Internet Draft Michael Behringer <draft-behringer-mpls-vpn-auth-00.txt> Jim Guichard Category: Standards Track Cisco Systems, Inc. October 2002 Expires: April 2003 MPLS VPN Authentication Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract Authentication in current MPLS/VPN networks [RFC2547] is based on routing authentication between CE-PE, PE-PE and again PE-CE, which authenticates the routing peer and verifies the integrity of routing updates from that peer. This does not provide CE-CE authentication. Here we propose a CE-CE authentication scheme that combines the three separate steps above and protects against misconfiguration of the PE routers by the Service Provider on the MPLS network. The proposed changes affect only the PE routers. 1. Overview The current MPLS standards do not provide authentication from CE to CE to determine whether an imported route originated within a trusted site. This opens a potential security hole where the MPLS Service Internet Draft MPLS VPN Authentication October 2002 Provider could mistakenly assign on a PE wrong route targets, thus inadvertently bringing a connected customer site into a wrong VPN. Misconfigurations of this type are hard to detect, and a real issue in MPLS networks. One possible solution to this problem is to mount IPsec on all CEs, 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 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, called "UPDATE authenticator". This attribute contains a keyed HMAC MD5 signature of the UPDATE message, 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 the routing key of the CE router that is to receive these routing updates. If the result is different from the value transmitted in the UPDATE authenticator attribute, the UPDATE is discarded 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, 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. 2. 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 draft-behringer-mpls-vpn-auth-00.txt page 2 Internet Draft MPLS VPN Authentication October 2002 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 PE's VRF, unless they are verified to belong to the VPN. Also in case of misconfigurations the Service Provider must be alerted. 3. CE-to-CE Authentication The proposal is to re-use the MD5 key that is being used for CE-PE routing authentication. For this proposal to work each VPN MUST use the same MD5 key on all CE-PE links for routing authentication. The MD5 key MUST be unique to each VPN. The VPN customer configures thus all their CE routers with this MD5 key. The MPLS Service Provider configures the PEs also with this key on all links to the customers CEs. In normal MPLS 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. The problem is that there is no linkage between the MD5 key used for CE-PE authentication and the one for PE-PE authentication. This proposal does not affect the CE-PE routing authentication, but the authentication MUST be used for this scheme to work. The BGP UPDATE message, as defined in [RFC1771] is sent between PEs, and carries the 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 the attached CE router. This prevents any connected interface or static routes from leaking into other VPNs without first confirming the local configuration of the PE router. 2. The PE sending an MP-BGP UPDATE calculates a keyed HMAC-MD5, as defined in [RFC2104], over the UPDATE message, using the key of one of the CEs that is connected to the corresponding VRF. The following sections of the BGP UPDATE message are included in the keyed HMAC-MD5 hash: - The BGP message header, consisting of marker, length and type. - The UPDATE message, including the MP_REACH_NLRI attribute 3. The UPDATE message is sent to the MP-BGP peers (other PE routers or BGP route reflectors). draft-behringer-mpls-vpn-auth-00.txt page 3 Internet Draft MPLS VPN Authentication October 2002 4. The route targets in the MP-BGP update determine which VRF/s the UPDATE refers to. 5. 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 received prefix if it does not contain the UPDATE-authenticator attribute, and the local VRF is configured for authentication. 6. The receiving PE makes the same calculation as in step 2, 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. 7. If the values are identical, the UPDATE is processed as normal and the NLRI installed in the VRF. If not, 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. From a configuration point of view this feature could be enabled on PE routers with a single command, which can be either globally valid for all VPNs or separate for each VPN. 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. 4. 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 16 octets, which is the length of the output of an MD5 function. The value of the UPDATE-authenticator attribute is the MD5 hash described above. 5. IANA considerations draft-behringer-mpls-vpn-auth-00.txt page 4 Internet Draft MPLS VPN Authentication October 2002 The UPDATE-authenticator BGP attribute type will need to be registered with IANA, according to the procedures defined in [RFC2042]. 6. Security Considerations This modification to the behaviour 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. 7. Open Questions Questions that still need to be addressed: 1 - Performance: Will the use of MD5 signatures for every Update message impact router performance too much? 2 - Should the proposal be open to use other signature algorithms such as SHA-1? 3 - Transition from normal operations to this mode: The feature needs to be enabled on all PEs of the same VPN at the same time to maintain connectivity. This may need to be addressed. 4. - do we need to state which protocols can be used with this scheme e.g. does IS-IS and EIGRP support MD-5 ? Authors' Addresses Michael H. Behringer <mbehring@cisco.com> Cisco Systems Avda de la Vega, 15; 28100 Alcobendas, Madrid; Spain Jim Guichard <jguichar@cisco.com> Cisco Systems 300 Apollo Drive; Chelmsford MA, 01824; USA References draft-behringer-mpls-vpn-auth-00.txt page 5 Internet Draft MPLS VPN Authentication October 2002 [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. Full Copyright Statement Copyright (C) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." draft-behringer-mpls-vpn-auth-00.txt page 6