OSPF Working Group Y. Yang Internet-Draft Cisco Systems Intended status: Standards Track A. Retana Expires: November 2, 2012 Hewlett-Packard Co. A. Roy Cisco Systems May 1, 2012 Hiding Transit-only Networks in OSPF Abstract A transit-only network is defined as a network connecting routers only. In OSPF, transit-only networks are usually configured with routable IP addresses, which are advertised in Link State Advertisements (LSAs) but not needed for data traffic. In addition, remote attacks can be launched against routers by sending packets to these transit-only networks. This document presents a mechanism to hide transit-only networks to speed up network convergence and minimize remote attack vulnerability. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on November 2, 2012. Yang, et al. Expires November 2, 2012 [Page 1] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Yang, et al. Expires November 2, 2012 [Page 2] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements notation . . . . . . . . . . . . . . . . . . . 3 2. Hiding IPv4 Transit-only Networks in OSPFv2 . . . . . . . . . . 4 2.1. Point-to-Point Networks . . . . . . . . . . . . . . . . . . 4 2.1.1. Advertising Point-to-Point Networks . . . . . . . . . 4 2.1.2. Hiding Point-to-Point Networks . . . . . . . . . . . . 5 2.2. Broadcast Networks . . . . . . . . . . . . . . . . . . . . 5 2.2.1. Advertising Broadcast Networks . . . . . . . . . . . . 5 2.2.2. Hiding Broadcast Networks . . . . . . . . . . . . . . 6 2.2.2.1. Sending Network-LSA . . . . . . . . . . . . . . . 6 2.2.2.2. Receiving Network-LSA . . . . . . . . . . . . . . 6 2.2.2.2.1. Backward Compatibility . . . . . . . . . . . 6 2.3. Non-Broadcast Networks . . . . . . . . . . . . . . . . . . 7 2.3.1. NBMA . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3.2. Point-to-MultiPoint . . . . . . . . . . . . . . . . . 7 2.3.2.1. Advertising Point-to-MultiPoint Networks . . . . 8 2.3.2.2. Hiding Point-to-MultiPoint Networks . . . . . . . 8 3. Hiding IPv6 Transit-only Networks in OSPFv3 . . . . . . . . . . 9 4. Hiding AF Enabled Transit-only Networks in OSPFv3 . . . . . . . 9 5. Forwarding Address . . . . . . . . . . . . . . . . . . . . . . 10 6. Virtual Links . . . . . . . . . . . . . . . . . . . . . . . . 10 7. Operational Considerations . . . . . . . . . . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . 11 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 11.1. Normative References . . . . . . . . . . . . . . . . . . 11 11.2. Informative References . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction A transit-only network is defined as a network connecting routers only. In OSPF, transit-only networks are usually configured with routable IP addresses, which are advertised in LSAs but not needed for data traffic. In addition, remote attacks can be launched against routers by sending packets to these transit-only networks. This document presents a mechanism to hide transit-only networks to speed up network convergence and minimize remote attack vulnerability. Hiding transit-only networks will not impact reachability to the end hosts. 1.1. Requirements notation Yang, et al. Expires November 2, 2012 [Page 3] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 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 [KEYWORD]. 2. Hiding IPv4 Transit-only Networks in OSPFv2 In [OSPFv2], networks are classified as point-to-point, broadcast, or non-broadcast. In the following sections, we will review how these OSPF networks are being advertised and discuss how to hide them consequently. 2.1. Point-to-Point Networks A point-to-point network joins a single pair of routers. Figure 1 shows a point-to-point network connecting routers RT1 and RT2. +---+.1 10.1.1.0/30 .2+---+ |RT1|---------------------------|RT2| +---+ +---+ Figure 1 Physical point-to-point network 2.1.1. Advertising Point-to-Point Networks For each numbered point-to-point network, a router has 2 link descriptions in its router-LSA, one Type 1 link (point-to-point) regarding the neighboring router, and one Type 3 link (stub) regarding the assigned IPv4 address. An example of router-LSA originated by RT1 would look like LS age = 0 ;newly (re)originated LS type = 1 ;router-LSA Link State ID = 1.1.1.1 ;RT1's Router ID Advertising Router = 1.1.1.1 ;RT1's Router ID #links = 2 Link ID = 2.2.2.2 ;RT2's Router ID Link Data = 10.1.1.1 ;Interface IP address Type = 1 ;connects to RT2 Metric = 10 Link ID= 10.1.1.0 ;IP network/subnet number Link Data = 255.255.255.252 ;Subnet's mask Type = 3 ;Connects to stub network Metric = 10 Yang, et al. Expires November 2, 2012 [Page 4] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 The Type 1 link will be used for SPF calculation while the Type 3 link will be used for Routing Information Base (RIB) installation. 2.1.2. Hiding Point-to-Point Networks To hide a transit-only point-to-point network, the Type 3 link MUST be omitted from the router-LSA. An example of router-LSA originated by RT1, hiding the point-to-point network depicted in Figure 1, would look like LS age = 0 ;newly (re)originated LS type = 1 ;router-LSA Link State ID = 1.1.1.1 ;RT1's Router ID Advertising Router = 1.1.1.1 ;RT1's Router ID #links = 1 Link ID = 2.2.2.2 ;RT2's Router ID Link Data = 10.1.1.1 ;Interface IP address Type = 1 ;connects to RT2 Metric = 10 2.2. Broadcast Networks A broadcast networks joins many (more than two) routers, and supports the capability to address a single physical message to all of the attached routers. Figure 2 shows a broadcast network connecting router RT3, RT4, and RT5. +---+ +---+ |RT3| |RT4| +---+ +---+ |.3 10.2.2.0/24 .4| +---------------------------+ |.5 +---+ |RT5| +---+ Figure 2 Broadcast network 2.2.1. Advertising Broadcast Networks For each broadcast network, a designated router (DR) describes it in its network-LSA. Assuming RT3 is elected as the DR in Figure 2, an example of the network-LSA originated by RT3 would look like Yang, et al. Expires November 2, 2012 [Page 5] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 LS age = 0 ;newly (re)originated LS type = 2 ;network-LSA Link State ID = 10.2.2.3 ;IP address of the DR (RT3) Advertising Router = 3.3.3.3 ;RT3's Router ID Network Mask = 255.255.255.0 Attached Router = 3.3.3.3 ;Router ID Attached Router = 4.4.4.4 ;Router ID Attached Router = 5.5.5.5 ;Router ID OSPF obtains the IP network number from the combination of the Link State ID and the Network Mask. In addition, the Link State ID is also being used for 2-way connectivity check. 2.2.2. Hiding Broadcast Networks 2.2.2.1. Sending Network-LSA To hide a transit-only broadcast network, a special network mask value 255.255.255.255 MUST be used in the network-LSA. While a broadcast network connects more than routers, using 255.255.255.255 will not hide an access broadcast network accidentally. As there is no change of the Link State ID, the 2-way connectivity check would proceed normally. An example of network-LSA originated by RT3, hiding the broadcast network depicted in Figure 2, would look like LS age = 0 ;newly (re)originated LS type = 2 ;network-LSA Link State ID = 10.2.2.3 ;IP address of the DR (RT3) Advertising Router = 3.3.3.3 ;RT3's Router ID Network Mask = 255.255.255.255 ;special subnet mask Attached Router = 3.3.3.3 ;Router ID Attached Router = 4.4.4.4 ;Router ID Attached Router = 5.5.5.5 ;Router ID 2.2.2.2. Receiving Network-LSA It's RECOMMENDED that all routers in an area be upgraded at the same time to process the modified network-LSA correctly and consistently. When a router receives a network-LSA, it MUST check the 2-way connectivity as normal. However, if the network mask in the network- LSA is 255.255.255.255, the router MUST NOT install the route in the RIB. 2.2.2.2.1. Backward Compatibility Yang, et al. Expires November 2, 2012 [Page 6] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 When a not-yet-upgraded router receives a modified network-LSA, as specified in section 2.2.2.1, a host route to the originating DR will be installed. This is not ideal but better than the current result, which exposes the whole subnet. In a partial deployment scenario, upgraded routers and not-yet- upgraded routers may coexist. The former do not have the host routes aforementioned, while the latter do have. Such inconsistencies create routing black holes, which should normally be avoided. In this case, however, as packets destined for the transit-only networks are dropped somewhere in the network, the black holes actually help DRs defend from the remote attacks. In summary, the modification of the network-LSA, as specified in section 2.2.2.1, is backward compatible with the current specification of [OSPFv2], even in a partial deployment scenario. 2.3. Non-Broadcast Networks A non-broadcast networks joins many (more than two) routers, but does NOT support the capability to address a single physical message to all of the attached routers. As mentioned in [OSPFv2], OSPF runs in one of two modes over non-broadcast networks: Non-Broadcast Multi- Access (NBMA) or Point-to-MultiPoint. 2.3.1. NBMA In NBMA mode, OSPF emulates operation over a broadcast network: a Designated Router is elected for the NBMA network, and the Designated Router originates an LSA for the network. To hide a NBMA transit-only network, OSPF adopts the same modification over the broadcast transit-only network, as defined in section 2.2.2. 2.3.2. Point-to-MultiPoint In point-to-MultiPoint mode, OSPF treats the non-broadcast network as a collection of point-to-point links. Figure 3 shows a non-broadcast network connecting router RT6, RT7, RT8, and RT9. In this network, all routers can communicate directly, except for routers RT7 and RT8. Yang, et al. Expires November 2, 2012 [Page 7] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 +---+ +---+ |RT6| |RT7| +---+ +---+ |.6 10.3.3.0/24 .7| +---------------------------+ |.8 .9| +---+ +---+ |RT8| |RT9| +---+ +---+ Figure 3 Non-Broadcast network 2.3.2.1. Advertising Point-to-MultiPoint Networks For a point-to-multipoint network, a router has multiple link descriptions in its router-LSA, one Type 1 link (point-to-point) for EACH directly communicable router, and one Type 3 link (stub) advertising its interface IPv4 address with a subnet mask of 255.255.255.255. An example of router-LSA originated by RT7 would look like LS age = 0 ;newly (re)originated LS type = 1 ;router-LSA Link State ID = 7.7.7.7 ;RT7's Router ID Advertising Router = 7.7.7.7 ;RT7's Router ID #links = 3 Link ID = 6.6.6.6 ;RT6's Router ID Link Data = 10.3.3.7 ;Interface IP address Type = 1 ;connects to RT6 Metric = 10 Link ID = 9.9.9.9 ;RT9's Router ID Link Data = 10.3.3.7 ;Interface IP address Type = 1 ;connects to RT9 Metric = 10 Link ID= 10.3.3.7 ;Interface IP address Link Data = 255.255.255.255 ;Subnet's mask Type = 3 ;Connects to stub network Metric = 0 2.3.2.2. Hiding Point-to-MultiPoint Networks To hide a transit-only point-to-multipoint network, the Type 3 link MUST be omitted from the router-LSA. Yang, et al. Expires November 2, 2012 [Page 8] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 An example of router-LSA originated by RT7, hiding the point-to- point network depicted in Figure 3, would look like LS age = 0 ;newly (re)originated LS type = 1 ;router-LSA Link State ID = 7.7.7.7 ;RT7's Router ID Advertising Router = 7.7.7.7 ;RT7's Router ID #links = 2 Link ID = 6.6.6.6 ;RT6's Router ID Link Data = 10.3.3.7 ;Interface IP address Type = 1 ;connects to RT6 Metric = 10 Link ID = 9.9.9.9 ;RT9's Router ID Link Data = 10.3.3.7 ;Interface IP address Type = 1 ;connects to RT9 Metric = 10 3. Hiding IPv6 Transit-only Networks in OSPFv3 In [OSPFv3], addressing semantics have been removed from the OSPF protocol packets and the main LSA types, leaving a network-protocol- independent core. More specifically, router-LSAs and network-LSAs no longer contain network addresses, but simply express topology information. Instead, two new LSA types, link-LSA and intra-area-prefix-LSA, have been introduced. A link-LSA associates a list of IPv6 address to a link and has local-link flooding scope, and an intra-area-prefix-LSA either associates a list of IPv6 address with a router by referencing a router-LSA, or associates a list of IPv6 address with a broadcast/NBMA network by referencing a network-LSA. In the latter case, the prefixes in the link-LSAs from adjacent neighbors are copied into the intra-area-prefix-LSA by the Designated Router. To hide a transit-only network in [OSPFv3], the associated IPv6 address prefixes MUST be omitted from the link-LSA. Consequently, when a Designated Router builds an intra-area-prefix-LSA referencing a network-LSA, these IPv6 address prefixes will be omitted. In addition, when a router builds an intra-area-prefix-LSA that is referencing a router-LSA, the associated IPv6 address prefixes of the transit-only network MUST also be omitted from the intra-area-prefix- LSA. 4. Hiding AF Enabled Transit-only Networks in OSPFv3 Yang, et al. Expires November 2, 2012 [Page 9] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 [OSPF-AF] supports multiple Address Families (AFs) by mapping each AF to a separate Instance ID and OSPFv3 instance. In the meantime, each prefix advertised in OSPFv3 has a prefix Length field [OSPFV3], which facilitates advertising prefixes of different lengths in different AFs. The existing LSAs defined in OSPFv3 are used for prefix advertising and there is no need to define new LSAs. In other words, as link-LSAs and intra-area-prefix-LSAs are still being used, same method as explained in section 3 can be used to hide those AF enabled transit-only networks as well. 5. Forwarding Address Non-zero forwarding address can be advertised in As-external-LSAs and NSSA-LSAs to achieve optimal routing to AS external routes. The matching routing table entry of the forwarding address must exist to facilitate the SPF calculation. In other words, when prefix-hiding is configured on the next-hop interface, the next-hop interface address MUST NOT be used as forwarding address. Consequently, sub-optimal routing to these AS external routes may exist when prefix-hiding is configured. 6. Virtual Links Virtual links are used to connect physically separate components of the backbone. The virtual link's viability is determined by the existence of an intra-area path between two endpoints. The matching routing table entries of the endpoints must exist to ensure the virtual link's operation. In other words, if prefix-hiding is configured on an interface, the virtual link endpoint MUST NOT use that interface's IP address as the virtual interface's IP address. 7. Operational Considerations By eliminating the ability to reach transit-only networks, the ability to manage these interfaces may be reduced. In order to not reduce the functionality and capability of the overall network, it is recommended that extensions such as RFC5837 be also implemented. Yang, et al. Expires November 2, 2012 [Page 10] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 8. Security Considerations One motivation for this document is to reduce remote attack vulnerability by hiding transit-only networks. The result should then be that fewer OSPF core networks will be exposed to un- authorized access. While the steps described in this document are meant to be applied to transit-only networks ONLY, they could be used to hide other networks as well. It is expected that the same care that users put on the configuration of other routing protocol parameters is used in the configuration of this extension. 9. IANA Considerations No actions are required from IANA as result of the publication of this document. 10. Acknowledgments The draft text was produced using Stefan Santesson's NroffEdit application. The idea of using a special subnet mask to hide broadcast networks in OSPF was originally introduced in the US patent "Apparatus and method to hide transit only multi-access networks in OSPF" (patent number: 7,929,524), by Yi Yang, Alvaro Retana, James Ng, Abhay Roy, Alfred Lindem, Sina Mirtorabi, Timothy Gage, and Khalid Raza. The authors would like to thank Acee Lindem, Shraddha Hegde, Rajesh Shetty, Marek Karasek, Michael Barnes, and Paull Wells, for their feedback on the document. 11. References 11.1. Normative References [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [NSSA] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option", RFC 3101, January 2003. [OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998. Yang, et al. Expires November 2, 2012 [Page 11] Internet-Draft Hiding Transit-only Networks in OSPF May 1, 2012 [OSPFv3] Coltun, R., Ferguson, D., Moy, J., and A. Lindem , "OSPF for IPv6", RFC 5340, July 2008. [OSPF-AF] Lindem, A., Mirtorabi, S., Roy, A., Barnes, M., and R. Aggarwal, "Support of Address Families in OSPFv3", RFC5838, April 2010. 11.2. Informative References [RFC5837] Atlas, A., Bonica, R., Pignataro, C., Shen, N., and JR. Rivers, "Extending ICMP for Interface and Next-Hop Identification", RFC5837, April 2010. Authors' Addresses Yi Yang Cisco Systems 7025 Kit Creek Road RTP, NC 27709 USA Email: yiya@cisco.com Alvaro Retana Hewlett-Packard Co. 2610 Wycliff Road Raleigh, NC 27607 USA Email: alvaro.retana@hp.com Abhay Roy Cisco Systems 225 West Tasman Drive San Jose, CA 95134 USA Email: akr@cisco.com Yang, et al. Expires November 2, 2012 [Page 12]