Network Working Group M. Gupta Internet Draft Tropos Networks Document: draft-gupta-ospf-ospfv2-sec-00.txt N. Melam Expires: April 2007 Juniper Networks October 2006 Authentication/Confidentiality for OSPFv2 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 This document describes means and mechanisms to provide authentication/confidentiality to OSPFv2 using IPsec (IP Security). Copyright Notice Copyright (C) The Internet Society (2006). 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 [N7]. M. Gupta/N. Melam Expires - April 2007 [Page 1]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 Table of Contents 1. Introduction...................................................2 2. Transport Mode vs Tunnel Mode..................................3 3. Authentication.................................................3 4. Confidentiality................................................3 5. Distinguishing OSPFv2 from OSPFv3 [N2].........................4 6. IPsec Requirements.............................................4 7. Key Management.................................................5 8. SA Granularity and Selectors...................................7 9. Virtual Links..................................................7 10. Rekeying......................................................8 10.1 . Rekeying Procedure......................................8 10.2 . KeyRolloverInterval.....................................8 10.3 . Rekeying Interval.......................................9 11. IPsec rules...................................................9 12. Entropy of Manual Keys.......................................11 13. Replay Protection............................................11 Security Considerations..........................................11 IANA Considerations..............................................12 Normative References.............................................12 Informative References...........................................13 Acknowledgments..................................................13 Authors' Addresses...............................................13 1. Introduction OSPF (Open Shortest Path First) Version 2 [N1] defines the fields AuType and Authentication in its protocol header to provide security. These fields do not provide any confidentiality and also the authentication provided by these fields is weak [specific problems here]. The demands for securing the routing protocols have increased since the OSPFv2 protocol was designed. This document describes how IP Security (Encapsulating Security Payload and Authentication Header protocols) can be used to provide integrity, authentication, and/or confidentiality to OSPFv2. It is assumed that the reader is familiar with OSPFv2 [N1], Authentication Header (AH) [N5], Encapsulating Security Payload (ESP) [N4], the concept of security associations, tunnel and transport mode of IPsec, and the key management options available for AH and ESP (manual keying [N3] and Internet Key Exchange (IKE)[I1]). M. Gupta/N. Melam Expires - April 2007 [Page 2]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 2. Transport Mode vs Tunnel Mode The transport mode Security Association (SA) is generally used between two hosts or routers/gateways when they are acting as hosts. The SA must be a tunnel mode SA if either end of the security association is a router/gateway. Two hosts MAY establish a tunnel mode SA between themselves. OSPFv2 packets are exchanged between routers. However, since the packets are locally delivered, the routers assume the role of hosts in the context of tunnel mode SA. All implementations confirming to this specification MUST support transport mode SA to provide required IPsec security to OSPFv2 packets. They MAY also support tunnel mode SA to provide required IPsec security to OSPFv2 packets. 3. Authentication Implementations conforming to this specification MUST support authentication for OSPFv2. In order to provide authentication to OSPFv2, implementations MUST support ESP and MAY support AH. If ESP in transport mode is used, it will only provide authentication to OSPFv2 protocol packets excluding the IP header and IP options. If AH in transport mode is used, it will provide authentication to OSPFv2 protocol packet, selected portions of IP header and selected IP options. When OSPFv2 authentication is enabled, o OSPFv2 packets that are not protected with AH or ESP MUST be silently discarded. o OSPFv2 packets that fail the authentication checks MUST be silently discarded. 4. Confidentiality Implementations conforming to this specification SHOULD support confidentiality for OSPFv2. If confidentiality is provided, ESP MUST be used. When OSPFv2 confidentiality is enabled, o OSPFv2 packets that are not protected with ESP MUST be silently M. Gupta/N. Melam Expires - April 2007 [Page 3]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 discarded. o OSPFv2 packets that fail the confidentiality checks MUST be silently discarded. 5. Distinguishing OSPFv2 from OSPFv3 [N2] The IP/IPv6 Protocol Type for OSPFv2 and OSPFv3 is the same (89) and OSPF distinguishes them based on the OSPF header version number. However, current IPsec standards do not allow using arbitrary protocol-specific header fields as the selectors. Therefore, the OSPF version field in the OSPF header cannot be used in order to distinguish OSPFv2 packets from OSPFv3 packets. As OSPFv2 is only for IPv4 and OSPFv3 is only for IPv6, the version field in the IP header can be used to distinguish OSPFv2 packets from OSPFv3 packets. 6. IPsec Requirements In order to implement this specification, the following IPsec capabilities are required. Transport mode IPsec in transport mode MUST be supported. [N3] Multiple Security Policy Databases (SPDs) The implementation MUST support multiple SPDs with a specific SPD selection function. [N3] Selectors The implementation MUST be able to use source address, destination address, protocol, and direction as selectors in the SPD. Interface ID tagging The implementation MUST be able to tag the inbound packets with the ID of the interface (physical or virtual) via which it arrived. [N3] Manual key support Manually configured keys MUST be able to secure the specified traffic. [N3] Encryption and authentication algorithms The implementation MUST NOT allow the user to choose stream ciphers as the encryption algorithm for securing OSPFv2 packets since the stream ciphers are not suitable for manual keys. M. Gupta/N. Melam Expires - April 2007 [Page 4]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 Except when in conflict with the above statement, the key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", and "SHOULD NOT" that appear in the [N6] document for algorithms to be supported are to be interpreted as described in [N7] for OSPFv2 support as well. Dynamic IPsec rule configuration The routing module SHOULD be able to configure, modify and delete IPsec rules on the fly. This is needed mainly for securing virtual links. Encapsulation of ESP packet IP encapsulation of ESP packets MUST be supported. For simplicity, UDP encapsulation of ESP packets SHOULD NOT be used. Different SAs for different Differentiated Services Code Points (DSCPs) As per [N3], the IPsec implementation MUST support the establishment and maintenance of multiple SAs with the same selectors between a given sender and receiver. This allows the implementation to associate different classes of traffic with the same selector values in support of Quality of Service (QoS). 7. Key Management OSPFv2 exchanges both multicast and unicast packets. While running OSPFv2 over a broadcast interface, the authentication/confidentiality required is "one to many". Since IKE is based on the Diffie-Hellman key agreement protocol and works only for two communicating parties, it is not possible to use IKE for providing the required "one to many" authentication/confidentiality. This specification mandates the usage of Manual Keying with current IPsec implementations. Future specifications can explore the usage of protocols like Kerberized Internet Negotiation of Keys/Group Secure Association Key Management Protocol (KINK/GSAKMP) when they are widely available. In manual keying, SAs are statically installed on the routers and these static SAs are used to authenticate/encrypt packets. The following discussion explains that it is not scalable and is practically infeasible to use different security associations for inbound and outbound traffic to provide the required "one to many" security. Therefore, the implementations MUST use manually configured keys with the same SA parameters (Security Parameter Index (SPI), keys etc.,) for both inbound and outbound SAs (as shown in Figure 3). M. Gupta/N. Melam Expires - April 2007 [Page 5]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 A | SAa ------------>| SAb <------------| | B | SAb ------------>| SAa <------------| Figure: 1 | C | SAa/SAb ------------>| SAa/SAb <------------| | Broadcast Network If we consider communication between A and B in Figure 1, everything seems to be fine. A uses security association SAa for outbound packets and B uses the same for inbound packets and vice versa. Now if we include C in the group and C sends a packet using SAa, then only A will be able to understand it. Similarly, if C sends a packet using SAb, then only B will be able to understand it. Since the packets are multicast and they are going to be processed by both A and B, there is no SA for C to use so that both A and B can understand them. A | SAa ------------>| SAb <------------| SAc <------------| | B | SAb ------------>| SAa <------------| Figure: 2 SAc <------------| | C | SAc ------------>| SAa <------------| SAb <------------| | Broadcast Network The problem can be solved by configuring SAs for all the nodes on every other node as shown in Figure 2. So A, B, and C will use SAa, M. Gupta/N. Melam Expires - April 2007 [Page 6]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 SAb, and Sac, respectively, for outbound traffic. Each node will lookup the SA to be used based on the source (A will use SAb and SAc for packets received from B and C, respectively). This solution is not scalable and practically infeasible because a large number of SAs will need to be configured on each node. Also, the addition of a node in the broadcast network will require the addition of another SA on every other node. A | SAo ------------>| SAi <------------| | B | SAo ------------>| SAi <------------| Figure: 3 | C | SAo ------------>| SAi <------------| | Broadcast Network The problem can be solved by using the same SA parameters (SPI, Keys, etc.) for both inbound (SAi) and outbound (SAo) SAs as shown in Figure 3. 8. SA Granularity and Selectors The user SHOULD be given the choice of sharing the same SA among multiple interfaces or using a unique SA per interface. 9. Virtual Links A different SA than the SA of the underlying interface MUST be provided for virtual links. The source IP address of the OSPF packets sent over the virtual links does not belong to the same subnet as the interface running OSPFv2. The source IP address of all the other OSPF packets, however, lies in the same subnet. This difference in the IP source address differentiates the packets sent on virtual links from other OSPFv2 interface types. As the virtual link end point IP addresses are not known, it is not possible to install SPD/Security Association Database (SAD) entries at the time of configuration. The virtual link end point IP addresses are learned during the routing table computation process. M. Gupta/N. Melam Expires - April 2007 [Page 7]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 The packet exchange over the virtual links starts only after the discovery of the end point IP addresses. In order to protect these exchanges, the routing module must install the corresponding SPD/SAD entries before starting these exchanges. Note that manual SA parameters are preconfigured but not installed in the SAD until the end point addresses are learned. 10. Rekeying To maintain the security of a link, the authentication and encryption key values SHOULD be changed from periodically. 10.1. Rekeying Procedure The following three-step procedure SHOULD be provided to rekey the routers on a link without dropping OSPFv2 protocol packets or disrupting the adjacency. (1) For every router on the link, create an additional inbound SA for the interface being rekeyed using a new SPI and the new key. (2) For every router on the link, replace the original outbound SA with one using the new SPI and key values. The SA replacement operation should be atomic with respect to sending OSPFv2 packets on the link so that no OSPFv2 packets are sent without authentication/encryption. (3) For every router on the link, remove the original inbound SA. Note that all routers on the link must complete step 1 before any begin step 2. Likewise, all the routers on the link must complete step 2 before any begin step 3. One way to control the progression from one step to the next is for each router to have a configurable time constant KeyRolloverInterval. After the router begins step 1 on a given link, it waits for this interval and then moves to step 2. Likewise, after moving to step 2, it waits for this interval and then moves to step 3. In order to achieve smooth key transition, all routers on a link should use the same value for KeyRolloverInterval and should initiate the key rollover process within this time period. At the end of this procedure, all the routers on the link will have a single inbound and outbound SA for OSPFv2 with the new SPI and key values. 10.2. KeyRolloverInterval M. Gupta/N. Melam Expires - April 2007 [Page 8]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 The configured value of KeyRolloverInterval should be long enough to allow the administrator to change keys on all the OSPFv2 routers. As this value can vary significantly depending upon the implementation and the deployment, it is left to the administrator to choose the appropriate value. 10.3. Rekeying Interval This section analyzes the security provided by manual keying and recommends that the encryption and authentication keys SHOULD be changed at least every 90 days. The weakest security provided by the security mechanisms discussed in this specification is when NULL encryption (for ESP) or no encryption (for AH) is used with the HMAC-MD5 authentication. Any other algorithm combinations will at least be as hard to break as the ones mentioned above. This is shown by the following reasonable assumptions: o NULL Encryption and HMAC-SHA-1 Authentication will be more secure as HMAC-SHA-1 is considered to be more secure than HMAC-MD5. o NON-NULL Encryption and NULL Authentication combination is not applicable as this specification mandates authentication when OSPFv2 security is enabled. o Data Encryption Security (DES) Encryption and HMAC-MD5 Authentication will be more secure because of the additional security provided by DES. o Other encryption algorithms like 3DES and the Advanced Encryption Standard (AES) will be more secure than DES. RFC 3562 [I4] analyzes the rekeying requirements for the TCP MD5 signature option. The analysis provided in RFC 3562 is also applicable to this specification as the analysis is independent of data patterns. 11. IPsec rules The following set of transport mode rules can be installed in the SPD to provide the authentication/confidentiality to OSPFv2 packets. Outbound Rules for interfaces running OSPFv2 security: No. source destination protocol action 1 intfPrefix any OSPF apply Outbound Rules for virtual links running OSPFv2 security: M. Gupta/N. Melam Expires - April 2007 [Page 9]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 No. source destination protocol action 2 src/32 dst/32 OSPF apply Inbound Rules for interfaces running OSPFv2 security: No. source destination protocol action 3 intfPrefix any ESP/OSPF or AH/OSPF apply 4 intfPrefix any OSPF drop Inbound Rules for virtual links running OSPFv2 security: No. source destination protocol action 5 src/32 dst/32 ESP/OSPF or AH/OSPF apply 6 src/32 dst/32 OSPF drop "intfPrefix" means the prefix of the interface that OSPFv2 is running on. For example, if the IP address of the interface where OSPFv2 is configured is 10.1.1.5/24, the value of "intfPrefix" would be "10.1.1.0/24". For outbound rules, action "apply" means encrypting/calculating ICV and adding an ESP or AH header. For inbound rules, action "apply" means decrypting/authenticating the packets and stripping the ESP or AH header. Rules 4 and 6 are to drop the insecure OSPFv2 packets without ESP/AH headers. ESP/OSPF or AH/OSPF in rules 3 and 5 mean that it is an OSPF packet secured with ESP or AH. Rules 1, 3 and 4 are meant to secure the unicast and multicast OSPF packets that are not being exchanged over the virtual links. These rules MUST only be installed in the security policy database (SPD) of the interface running OSPFv2 security. Rules 2, 5 and 6 are meant to secure the packets being exchanged over virtual links. These rules are installed after learning the virtual link end point IPv6 addresses. These rules MUST be installed on at least the interfaces that are connected to the transit area for the virtual link. These rules MAY alternatively be installed on all the interfaces. If these rules are not installed on all the interfaces, clear text or malicious OSPFv2 packets with the same source and destination addresses as the virtual link end point IPv6 addresses will be delivered to OSPFv2. Though OSPFv2 drops these packets because they were not received on the right interface, OSPFv2 receives some clear text or malicious packets even when the security M. Gupta/N. Melam Expires - April 2007 [Page 10]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 is enabled. Installing these rules on all the interfaces insures that OSPFv2 does not receive these clear text or malicious packets when security is turned enabled. On the other hand, installing these rules on all the interfaces increases the processing overhead on the interfaces where there is no other IPsec processing. The decision of installing these rules on all the interfaces or on just the interfaces that are connected to the transit area is a private decision and doesn't affect the interoperability in any way. Hence it is an implementation choice. 12. Entropy of Manual Keys The implementations MUST allow the administrator to configure the cryptographic and authentication keys in hexadecimal format rather than restricting it to a subset of ASCII characters (letters, numbers etc.). A restricted character set will reduce key entropy significantly as discussed in [I2]. 13. Replay Protection Since it is not possible using the current standards to provide complete replay protection while using manual keying, the proposed solution will not provide protection against replay attacks. Detailed analysis of various vulnerabilities of the routing protocols and OSPF in particular is discussed in [I3] and [I2]. The conclusion is that replay of OSPF packets can cause adjacencies to be disrupted, which can lead to a DoS attack on the network. It can also cause database exchange process to occur continuously thus causing CPU overload as well as micro loops in the network. Security Considerations This memo discusses the use of IPsec AH and ESP headers in order to provide security to OSPFv2 for IPv6. Hence, security permeates throughout this document. OSPF Security Vulnerabilities Analysis [I2] identifies OSPF vulnerabilities in two scenarios -- one with no authentication or simple password authentication and the other with cryptographic authentication. The solution described in this specification provides protection against all the vulnerabilities identified for scenarios with cryptographic authentication with the following exceptions: Limitations of manual key: This specification mandates the usage of manual keys. The following are the known limitations of the usage of manual keys. M. Gupta/N. Melam Expires - April 2007 [Page 11]
Internet Draft Authentication/Confidentiality for OSPFv2 Oct 2006 o As the sequence numbers cannot be negotiated, replay protection can not be provided. This leaves OSPF insecure against all the attacks that can be performed by replaying OSPF packets. o Manual keys are usually long lived (changing them often is a tedious task). This gives an attacker enough time to discover the keys. o As the administrator is manually configuring the keys, there is a chance that the configured keys are weak (there are known weak keys for DES/3DES at least). Impersonating attacks: The usage of the same key on all the OSPF routers connected to a link leaves them all insecure against impersonating attacks if any one of the OSPF routers is compromised, malfunctioning or misconfigured. Detailed analysis of various vulnerabilities of routing protocols is discussed in [I3]. IANA Considerations This document has no IANA considerations. This section should be removed by the RFC Editor to final publication. Normative References [N1] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. [N2] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6", RFC 2740, December 1999. [N3] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [N4] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [N5] Kent, S., "IP Authentication Header", RFC 4302, December 2005. [N6] Eastlake 3rd, D., "Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH)", RFC 4305, December 2005. M. Gupta/N. Melam Expires - April 2007 [Page 12]
Internet Draft Authentication/Confidentiality to OSPFv2 Oct 2006 [N7] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Informative References [I1] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. [I2] Jones, E. and O. Moigne, "OSPF Security Vulnerabilities Analysis", Work in Progress. [I3] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to Routing Protocols", Work in Progress. [I4] Leech, M., "Key Management Considerations for the TCP MD5 Signature Option", RFC 3562, July 2003. [I5] Gupta, M. and N. Melam, "Authentication/Confidentiality for OSPFv3", RFC 4552, June 2006. Acknowledgments This document is widely derived from Authentication/Confidentiality to OSPFv3 [I5]. Authors' Addresses Mukesh Gupta Tropos Networks 555 Del Rey Ave Sunnyvale, CA 94085 Phone: 408-331-6889 EMail: mukesh.gupta@tropos.com Nagavenkata Suresh Melam Juniper Networks 1194 N. Mathilda Ave Sunnyvale, CA 94089 Phone: 408-505-4392 EMail: nmelam@juniper.net Full Copyright Statement This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. M. Gupta/N. Melam Expires - April 2007 [Page 13]
Internet Draft Authentication/Confidentiality to OSPFv2 Oct 2006 This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. M. Gupta/N. Melam Expires - April 2007 [Page 14]