Network Working Group V. Devarapalli
Internet-Draft WiChorus
Intended status: Standards Track K. Weniger
Expires: October 15, 2009 April 13, 2009
Redirect Mechanism for IKEv2
draft-ietf-ipsecme-ikev2-redirect-08.txt
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Abstract
IKEv2 is a protocol for setting up VPN tunnels from a remote location
to a gateway so that the VPN client can access services in the
network behind the gateway. Currently there is no standard mechanism
specified that allows an overloaded VPN gateway or a VPN gateway that
is being shut down for maintenance to redirect the VPN client to
attach to another gateway. This document proposes a redirect
mechanism for IKEv2. The proposed mechanism can also be used in
Mobile IPv6 to enable the home agent to redirect the mobile node to
another home agent.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. IKEv2 Initial Exchange with Redirect . . . . . . . . . . . . . 4
4. Use of Anycast Addresses with the Redirect Mechanism . . . . . 5
5. Gateway Initiated Redirect . . . . . . . . . . . . . . . . . . 6
6. Redirect During IKE_AUTH Exchange . . . . . . . . . . . . . . 7
7. Redirect Messages . . . . . . . . . . . . . . . . . . . . . . 8
7.1. REDIRECT_SUPPORTED . . . . . . . . . . . . . . . . . . . . 8
7.2. REDIRECT . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.3. REDIRECTED_FROM . . . . . . . . . . . . . . . . . . . . . 10
8. Use of the Redirect Mechanism between IKEv2 Peers . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . . 12
12.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
IKEv2 [2] is used for setting up IPsec-based VPNs. The IP address of
the VPN gateway can be configured on the VPN client. But this does
not scale well, when the number of VPN gateways is large. Dynamic
discovery of VPN gateways using DNS is quite widely used too.
However, using DNS is not flexible when it comes to assigning a VPN
gateway to the VPN client based on the load on the VPN gateways. The
VPN client typically tries to connect to the IP address of the VPN
gateways that appears first in the DNS response. If the VPN tunnel
setup fails, then the VPN client tries to attach to the other VPN
gateways returned in the DNS response.
This document proposes a redirect mechanism for IKEv2 that enables a
VPN gateway to redirect the VPN client to another VPN gateway, for
example, based on the load condition. The redirect can be done
during the IKE_SA_INIT or the IKE_AUTH exchange. Gateway-initiated
redirect in the middle of a session is also supported. The redirect
mechanism can also be used in conjunction with anycast addresses. In
this case, anycast address for the cluster of VPN gateways is stored
in the DNS instead of a list of unicast IP addresses of the VPN
gateways.
The redirect can also happen because of administrative or optimal
routing reasons. This document does not attempt to provide an
exhaustive list of reasons for redirecting a VPN client to another
VPN gateway.
Mobile IPv6 [3] may use IKEv2 for mutual authentication between the
mobile node and the home agent. IKEv2 may also be used for home
address configuration and setting up IPsec security associations for
protecting Mobile IPv6 signaling messages [4]. The IKEv2 exchange
precedes the exchange of Mobile IPv6 signaling messages. Therefore
the mechanism described in this document can also be used by a Mobile
IPv6 home agent to redirect a mobile node to another home agent.
There is a Home Agent Switch mechanism available for redirecting a
mobile node to another home agent, described in [5]. The Home Agent
Switch mechanism can only be used after the binding cache had been
created at the home agent for the mobile node. The disadvantage with
this is that quite a bit of state is created on the home agent before
the mobile node can be redirected to another home agent. The
mechanism described in this document can be used for redirecting a
mobile node before any state related to the Mobile IPv6 binding is
created on the home agent.
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2. Terminology
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 [1].
3. IKEv2 Initial Exchange with Redirect
This section describes the use of Redirect mechanism during the
IKE_SA_INIT exchange. Gateway-initiated redirect and the use of
redirect during IKE_AUTH exchange are explained in subsequent
sections.
To redirect an IKEv2 session to another VPN gateway, the VPN gateway
that initially received the IKE_SA_INIT request selects another VPN
gateway and responds to the VPN client with a REDIRECT Notification
payload. The mechanism by which the initial VPN gateway selects
another VPN gateway is out of scope for this document. The IP
address of the selected VPN gateway is sent in the REDIRECT payload.
The gateway MUST include the nonce data from the Ni payload sent by
the initiator in the REDIRECT payload. This prevents certain Denial-
of-Service attacks on the initiator that could be caused by an
attacker injecting IKE_SA_INIT responses with REDIRECT payloads.
The VPN client indicates support for the IKEv2 redirect mechanism and
the willingness to be redirected by including a REDIRECT_SUPPORTED
notification message in the initial IKE_SA_INIT request. If the
IKE_SA_INIT request did not include the REDIRECT_SUPPORTED payload,
the responder MUST NOT send the REDIRECT payload to the VPN client.
Initiator Responder (initial VPN GW)
--------- -------------------------
(IP_I:500 -> Initial_IP_R:500)
HDR(A,0), SAi1, KEi, Ni, -->
N(REDIRECT_SUPPORTED)
(Initial_IP_R:500 -> IP_I:500)
<-- HDR(A,0), N(REDIRECT, IP_R)
When the VPN client receives the IKE_SA_INIT response with the
REDIRECT payload, it initiates a new IKE_SA_INIT exchange with the
VPN gateway listed in the REDIRECT payload provided this is allowed
by its IPsec policy. The VPN client includes the IP address of the
original VPN gateway that redirected the client. The IKEv2 exchange
then proceeds as normal with the selected VPN gateway.
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Initiator Responder (Selected VPN GW)
--------- ---------------------------
(IP_I:500 -> IP_R:500)
HDR(A,0), SAi1, KEi, Ni, -->
N(REDIRECTED_FROM, Initial_IP_R)
(IP_R:500 -> IP_I:500)
<-- HDR(A,B), SAr1, KEr, Nr,[CERTREQ]
(IP_I:500 -> IP_R:500)
HDR(A,B), SK {IDi, [CERT,] [CERTREQ,]
[IDr,]AUTH, SAi2, TSi, TSr} -->
(IP_R:500 -> IP_I:500)
<-- HDR(A,B), SK {IDr, [CERT,] AUTH,
SAr2, TSi, TSr}
When this mechanism is used with Mobile IPv6, care must be taken to
ensure that the home agent information is consistent with the IKEv2
gateway information. The Mobile IPv6 home agent discovery mechanisms
(for instance, RFC 5026 [4]) would have configured the mobile node
with a particular home agent. When the mobile node initiates an
IKEv2 exchange with the home agent and is redirected to another
gateway, the home agent information should also be updated, subject
to the policy on the mobile node.
4. Use of Anycast Addresses with the Redirect Mechanism
The use of anycast addresses will avoid having to configure a
particular VPN gateway's IP address in the DNS. Instead, the anycast
address that represents the group of VPN gateways is stored in the
DNS. When the VPN client performs a DNS lookup for the VPN gateway,
it receives the anycast address of the VPN gateway in the DNS
response.
If an anycast address is returned in response to DNS resolution of an
FQDN, the VPN client sends the IKE_SA_INIT request to the anycast
address. The IKE_SA_INIT request is routed to one of the VPN
gateways that is part of the anycast group. The VPN gateway that
receives the IKE_SA_INIT request responds with an IKE_SA_INIT reply
from the anycast address.
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Initiator Responder (any VPN GW)
--------- -------------------------
(IP_I:500 -> ANYCAST:500)
HDR(A,0), SAi1, KEi, Ni) -->
N(REDIRECT_SUPPORTED)
(ANYCAST:500 -> IP_I:500)
<-- HDR(A,0), N(REDIRECT, IP_R)
If the destination address on the IKE_SA_INIT request is an anycast
address, the VPN gateway that received the IKE_SA_INIT request MUST
include the REDIRECT payload to redirect the VPN client to a unicast
address of one of the VPN gateway. The VPN gateway that received the
IKE_SA_INIT request MAY redirect the client to its own unicast
address, if it is not overloaded.
The rest of the IKEv2 exchange is the same as described in Section 3.
5. Gateway Initiated Redirect
The redirect mechanism may also be used by a VPN gateway to redirect
the client to another VPN gateway in middle of a session. To
redirect a client, the gateway should send an INFORMATIONAL message
with the REDIRECT Notify payload. The REDIRECT payload MUST carry
information about the new VPN gateway. The gateway MUST NOT include
any nonce data in the REDIRECT payload, since it is a gateway-
initiated message and is protected by the IKEv2 security association.
When the client receives this message, it MUST send an empty message
as an acknowledgement. Until the client responds with an
acknowledgement, the gateway SHOULD re-transmit the redirect
INFORMATIONAL message as described in [2]. The following illustrates
the INFORMATIONAL message exchange for gateway-initiated redirect.
Initiator (VPN client) Responder (VPN GW)
---------------------- ------------------
<-- HDR, SK {N[REDIRECT, IP_R/FQDN_R]}
HDR, SK {} -->
The INFORMATIONAL message exchange described above is protected by
the existing IKEv2 SA between the client and the gateway.
Once the client sends an acknowledgement to the gateway, it SHOULD
delete the existing security associations with the old gateway by
sending an Informational message with a DELETE payload. The gateway
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MAY also decide to delete the security associations without any
signaling from the client, again by sending an Informational message
with a DELETE payload. However, it should allow sufficient time for
the client to setup the required security associations with the new
security gateway. This time period should be configurable on the
gateway.
6. Redirect During IKE_AUTH Exchange
If the gateway decides to redirect the client during the IKE_AUTH
exchange, based on the identity presented by the client in the
IKE_AUTH request message, it prevents the creation of a CHILD SA and
sends the REDIRECT payload in the IKE_AUTH response. When the client
receives the IKE_AUTH response with the REDIRECT payload, it SHOULD
delete the existing IKEv2 security association with the gateway. The
gateway MUST verify the client's AUTH payload before sending the
Redirect payload, and the client MUST verify the gateway's AUTH
payload before acting on the Redirect payload.
Initiator Responder ( VPN GW)
--------- -------------------
(IP_I:500 -> IP_R:500)
HDR(A,0), SAi1, KEi, Ni, -->
N(REDIRECTED_SUPPORTED)
(IP_R:500 -> IP_I:500)
<-- HDR(A,B), SAr1, KEr, Nr,[CERTREQ]
(IP_I:500 -> IP_R:500)
HDR(A,B), SK {IDi, [CERT,] [CERTREQ,]
[IDr,]AUTH, SAi2, TSi, TSr} -->
(IP_R:500 -> IP_I:500)
<-- HDR(A,B), SK {IDr, [CERT,] AUTH,
N[REDIRECT, IP_R/FQDN_R]}
In case the IKE_AUTH exchange involves EAP authentication as
described in Section 2.16 of RFC 4306 [2] or multiple authentication
methods as described in RFC 4739 [6], the IKE_AUTH exchange is more
complicated. The identity presented by the client in the first
IKE_AUTH request might be a temporary one. In addition, the gateway
might decide to redirect the client based on the interaction with the
the AAA server, when EAP authentication is used or the external
authentication server, when multiple authentication methods are used.
In such cases, the gateway should send the REDIRECT notification
payload in the final IKE_AUTH response message that carries the AUTH
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payload and the traffic selectors. The gateway MUST NOT send and the
client MUST NOT accept a redirect in an earlier IKE_AUTH message.
7. Redirect Messages
7.1. REDIRECT_SUPPORTED
The REDIRECT_SUPPORTED payload is included in the initial IKE_SA_INIT
request by the initiator to indicate support for the IKEv2 redirect
mechanism described in this document.
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 (=0) | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 'Next Payload', 'Payload Length', 'Protocol ID', 'SPI Size' and
the 'Notify Message Type' fields are the same as described in Section
3.10 of [2]. The 'SPI Size' field MUST be set to 0 to indicate that
the SPI is not present in this message. The 'Protocol ID' MUST be
set to 0, since the notification is not specific to a particular
security association.
The 'Payload Length' field MUST be set to '8'. The 'Notify Message
Type' field is set to indicate the REDIRECT_SUPPORTED payload <value
to be assigned by IANA>.
7.2. REDIRECT
The REDIRECT payload is included in an IKE_SA_INIT response from the
responder or an INFORMATIONAL message from the responder, when the
responder wants to redirect the initiator to another VPN gateway.
The message includes the new responder's IP address.
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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 (=0) | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GW Ident Type | GW Ident Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~
~ New Responder GW Identity ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Nonce Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 'Next Payload', 'Payload Length', 'Protocol ID', 'SPI Size' and
the 'Notify Message Type' fields are the same as described in Section
3.10 of [2]. The 'SPI Size' field MUST be set to 0 to indicate that
the SPI is not present in this message. The 'Protocol ID' MUST be
set to 0, since the notification is not specific to a particular
security association.
The 'Payload Length' field is set to the length in octets of the
entire payload, including the generic payload header. 'Notify
Message Type' field is set to indicate the REDIRECT payload <value to
be assigned by IANA>. The 'GW Identity Type' field indicates the
type of information that is sent to identify the new VPN gateway.
The following values are reserved by this document.
1 - IPv4 address of the new VPN gateway
2 - IPv6 address of the new VPN gateway
3 - FQDN of the new VPN gateway
All other values for this field are reserved and MUST NOT be used.
The 'GW Ident Len' field is set to the length of the gateway identity
information. The identity of the new VPN gateway is carried in the
'New Responder GW Identity' field.
The 'Nonce Data' field carries the nonce data from the Ni payload
sent by the initiator. The size of the nonce MUST be between 16 and
256 bytes as described in Section 3.9 of [2]. The 'Nonce Data' field
is present in the REDIRECT payload only when the REDIRECT payload is
sent in the IKE_SA_INIT response message. It MUST NOT be included in
the REDIRECT payload if sent in an IKE_AUTH response or in a gateway-
initiated redirect message.
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7.3. REDIRECTED_FROM
The REDIRECTED_FROM message type is included in the IKE_SA_INIT
request from the initiator to the new VPN gateway to indicate the IP
address of the original VPN gateway that redirected the initiator.
The original VPN gateway's IP address is included in the message.
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 (=0) | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GW Ident Type | GW Ident Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~
~ Original Responder GW Identity ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 'Next Payload', 'Payload Length', 'Protocol ID', 'SPI Size' and
the 'Notify Message Type' fields are the same as described in Section
3.10 of [2] The 'SPI Size' field MUST be set to 0 to indicate that
the SPI is not present in this message. The 'Protocol ID' MUST be
set to 0, since the notification is not specific to a particular
security association.
The 'Payload Length' field MUST be set to either '13' or '25'
depending on whether an IPv4 or IPv6 address of the original VPN
gateway is sent in the message. The 'Notify Message Type' field is
set to indicate the REDIRECTED_FROM payload <value to be assigned by
IANA>. The 'GW Identity Type' field indicates the type of
information that is sent to identify the new VPN gateway. The
following values are reserved by this document.
1 - IPv4 address of the original VPN gateway
2 - IPv6 address of the original VPN gateway
All other values for this field are reserved and MUST NOT be used.
The 'GW Ident Len' field is set to the length of the gateway identity
information. The identity of the original VPN gateway is carried in
the 'Original Responder GW Identity' field.
8. Use of the Redirect Mechanism between IKEv2 Peers
The Redirect mechanism described in this document is mainly intended
for use in client-gateway scenarios. However, the mechanism can also
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be used between any two IKEv2 peers. But this protocol is
asymmetric, meaning that only the original responder can redirect the
original initiator to another server.
9. Security Considerations
An eavesdropper on the path between VPN client and server may send a
redirect to the client upon receiving an IKE_SA_INIT message from
this client. This is no problem regarding DoS attacks for the VPN
connection, since an on-path-attacker can as well drop the
IKE_SA_INIT requests to prevent VPN access for the client. But an
eavesdropper on the path between VPN client and server can redirect a
large number of clients to a victim, which is then flooded with
IKE_SA_INIT requests. Flooding only happens if many clients initiate
IKEv2 exchange at almost the same time, which is considered a rare
event. However, this may happen if a Home Agent/VPN server is
shutdown for maintenance and all clients need to re-establish VPN
connections with another Home Agent/VPN server or if the on-path
attacker forces all IPsec security associations to expire by dropping
all received IKEv2 messages.
The use of REDIRECTED_FROM payload is intended to discourage a rogue
VPN gateway from redirecting a large number of VPN clients to a
particular VPN gateway. It does not prevent such a DoS attack.
Since the redirect message is not always sent as a secure message, it
MUST NOT result in the modification of the PAD entries on the client.
The new gateway, to which the client is redirected to should be
subject to the same authentication and authorization requirements as
the original gateway. To support a scenario where the FQDN of the
gateway is in the client's PAD entry and the client is redirected to
another gateway in the same administrative domain, one can either
configure all the possible gateways from the domain or use a wildcard
entry like, for example, GW*.example.com, in the client's
corresponding PAD entry.
10. IANA Considerations
This document defines four new IKEv2 Notification Message types as
described in Section 7. The three Notify Message Types must be
assigned values between 16396 and 40959.
o REDIRECT_SUPPORTED
o REDIRECT
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o REDIRECTED_FROM
11. Acknowledgements
The use of anycast address with IKEv2 was first described in [7]. It
was then added to an early draft version of RFC 5026 and later
removed before the RFC was published. Therefore the authors of [7]
and RFC 5026 are acknowledged.
Thanks to Pasi Eronen, with whom the solution described in this
document was extensively discussed. Thanks to Tero Kivinen for
suggesting the use of REDIRECTED_FROM payload and other comments
which helped improve the document. The authors would also like to
thank Yaron Sheffer, Sunil Kumar, Fan Zhao, Yoav Nir, Richard
Graveman, Kanagavel Rajan, Srini Addepalli, and Arnaud Ebalard for
their reviews and comments.
12. References
12.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306,
December 2005.
12.2. Informative References
[3] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[4] Giaretta, G., Kempf, J., and V. Devarapalli, "Mobile IPv6
Bootstrapping in Split Scenario", RFC 5026, October 2007.
[5] Haley, B., Devarapalli, V., Deng, H., and J. Kempf, "Mobility
Header Home Agent Switch Message", RFC 5142, January 2008.
[6] Eronen, P. and J. Korhonen, "Multiple Authentication Exchanges
in the Internet Key Exchange (IKEv2) Protocol", RFC 4739,
November 2006.
[7] Weniger, K. and F. Dupont, "IKEv2-based Home Agent Assignment in
Mobile IPv6/NEMO Bootstrapping", draft-dupont-ikev2-haassign-02
(work in progress), January 2007.
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Authors' Addresses
Vijay Devarapalli
WiChorus
3590 North First St
San Jose, CA 95134
USA
Email: vijay@wichorus.com
Kilian Weniger
Email: kilian.weniger@googlemail.com
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