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Internet Engineering Task Force (IETF)                      M. BehringerRequest for Comments: 7404                                     E. VynckeCategory: Informational                                            CiscoISSN: 2070-1721                                            November 2014Using Only Link-Local Addressing inside an IPv6 NetworkAbstract   In an IPv6 network, it is possible to use only link-local addresses   on infrastructure links between routers.  This document discusses the   advantages and disadvantages of this approach to facilitate the   decision process for a given network.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7404.Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Behringer & Vyncke            Informational                     [Page 1]

RFC 7404                     Link-Local Only               November 2014Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Using Link-Local Addressing on Infrastructure Links . . . . .22.1.  The Approach  . . . . . . . . . . . . . . . . . . . . . .32.2.  Advantages  . . . . . . . . . . . . . . . . . . . . . . .42.3.  Caveats . . . . . . . . . . . . . . . . . . . . . . . . .52.4.  Internet Exchange Points  . . . . . . . . . . . . . . . .62.5.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .73.  Security Considerations . . . . . . . . . . . . . . . . . . .84.  Informative References  . . . . . . . . . . . . . . . . . . .8   Acknowledgments   . . . . . . . . . . . . . . . . . . . . . . . .10   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .101.  Introduction   An infrastructure link between a set of routers typically does not   require global or unique local addresses [RFC4193].  Using only link-   local addressing on such links has a number of advantages; for   example, routing tables do not need to carry link addressing and can   therefore be significantly smaller.  This helps to decrease failover   times in certain routing convergence events.  An interface of a   router is also not reachable beyond the link boundaries, therefore   reducing the attack surface.   This document discusses the advantages and caveats of this approach.   Note that some traditional techniques used to operate a network, such   as pinging interfaces or seeing interface information in a   traceroute, do not work with this approach.  Details are discussed   below.   During WG and IETF last call, the technical correctness of the   document was reviewed; however, debate exists as to whether to   recommend this technique.  The deployment of this technique is   appropriate where it is found to be necessary.2.  Using Link-Local Addressing on Infrastructure Links   This document discusses the approach of using only link-local   addresses (LLAs) on all router interfaces on infrastructure links.   Routers don't typically need to receive packets from hosts or nodes   outside the network.  For a network operator, there may be reasons to   use addresses that are greater than link-local scope on   infrastructure interfaces for certain operational tasks, such as   pings to an interface or traceroutes across the network.  This   document discusses such cases and proposes alternative procedures.Behringer & Vyncke            Informational                     [Page 2]

RFC 7404                     Link-Local Only               November 20142.1.  The Approach   In this approach, neither globally routed IPv6 addresses nor unique   local addresses are configured on infrastructure links.  In the   absence of specific global or unique local address definitions, the   default behavior of routers is to use link-local addresses, notably   for routing protocols.   The sending of ICMPv6 [RFC4443] error messages ("packet-too-big",   "time-exceeded", etc.) is required for routers.  Therefore, another   interface must be configured with an IPv6 address that has a greater   scope than link-local.  This address will usually be a loopback   interface with a global scope address belonging to the operator and   part of an announced prefix (with a suitable prefix length) to avoid   being dropped by other routers implementing ingress filtering   [RFC3704].  This is implementation dependent.  For the remainder of   this document, we will refer to this interface as a "loopback   interface".   [RFC6724] recommends that IPv6 addresses that are greater than link-   local scope be used as the source IPv6 address for all generated   ICMPv6 messages sent to a non-link-local address, with the exception   of ICMPv6 redirect messages (as defined inSection 4.5 of [RFC4861]).   The effect on specific traffic types is as follows:   o  Most control plane protocols (such as BGP [RFC4271], IS-IS      [IS-IS], OSPFv3 [RFC5340], Routing Information Protocol Next      Generation (RIPng) [RFC2080], and PIM [RFC4609]) work by default      or can be configured to work with link-local addresses.      Exceptions are explained in the caveats section (Section 2.3).   o  Management plane traffic (such as Secure SHell (SSH) Protocol      [RFC4251], Telnet [RFC0495], Simple Network Management Protocol      (SNMP) [RFC1157], and ICMPv6 Echo Request [RFC4443]) can use the      address of the router loopback interface as the destination      address.  Router management can also be done over out-of-band      channels.   o  ICMP error messages are usually sourced from a loopback interface      with a scope that is greater than link-local.Section 4.5 of      [RFC4861] explains one exception: ICMP redirect messages can also      be sourced from a link-local address.   o  Data plane traffic is forwarded independently of the link address      type.Behringer & Vyncke            Informational                     [Page 3]

RFC 7404                     Link-Local Only               November 2014   o  Neighbor discovery (neighbor solicitation and neighbor      advertisement) is done by using link-local unicast and multicast      addresses.  Therefore, neighbor discovery is not affected.   Thus, we conclude that it is possible to construct a working network   in this way.2.2.  Advantages   The following list of advantages is in no particular order.   Smaller routing tables: Since the routing protocol only needs to   carry one global address (the loopback interface) per router, it is   smaller than the traditional approach where every infrastructure link   address is carried in the routing protocol.  This reduces memory   consumption and increases the convergence speed in some routing   failover cases.  Because the Forwarding Information Base to be   downloaded to line cards is smaller, and there are fewer prefixes in   the Routing Information Base, the routing algorithm is accelerated.   Note that smaller routing tables can also be achieved by putting   interfaces in passive mode for the Interior Gateway Protocol (IGP).   Simpler address management: Only loopback interface addresses need to   be considered in an addressing plan.  This also allows for easier   renumbering.   Lower configuration complexity: Link-local addresses require no   specific configuration, thereby lowering the complexity and size of   router configurations.  This also reduces the likelihood of   configuration mistakes.   Simpler DNS: Less routable address space in use also means less   reverse and forward mapping DNS resource records to maintain.  Of   course, if the operator selects not to enter any global interface   addresses in the DNS anyway, then this is less of an advantage.   Reduced attack surface: Every routable address on a router   constitutes a potential attack point; a remote attacker can send   traffic to that address, for example, a TCP SYN flood (see   [RFC4987]).  If a network only uses the addresses of the router   loopback interface(s), only those addresses need to be protected from   outside the network.  This may ease protection measures, such as   Infrastructure Access Control Lists (iACL).  Without using link-local   addresses, it is still possible to achieve the simple iACL if the   network addressing scheme is set up such that all link and loopback   interfaces have addresses that are greater than link-local and are   aggregatable, and if the infrastructure access list covers that   entire aggregated space.  See also [RFC6752] for further discussionBehringer & Vyncke            Informational                     [Page 4]

RFC 7404                     Link-Local Only               November 2014   on this topic.  [RFC6860] describes another approach to hide   addressing on infrastructure links for OSPFv2 and OSPFv3 by modifying   the existing protocols.  This document does not modify any protocol   and applies only to IPv6.2.3.  Caveats   The caveats listed in this section are in no particular order.   Interface ping: If an interface doesn't have a routable address, it   can only be pinged from a node on the same link.  Therefore, it is   not possible to ping a specific link interface remotely.  A possible   workaround is to ping the loopback address of a router instead.  In   most cases today, it is not possible to see which link the packet was   received on; however, [RFC5837] suggests including the interface   identifier of the interface a packet was received on in the ICMPv6   response.  It must be noted that there are few implementations of   this ICMPv6 extension.  With this approach, it would be possible to   ping a router on the addresses of loopback interfaces, yet see which   interface the packet was received on.  To check liveliness of a   specific interface, it may be necessary to use other methods, such as   connecting to the router via SSH and checking locally or using SNMP.   Traceroute: Similar to the ping case, a reply to a traceroute packet   would come from the address of a loopback interface, and current   implementations do not display the specific interface the packets   came in on.  Again, [RFC5837] provides a solution.  As in the ping   case above, it is not possible to traceroute to a particular   interface if it only has a link-local address.  Conversely, this   approach may make network topology discovery from outside the network   simpler: instead of responding with multiple different interface IP   addresses, which have to be correlated by the outsider, a router will   always respond with the same loopback address.  If reverse DNS   mapping is used, the mapping is trivial in either case.   Hardware dependency: LLAs have usually been based on 64-bit Extended   Unique Identifiers (EUI-64); hence, they change when the Message   Authentication Code (MAC) address is changed.  This could pose a   problem in a case where the routing neighbor must be configured   explicitly (e.g., BGP) and a line card needs to be physically   replaced, hence changing the EUI-64 LLA and breaking the routing   neighborship.  LLAs can be statically configured, such as fe80::1 and   fe80::2, which can be used to configure any required static routing   neighborship.  However, this static LLA configuration may be more   complex to operate than statically configured addresses that are   greater than link-local scope.  This is because LLAs are inherently   ambiguous.  For a multi-link node, such as a router, to deal with theBehringer & Vyncke            Informational                     [Page 5]

RFC 7404                     Link-Local Only               November 2014   ambiguity, the link zone index must also be considered explicitly,   e.g., using the extended textual notation described in [RFC4007], as   in this example, 'BGP neighbor fe80::1%eth0 is down'.   Network Management System (NMS) toolkits: If there is any NMS tool   that makes use of an interface IP address of a router to carry out   any of its NMS functions, then it would no longer work if the   interface does not have a routable address.  A possible workaround   for such tools is to use the routable address of the router loopback   interface instead.  Most vendor implementations allow the   specification of loopback interface addresses for SYSLOG, IPFIX, and   SNMP.  The Link Layer Discovery Protocol (LLDP) (IEEE 802.1AB-2009)   runs directly over Ethernet and does not require any IPv6 address, so   dynamic network discovery is not hindered by using only LLA when   using LLDP.  But, network discovery based on Neighbor Discovery   Protocol (NDP) cache content will only display the link-local   addresses and not the addresses of the loopback interfaces;   therefore, network discovery should rather be based on the Route   Information Base to detect adjacent nodes.   MPLS and RSVP-Traffic Engineering (RSVP-TE) [RFC3209] allow the   establishment of an MPLS Label Switched Path (LSP) on a path that is   explicitly identified by a strict sequence of IP prefixes or   addresses (each pertaining to an interface or a router on the path).   This is commonly used for Fast Reroute (FRR).  However, if an   interface uses only a link-local address, then such LSPs cannot be   established.  At the time of writing this document, there is no   workaround for this case; therefore, where RSVP-TE is being used, the   approach described in this document does not work.2.4.  Internet Exchange Points   Internet Exchange Points (IXPs) have a special importance in the   global Internet because they connect a high number of networks in a   single location and because a significant part of Internet traffic   passes through at least one IXP.  An IXP requires, therefore, a very   high level of security.  The address space used on an IXP is   generally known, as it is registered in the global Internet Route   Registry, or it is easily discoverable through traceroute.  The IXP   prefix is especially critical because practically all addresses on   this prefix are critical systems in the Internet.Behringer & Vyncke            Informational                     [Page 6]

RFC 7404                     Link-Local Only               November 2014   Apart from general device security guidelines, there are basically   two additional ways to raise security (see also [BGP-OPSEC]):   1.  Not to announce the prefix in question, and   2.  To drop all traffic from remote locations destined to the IXP       prefixes.   Not announcing the prefix of the IXP would frequently result in   traceroute and similar packets (required for Path MTU Discovery   (PMTUD)) being dropped due to unicast Reverse Path Forwarding (uRPF)   checks.  Given that PMTUD is critical, this is generally not   acceptable.  Dropping all external traffic to the IXP prefix is hard   to implement because if only one service provider connected to an IXP   does not filter correctly, then all IXP routers are reachable from at   least that service provider network.   As the prefix used in the IXP is usually longer than a /48, it is   frequently dropped by route filters on the Internet having the same   net effect as not announcing the prefix.   Using link-local addresses on the IXP may help in this scenario.  In   this case, the generated ICMPv6 packets would be generated from   loopback interfaces or from any other interface with a globally   routable address without any configuration.  However, in this case,   each service provider would use their own address space, making a   generic attack against all devices on the IXP harder.  All of an   IXP's loopback interface addresses can be discovered by a potential   attacker with a simple traceroute; a generic attack is, therefore,   still possible, but it would require more work.   In some cases, service providers carry the IXP addresses in their IGP   for certain forms of traffic engineering across multiple exit points.   Link-local addresses cannot be used for this purpose; in this case,   the service provider would have to employ other methods of traffic   engineering.   If an Internet Exchange Point is using a global prefix registered for   this purpose, a traceroute will indicate whether the trace crosses an   IXP rather than a private interconnect.  If link-local addressing is   used instead, a traceroute will not provide this distinction.2.5.  Summary   Exclusively using link-local addressing on infrastructure links has a   number of advantages and disadvantages, both of which are described   in detail in this document.  A network operator can use this document   to evaluate whether or not using link-local addressing onBehringer & Vyncke            Informational                     [Page 7]

RFC 7404                     Link-Local Only               November 2014   infrastructure links is a good idea in the context of his/her   network.  This document makes no particular recommendation either in   favor or against.3.  Security Considerations   Using only LLAs on infrastructure links reduces the attack surface of   a router.  Loopback interfaces with routed addresses are still   reachable and must be secured, but infrastructure links can only be   attacked from the local link.  This simplifies security of control   and management planes.  The approach does not impact the security of   the data plane.  The link-local-only approach does not address   control plane [RFC6192] attacks generated by data plane packets (such   as hop-limit expiration or packets containing a hop-by-hop extension   header).   For additional security considerations, as previously stated, see   also [RFC5837] and [BGP-OPSEC].4.  Informative References   [BGP-OPSEC]              Durand, J., Pepelnjak, I., and G. Doering, "BGP operations              and security", Work in Progress,draft-ietf-opsec-bgp-security-05, August 2014.   [IS-IS]    International Organization for Standardization,              "Intermediate System to Intermediate System intra-domain              routeing information exchange protocol for use in              conjunction with the protocol for providing the              connectionless-mode network service (ISO 8473)", ISO              Standard 10589, 2002.   [RFC0495]  McKenzie, A., "Telnet Protocol specifications",RFC 495,              May 1973, <http://www.rfc-editor.org/info/rfc0495>.   [RFC1157]  Case, J., Fedor, M., Schoffstall, M., and J. Davin,              "Simple Network Management Protocol (SNMP)", STD 15,RFC1157, May 1990, <http://www.rfc-editor.org/info/rfc1157>.   [RFC2080]  Malkin, G. and R. Minnear, "RIPng for IPv6",RFC 2080,              January 1997, <http://www.rfc-editor.org/info/rfc2080>.   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP              Tunnels",RFC 3209, December 2001,              <http://www.rfc-editor.org/info/rfc3209>.Behringer & Vyncke            Informational                     [Page 8]

RFC 7404                     Link-Local Only               November 2014   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed              Networks",BCP 84,RFC 3704, March 2004,              <http://www.rfc-editor.org/info/rfc3704>.   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and              B. Zill, "IPv6 Scoped Address Architecture",RFC 4007,              March 2005, <http://www.rfc-editor.org/info/rfc4007>.   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast              Addresses",RFC 4193, October 2005,              <http://www.rfc-editor.org/info/rfc4193>.   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)              Protocol Architecture",RFC 4251, January 2006,              <http://www.rfc-editor.org/info/rfc4251>.   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway              Protocol 4 (BGP-4)",RFC 4271, January 2006,              <http://www.rfc-editor.org/info/rfc4271>.   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control              Message Protocol (ICMPv6) for the Internet Protocol              Version 6 (IPv6) Specification",RFC 4443, March 2006,              <http://www.rfc-editor.org/info/rfc4443>.   [RFC4609]  Savola, P., Lehtonen, R., and D. Meyer, "Protocol              Independent Multicast - Sparse Mode (PIM-SM) Multicast              Routing Security Issues and Enhancements",RFC 4609,              October 2006, <http://www.rfc-editor.org/info/rfc4609>.   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,              "Neighbor Discovery for IP version 6 (IPv6)",RFC 4861,              September 2007, <http://rfc-editor.org/info/rfc4861>.   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common              Mitigations",RFC 4987, August 2007,              <http://www.rfc-editor.org/info/rfc4987>.   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF              for IPv6",RFC 5340, July 2008,              <http://www.rfc-editor.org/info/rfc5340>.   [RFC5837]  Atlas, A., Bonica, R., Pignataro, C., Shen, N., and JR.              Rivers, "Extending ICMP for Interface and Next-Hop              Identification",RFC 5837, April 2010,              <http://www.rfc-editor.org/info/rfc5837>.Behringer & Vyncke            Informational                     [Page 9]

RFC 7404                     Link-Local Only               November 2014   [RFC6192]  Dugal, D., Pignataro, C., and R. Dunn, "Protecting the              Router Control Plane",RFC 6192, March 2011,              <http://www.rfc-editor.org/info/rfc6192>.   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,              "Default Address Selection for Internet Protocol Version 6              (IPv6)",RFC 6724, September 2012,              <http://www.rfc-editor.org/info/rfc6724>.   [RFC6752]  Kirkham, A., "Issues with Private IP Addressing in the              Internet",RFC 6752, September 2012,              <http://www.rfc-editor.org/info/rfc6752>.   [RFC6860]  Yang, Y., Retana, A., and A. Roy, "Hiding Transit-Only              Networks in OSPF",RFC 6860, January 2013,              <http://www.rfc-editor.org/info/rfc6860>.Acknowledgments   The authors would like to thank Salman Asadullah, Brian Carpenter,   Bill Cerveny, Benoit Claise, Rama Darbha, Simon Eng, Wes George,   Fernando Gont, Jen Linkova, Harald Michl, Janos Mohacsi, Ivan   Pepelnjak, Alvaro Retana, Jinmei Tatuya, and Peter Yee for their   useful comments about this work.Authors' Addresses   Michael Behringer   Cisco   Building D, 45 Allee des Ormes   Mougins  06250   France   EMail: mbehring@cisco.com   Eric Vyncke   Cisco   De Kleetlaan, 6A   Diegem  1831   Belgium   EMail: evyncke@cisco.comBehringer & Vyncke            Informational                    [Page 10]