L3VPN WG M. Behringer
Internet-Draft J. Guichard
Expires: September 23, 2005 Cisco Systems Inc
P. Marques
Juniper Networks
March 22, 2005
Layer-3 VPN Import/Export Verification
draft-ietf-l3vpn-vpn-verification-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Configuration errors on Provider Edge (PE) routers in Layer-3 VPN
networks based on [RFC2547] can lead to security breaches of the
connected VPNs. For example, the PE router could be mistakenly
configured such that a connected Customer Edge (CE) router belongs to
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an incorrect VPN. Here we propose a scheme that verifies local and
remote routing information received by the PE router before it
installs new VPN routes into the Virtual Routing & Forwarding
Instance (VRF). The proposed changes affect only the PE routers.
Table of Contents
1. Changes to previous version, and to-do-list . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. Problem Statement and Overview . . . . . . . . . . . . . . . . 3
4. CE-CE Authentication . . . . . . . . . . . . . . . . . . . . . 4
4.1 PE-CE Authentication Behavior . . . . . . . . . . . . . . 5
4.2 Behaviour of PE sending the UPDATE-authenticator . . . . . 5
4.3 Behaviour of PE receiving the UPDATE-authenticator . . . . 6
5. Extranet VPN Processing . . . . . . . . . . . . . . . . . . . 7
6. The UPDATE-authenticator attribute . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
10. Informative References . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 11
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1. Changes to previous version, and to-do-list
This document is not much changed from the previous version,
draft-behringer-mpls-vpn-auth-04.txt. Some editorial changes were
made, and the title adapted to reflect better the intentions of the
document.
Issues to be addressed in the next version:
o Address differences to draft-ietf-l3vpn-l3vpn-auth-01.txt.
o Refer to the requirements draft, and clarify which requirements
are met by the approach described here.
o Address how this scheme works with Inter-AS.
o Look into alternatives to CE-PE routing, for deployments where
PE-CE routing is not feasible.
2. 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 [RFC-2119].
3. Problem Statement and Overview
The current Layer-3 VPN architecture as defined in [RFC2547] does not
provide any mechanism to determine whether an imported route on a PE
router originated from the correct VPN. This opens a potential
security hole where the VPN Service Provider could mistakenly assign
on a PE router the incorrect "route-target" values, thus
inadvertently bringing a connected CE router, with the network/s
behind it, into a wrong VPN.
[RFC2547] does not require that PE-CE sessions or PE-PE sessions be
authenticated. However, in the cases where this is deployed, route
authentication relies on a three-step configuration process; From the
CE router to the PE router, from that PE router to other PE routers
in the same VPN provider network, and from the other PE routers to
the corresponding CE routers.
Correct access control between VPNs relies on the accurate
configuration of "route-targets" on the PE routers. Because the 3
authentication steps above are essentially disjoint, the linkage
necessary to "glue" them together is the correct configuration of the
VPN provider network, and the corresponding "route-target" values. .
If the Service Provider has assigned the wrong "route-target" values
then this is hard to detect from within the customer's network, and a
real issue in [RFC2547] networks. One possible solution to this
problem is to mount IPsec [RFC2401] on all CE routers, but this is
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often perceived as too "heavy-weight". Therefore, a mechanism is
required which prevents routes from being passed into a PE router's
Virtual Routing & Forwarding Instance (VRF), unless they have been
verified to belong to the associated VPN. Also, in the case of such
configuration errors, the Service Provider must be alerted so that
the mistake can be rectified.
This proposal aims to solve the problem of accidental
misconfiguration of VPN parameters on PE routers. The approach is to
associate one or more authentication keys to a VPN, and use existing
routing protocol authentication mechanisms [RFC2082, 2154, 2385], to
provide PE-CE authentication. PE-PE routing exchanges are validated
via routing update signatures. Since a PE router can hold several
VRF's, the authentication between PEs will use the different MD5
keys, based on which VRF's routes need to be verified.
BGP UPDATE messages between PE routers will include a new BGP
attribute, hereby referred to as the "UPDATE-authenticator". This
attribute contains a keyed HMAC MD5 signature of a locally generated
per-VRF random number, using the MD5 key that is also used on this PE
router for the PE-CE routing authentication of that VPN.
The receiving PE router generates a keyed HMAC MD5 signature using
information from the "UPDATE-authenticator" attribute contained
within the BGP UPDATE message, and the routing key of the CE router
that is to receive the routes contained within the update. If the
result is different from the signature value transmitted in the
UPDATE-authenticator attribute, the routes within the UPDATE are not
imported and a warning is logged.
This proposal imposes some operational constraints to be workable;
Regardless of whether a routing protocol is used or not within the
VRF, at least one authentication key MUST be configured for each VRF
that wishes to use the mechanisms described within this document. If
a dynamic routing protocol is used, then routing with MD5
authentication [RFC2082, 2154, 2385] SHOULD be configured for all PE-
CE links of a particular VPN. All CE routers of the same VPN MAY use
the same or different MD5 keys and the PE router MUST indicate which
key has been used when advertising routes from the associated VRF.
If the Service Provider manages the CE routers on behalf of the
customer, then downstream C routers MUST also use the same MD5 key.
MD5 keys SHOULD be chosen to be unique to a VPN.
4. CE-CE Authentication
As previously stated, this document proposes to re-use the MD5 key
that is being used for PE-CE routing authentication. This has the
advantage that no changes or software upgrades are necessary at the
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CE routers or within the VPN site. For this proposal to work, each
PE router, on export of the routes from within a given VPN, MUST
indicate which MD5 key has been used to authenticate the local
routes. The MD5 key set SHOULD be unique to each VPN. The VPN
customer configures thus all their CE routers with an MD5 key. The
VPN Service Provider also configures the PE routers with this local
key on all links to the customers CE routers. This proposal does not
affect the CE-PE routing authentication, but the authentication MUST
be used for this scheme to work.
This proposal is orthogonal with MD5 authentication between PE
routers on the VPN network. Authentication of peering sessions
between PEs provides protection of the VPN routing information
without any validation of its origin.
While currently, the VPN service provider may choose to configure
routing authentication between the PE and CE, this information only
affects the local routing session between the two routers.
Conceptually, this proposal extends this key verification between the
local PE and CE to remote PE to CE connections.
Using the mechanisms described within this document, the BGP UPDATE
message, as defined in [RFC1771], is sent between PE routers (or BGP
route reflectors), and carries a new UPDATE-authenticator attribute,
which is used to verify the source of the routing information.
4.1 PE-CE Authentication Behavior
If a dynamic routing protocol is used between PE and CE routers, then
the routing protocol is secured with MD5 authentication. Routes are
only put into a VRF that is configured with Layer-3 VPN
"Import/Export Verification" if the MD5 authentication is successful.
If a VRF is configured at the PE router for Layer-3 VPN
"Import/Export Verification" using MD5 authentication, it is OPTIONAL
to confirm local route authentication prior to any route export from
the VRF. Route authentication involves checking whether the PE
router can confirm route receipt from each CE router that is attached
to the VRF.
4.2 Behaviour of PE sending the UPDATE-authenticator
When Layer-3 "Import/Export Verification" is enabled, the PE router
SHOULD calculate a random number, referred to as the 'Generator', for
each VRF that is configured for authentication. Alternatively a
combination of the local "route-target" values may be used to
generate this number. This is implementation specific.
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Having generated the VRF specific random number, the PE router on
sending a [RFC2858] BGP UPDATE calculates a keyed HMAC-MD5 signature,
as defined in [RFC2104], over the 'Generator', using the key of one
of the CEs that is connected to the corresponding VRF. The result of
this calculation is carried, along with the 'Generator' and an
identification of the key used against the 'Generator', in the "HMAC-
MD5 Signature" field within the UPDATE-authenticator attribute.
Each key within a VRF will have a corresponding 'key-identifier',
which SHOULD be configurable within the VRF, and MUST be unique
across VPNs. Every PE router that holds members of the VPN MUST
carry <key, key-identifier> mappings so that they can verify which
key to use when authenticating incoming routing updates. The
key-identifier MAY be the route-target.
The PE sending an [RFC2858] UPDATE will add a 'key-identifier' to the
UPDATE-authenticator attribute to indicate which key should be used
by a receiving PE router to verify the update. The UPDATE message is
sent to any [RFC2858] BGP peers (other PE routers or BGP route
reflectors). The "route-targets" in the [RFC2858] update determine
which VRF/s the UPDATE refers to, and these are used as described in
[RFC2547] to determine which PE routers will import which routes.
4.3 Behaviour of PE receiving the UPDATE-authenticator
A PE router that receives a [RFC2858] BGP update that contains the
UPDATE-authenticator attribute SHOULD verify the contents of the
update with the following algorithm. As an OPTIONAL step, the PE
router MAY perform this comparison only if it has authenticated local
routes from the CE router:
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IF target VRF is configured for Layer-3 VPN Import/Exp. Verification
THEN
IF UPDATE-authenticator attribute is present
THEN
subroutine determine_MD5-key
verify UPDATE-authenticator with MD5-key
IF result = signature of received UPDATE-authenticator
THEN
import route into VRF
ELSE
mark routes as 'not authenticated'; log error
ELSE
mark routes as 'not authenticated'; log error
ELSE
mark routes as 'not authenticated'; log error
subroutine determine_MD5-key
IF key-identifier = 0
THEN
MD5-key = the MD5 key used for routing authentication
with one of the routing peers of the VRF.
ELSE
MD5-key = lookup_in_config (key-identifier)
RETURN MD5-key
A router MAY verify whether all MD5 keys for a given VRF are the
same. If it does a warning message MUST be logged if it detects
differences.
In the case where the Service Provider manages the CE routers, the
Service Provider must also configure the key at the CE routers and
this should match with any directly connected downstream C routers
within the customer site. If the C routers have a different key than
the CE router then the CE will not authenticate any routes from
within the site, and will therefore not advertise any routing
information to the PE router. The PE router is thus able to use the
previously described mechanisms and will not import/export any routes
from/to the customers VRF.
5. Extranet VPN Processing
There are typically two types of Extranets that can be defined using
the [RFC2547] architecture; Central Services Extranet and Distributed
Extranet.
The Central Services Extranet provides connectivity between spoke VPN
sites through a central PE router. This PE router carries routes for
all members of the extranet whereas spoke PE routers carry only local
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routes, and a route to the central PE router. To support this type
of configuration, the central PE router needs to carry <key, key-
identifier> mappings for ALL members of the extranet. On receiving
an incoming update, the central PE is able to identify which key to
use on the UPDATE-authenticator attribute by looking at the key-
identifier carried within the update.
The Distributed Extranet model provides connectivity directly between
members of a given VPN. This means that each PE router that holds
members of the extranet is configured to import the relevant "route-
target" values used for export by other members of the VPN. Using
the key-identifier, a PE router is able to identify which key to use
on an incoming update to verify the source. This means that each PE
router within the extranet MUST carry <key, key-identifier> mappings
for all members of the VPN.
6. The UPDATE-authenticator attribute
The UPDATE-authenticator attribute is an optional, transitive BGP
attribute, with an attribute type code value to be assigned. Its
length is 24 octets, which is the length of the output of an MD5
function (16 octets), a 'Generator' field, and a 'Key-identifier', as
shown in the following figure.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC-MD5 Signature |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC-MD5 (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC-MD5 (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC-MD5 (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Generator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key-identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7. IANA Considerations
The UPDATE-authenticator BGP attribute type will need to be
registered with IANA, according to the procedures defined in
[RFC2042].
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8. Security Considerations
This modification to the behavior of the PE router aims at detecting
inadvertent configuration mistakes of the Service Provider, and at
isolating CE routers that appear not to belong to the VPN they were
configured for.
There is no protection against the Service Provider staff maliciously
adding a CE router to a VPN. However, the malicious engineer must
know the MD5 key of the VPN to be intruded. This threat can be
avoided with CE-CE IPsec authentication, which is configured by the
VPN customer, and to which the Service Provider does not have access.
9. Acknowledgements
Many thanks to Dan Tappan, David Allan and Eric Vyncke for their
contributions to this work.
10. Informative References
[1] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995.
[2] Manning, B., "Registering New BGP Attribute Types", RFC 2042,
January 1997.
[3] Baker, F., Atkinson, R. and G. Malkin, "RIP-2 MD5
Authentication", RFC 2082, January 1997.
[4] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[5] Murphy, S., Badger, M. and B. Wellington, "OSPF with Digital
Signatures", RFC 2154, June 1997.
[6] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[7] Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March 1999.
[8] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[9] Bates, T., Rekhter, Y., Chandra, R. and D. Katz, "Multiprotocol
Extensions for BGP-4", RFC 2858, June 2000.
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Authors' Addresses
Michael H. Behringer
Cisco Systems Inc
Village d'Entreprises Green Side
400, Avenue Roumanille, Batiment T 3
Biot - Sophia Antipolis 06410
France
Email: mbehring@cisco.com
URI: http://www.cisco.com
Jim Guichard
Cisco Systems Inc
300 Apollo Drive
Chelmsford, MA 01824
USA
Email: jguichar@cisco.com
URI: http://www.cisco.com
Pedro Marques
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
USA
Email: roque@juniper.net
URI: http://www.juniper.net
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