PPVPN Working Group Michael Behringer
Internet Draft Jim Guichard
Expiration Date: July 2003 Cisco Systems Inc
January 7th 2002
MPLS VPN Import/Export Verification
draft-behringer-mpls-vpn-auth-01.txt
Status of this Memo
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Abstract
Configuration errors on Provider Edge (PE) routers in MPLS VPN
[RFC2547] networks can lead to security breaches of the connected VPNs.
For example the PE could be mistakenly configured such that a connected
Customer Edge (CE) router belongs to 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 VRF. The proposed changes affect only the PE routers.
Table of Contents
Internet Draft MPLS VPN Authentication January 2003
1. Conventions used in this document 2
2. Overview 2
3. Problem Statement 3
4. CE-CE Authentication 3
5. Supported PE-CE Routing Protocols 5
6. The UPDATE-authenticator attribute 6
7. IANA Considerations 6
8. Security Considerations 6
9. Acknowledgements 7
10. References 7
11. Author's Addresses 7
12. Full Copyright Statement 7
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 [RFC-2119].
2. Overview
The current MPLS standards do 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 MPLS Service
Provider could mistakenly assign on a PE router the incorrect "route-
target" values, thus inadvertently bringing a connected customer site
into a wrong VPN.
Misconfigurations of this type by the Service Provider are hard to
detect, and a real issue in [RFC2547] networks. One possible solution
to this problem is to mount IPsec on all CE routers, but this is often
perceived as too "heavy-weight".
This proposal aims to solve the problem of accidental misconfiguration
of MPLS parameters on PE routers. The approach is to re-use the routing
MD5 key, which is used for PE-CE authentication [RFC2082, 2154, 2385],
also for the PE-PE routing authentication. Since a PE router can hold
several virtual routing and forwarding instances (VRFs), the
authentication between PEs will use the different MD5 keys, based on
which VRF's routes need to be authenticated.
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
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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 for the
incoming UPDATE message using information from the "UPDATE-
authenticator" attribute, and the routing key of the CE router that is
to receive these routing updates. 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.
The proposal imposes some operational constraints to be workable:
Routing with MD5 authentication [RFC2082, 2154, 2385] must be
configured for all PE-CE links of a particular VPN, and all CEs of the
same VPN must use the same MD5 key. 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 must be chosen to be unique to a
VPN.
3. Problem Statement
The current MPLS architecture as defined in [RFC2547] allows a three-
step configuration process for route authentication: From the CE to the
PE, from that PE to other PEs in the same MPLS network, and from the
other PEs to the corresponding CEs. The link between these three steps
depends on correct configuration of the MPLS core network, namely the
correct configuration of "route-targets" on the PE routers.
If an MPLS Service Provider inadvertently configures the wrong "route-
targets", they may introduce a random CE router with the network behind
it to another VPN. This error is hard to detect from within the
customer's network.
A mechanism is required which prevents routes from being passed into
a PEÆs VRF, unless they are verified to belong to the VPN. Also in
the case of misconfigurations, the Service Provider must be alerted
so that the mistake can be rectified.
4. CE-CE Authentication
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 CE routers or within the VPN
site. For this proposal to work each VPN MUST use the same MD5 key on
all PE-CE links for routing authentication. The MD5 key MUST be unique
to each VPN. The VPN customer configures thus all their CE routers with
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Internet Draft MPLS VPN Authentication January 2003
this MD5 key. The MPLS Service Provider configures the PEs also with
this key on all links to the customers CEs. This proposal does not
affect the CE-PE routing authentication, but the authentication MUST be
used for this scheme to work.
In normal [RFC2547] networks, the PE router that receives routes from a
CE advertises these routes via MP-BGP to the other PEs, using a PE
specific MD5 key. This is a problem as there is no linkage between the
MD5 key used for PE-CE authentication and the one used for PE-PE
authentication. This essentially introduces a 3-stage authentication
process, with each stage disjointed from the other two stages.
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 handled as follows:
1. The PE will only export routes from a local VRF that is configured
for MD5 authentication if it can confirm that at least 1 local route
has been received and authenticated from each CE router that is
attached to the VRF. This prevents any connected interface or static
routes from leaking into other VPNs without first confirming the local
configuration of the PE router. The exception to this rule is the
'Trusted PE' model (see section 5).
2. The PE will 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.
3. The PE sending an MP-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',
within the UPDATE-authenticator attribute.
4. The UPDATE message is sent to any MP-BGP peers (other PE routers or
BGP route reflectors).
5. The route targets in the MP-BGP update determine which VRF/s the
UPDATE refers to, and these are used as normal to determine which PE
routers will import which routes.
6. The receiving PE MUST not import a received prefix if it contains
the UPDATE-authenticator attribute, and the local VRF is NOT configured
for authentication. Likewise, the receiving PE MUST not import a
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Internet Draft MPLS VPN Authentication January 2003
received prefix if it does not contain the UPDATE-authenticator
attribute, and the local VRF is configured for authentication.
7. The receiving PE uses the local MD5 key for the importing VRF to
calculate a HMAC-MD5 signature, and compares the result to the value in
the received "UPDATE authenticator" path attribute. This comparison is
ONLY performed if the PE has at least 1 locally authenticated route
from the CE router.
8. If the values are identical, the UPDATE is processed as normal and
the routes are installed into the VRF. If not, the PE router MUST check
if any further keys are available within the VRF. If they are then it
should cycle through step 7 until either a) there are no more keys
available, or b) a match is found.
9. If no match is found then the BGP table is updated but the routes
are marked as 'not authenticated', and a warning is logged to indicate
a misconfiguration to the Service Provider.
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 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. Supported PE-CE Routing Protocols
This scheme in principle requires a dynamic routing protocol that
supports MD5 procedures. These currently include RIPv2, OSPF, ISIS,
EIGRP and BGP4. Static routing is considered a special case and is
serviced through the 'Trusted PE' model.
The 'Trusted PE' model provides the ability for the Service Provider to
authenticate routing information that is distributed between PE
routers, without the requirement of MD5 authentication on the PE-CE
links. This is useful when static routing is used between the PE and CE
for example.
Using this model, the PE router will export all routes from within a
VRF that has been configured for authentication, using the previous
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Internet Draft MPLS VPN Authentication January 2003
described mechanisms. However, the check to see whether a route has
been authenticated with the CE router is disabled. A receiving PE
router is able to authenticate the routes using the local VRF key,
which eliminates a misconfiguration of the route targets from causing
any leak of routing information between VPNs.
Since it is still possible for the Service Provider to a) misconfigure
the export route targets AND b) misconfigure the key, the Trusted PE
model is recommended only in exceptional cases.
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 20 octets, which is the length of the output of an MD5 function (16
octets), plus the 'generator' field, 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC-MD5 (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC-MD5 (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC-MD5 (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Generator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7. IANA Considerations
The UPDATE-authenticator BGP attribute type will need to be registered
with IANA, according to the procedures defined in [RFC2042].
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
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Internet Draft MPLS VPN Authentication January 2003
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
10. References
[RFC1771] "A Border Gateway Protocol 4 (BGP-4)". Y. Rekhter, T. Li.
March 1995
[RFC2042] "Registering New BGP Attribute Types". B. Manning. January
1997.
[RFC2082] "RIP-2 MD5 Authentication". F. Baker, R. Atkinson. January
1997.
[RFC2104] "HMAC: Keyed-Hashing for Message Authentication". H.
Krawczyk, M. Bellare, R. Canetti. February 1997.
[RFC2154] "OSPF with Digital Signatures". S. Murphy, M. Badger, B.
Wellington. June 1997.
[RFC2385] "Protection of BGP Sessions via the TCP MD5 Signature
Option". A. Heffernan. August 1998.
[RFC2547] "BGP/MPLS VPNs". E. Rosen, Y. Rekhter. March 1999.
11. Author's Addresses
Michael H. Behringer
Cisco Systems, Inc.
Avda de la Vega, 15; 28100 Alcobendas, Madrid; Spain
Email: mbehring@cisco.com
Jim Guichard
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
Email: jguichar@cisco.com
12. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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