Message Type (2 octets) - MUST be 16427, the value
assigned for ERX.
o Domain Name (variable) - contains the domain name or realm, as
these terms are used in [RFC6696] and encoded as ASCII, as
specified in [RFC4282].
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5. Operational Considerations
This specification changes the behavior of IKE peers, both initiators
and responders. The behavior of backend AAA servers is not changed
by this specification, but they are required to support [RFC6696].
The same goes for the EAP client, if it's not integrated into the IKE
initiator (for example, if the EAP client is an operating system
component).
This specification is silent about key storage and key lifetimes on
either the EAP client or the EAP server. These issues are covered in
Sections 3, 4, and 5 of [RFC6696]. The key lifetime may be
communicated from the AAA server to the EAP client via the Lifetime
attribute in the EAP-Finish/Re-auth message. If the server does not
have a valid key, while the client does have one, regular EAP is used
(see Section 3). This should not happen if lifetimes are
communicated. In such a case, the IKEv2 initiator / EAP client MAY
alert the user and MAY log the event. Note that this does not
necessarily indicate an attack. It could simply be a loss of state
on the AAA server.
6. Security Considerations
The protocol extension described in this document extends the
authentication from one EAP context, which may or may not be part of
IKEv2, to an IKEv2 context. Successful completion of the protocol
proves to the authenticator, which in our case is a VPN gateway, that
the supplicant or VPN client has authenticated in some other EAP
context.
The protocol supplies the authenticator with the domain name with
which the supplicant has authenticated, but does not supply it with a
specific identity. Instead, the gateway receives an EMSKName, which
is an ephemeral ID. With this variant of the IKEv2 protocol, the
initiator never sends its real identity on the wire while the server
does. This is different from the usual IKEv2 practice of the
initiator revealing its identity first.
If the domain name is sufficient to make access control decisions,
this is enough. If not, then the gateway needs to find out either
the real name or authorization information for that particular user.
This may be done using the AAA protocol or by some other federation
protocol, which is out of scope for this specification.
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7. IANA Considerations
IANA has assigned a notify message type of 16427 from the "IKEv2
Notify Message Types - Status Types" registry with the name
"ERX_SUPPORTED".
8. Acknowledgements
The authors would like to thank Yaron Sheffer for comments and
suggested text that have contributed to this document.
Thanks also to Juergen Schoenwaelder for his OPS-DIR review comments.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC5295] Salowey, J., Dondeti, L., Narayanan, V., and M. Nakhjiri,
"Specification for the Derivation of Root Keys from an
Extended Master Session Key (EMSK)", RFC 5295,
August 2008.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
[RFC6696] Cao, Z., He, B., Shi, Y., Wu, Q., and G. Zorn, "EAP
Extensions for the EAP Re-authentication Protocol (ERP)",
RFC 6696, July 2012.
9.2. Informative References
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC4511] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", RFC 4511, June 2006.
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
January 2010.
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[RFC6733] Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
"Diameter Base Protocol", RFC 6733, October 2012.
[SECBEAC] Sheffer, Y. and Y. Nir, "Secure Beacon: Securely Detecting
a Trusted Network", Work in Progress, June 2009.
Authors' Addresses
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 67897
Israel
EMail: ynir@checkpoint.com
Qin Wu
Huawei Technologies Co., Ltd.
101 Software Avenue, Yuhua District
Nanjing, JiangSu 210012
China
EMail: sunseawq@huawei.com
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