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Versions: 00 01 02 03 04 05 06 07 08 09 10 11              Informational
OPsec Working Group                                         M. Behringer
Internet-Draft                                                 E. Vyncke
Intended status: Informational                                     Cisco
Expires: March 29, 2015                               September 25, 2014

        Using Only Link-Local Addressing Inside an IPv6 Network


   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 help the decision
   process for a given network.

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
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   This Internet-Draft will expire on March 29, 2015.

Copyright Notice

   Copyright (c) 2014 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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Using Link-Local Addressing on Infrastructure Links . . . . .   2
     2.1.  The Approach  . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Advantages  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Caveats . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Internet Exchange Points  . . . . . . . . . . . . . . . .   6
     2.5.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  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, that 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 traditionally used techniques to operate a network
   such as pinging interfaces, or seeing interface information in a
   traceroute do not work with this approach.  Details are discussed

   During WG and IETF last call the technical correctness of the
   document has been 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 (LLA) 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 greater than link-local scope addresses 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.

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2.1.  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...) is required for routers.  Therefore, another interface
   must be configured with an IPv6 address with 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 [RFC3704].  This is
   implementation dependent.  For the remainder of this document we will
   refer to this interface as a "loopback interface".

   [RFC6724] recommends that greater than link-local scope IPv6
   addresses are 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 in [RFC4861] section 4.5.

   The effect on specific traffic types is as follows:

   o  Most control plane protocols, such as BGP [RFC4271], ISIS [IS-IS],
      OSPFv3 [RFC5340], RIPng [RFC2080], 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 SSH [RFC4251], Telnet [RFC0495],
      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

   o  ICMP error messages are usually sourced from a loopback interface
      with a greater than link-local address scope.  [RFC4861] section
      4.5 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

   o  Neighbor discovery (neighbor solicitation and neighbor
      advertisement) is done by using link-local unicast and multicast
      addresses.  Therefore neighbor discovery is not affected.

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   We therefore 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: 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

   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 greater than link-local addresses and are
   aggregatable, and if the infrastructure access list covers that
   entire aggregated space.  See also [RFC6752] for further discussion
   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, however it works only for IPv6.

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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.  Also here, [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; because 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 EUI-64 based, hence, they
   change when the MAC address is changed.  This could pose 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 greater than link-local scope addresses,
   because LLAs are inherently ambiguous for a multi-link node such as a
   router; to deal with the 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 interface IP address of a router to carry out any
   of its NMS functions, then it would no longer work if the interface

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   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
   protocol LLDP (IEEE 802.1AB-2009) runs directly over Ethernet and
   does not require any IPv6 address, so dynamic network discovery is
   not hindered when using LLDP.  But, network discovery based on 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-TE [RFC3209] allow establishing an MPLS 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 Re-Route (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.

   Apart from general device security guidelines, there are generally
   two additional ways to raise security (see also

   1.  Not to announce the prefix in question, and

   2.  To drop all traffic from remote locations destined to the IXP

   Not announcing the prefix of the IXP would frequently result in
   traceroute and similar packets (required for PMTUD) to be 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

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   all IXP routers are reachable from at least that service provider

   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 his 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

   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

   Using exclusively link-local addressing on infrastructure links has a
   number of advantages and disadvantages, which are both described in
   detail in this document.  A network operator can use this document to
   evaluate whether using link-local addressing on infrastructure links
   is a good idea in the context of his/her network or not.  This
   document makes no particular recommendation either in favour or

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

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   as hop-limit expiration or packets containing a hop-by-hop extension

   For additional security considerations, as previously stated, see
   also [RFC5837] and [I-D.ietf-opsec-bgp-security].

4.  IANA Considerations

   There are no IANA considerations or implications that arise from this

5.  Acknowledgements

   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.

6.  Informative References

              Durand, J., Pepelnjak, I., and G. Doering, "BGP operations
              and security", draft-ietf-opsec-bgp-security-05 (work in
              progress), August 2014.

   [IS-IS]    ISO/IEC 10589, , "Intermediate System to Intermediate
              System Intra-Domain Routing Exchange Protocol for use in
              Conjunction with the Protocol for Providing the
              Connectionless-mode Network Service (ISO 8473)", June

   [RFC0495]  McKenzie, A., "Telnet Protocol specifications", RFC 495,
              May 1973.

   [RFC1157]  Case, J., Fedor, M., Schoffstall, M., and J. Davin,
              "Simple Network Management Protocol (SNMP)", STD 15, RFC
              1157, May 1990.

   [RFC2080]  Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080,
              January 1997.

   [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.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, March 2004.

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   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              March 2005.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [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.

   [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.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, August 2007.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008.

   [RFC5837]  Atlas, A., Bonica, R., Pignataro, C., Shen, N., and JR.
              Rivers, "Extending ICMP for Interface and Next-Hop
              Identification", RFC 5837, April 2010.

   [RFC6192]  Dugal, D., Pignataro, C., and R. Dunn, "Protecting the
              Router Control Plane", RFC 6192, March 2011.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012.

   [RFC6752]  Kirkham, A., "Issues with Private IP Addressing in the
              Internet", RFC 6752, September 2012.

   [RFC6860]  Yang, Y., Retana, A., and A. Roy, "Hiding Transit-Only
              Networks in OSPF", RFC 6860, January 2013.

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Authors' Addresses

   Michael Behringer
   Building D, 45 Allee des Ormes
   Mougins  06250

   Email: mbehring@cisco.com

   Eric Vyncke
   De Kleetlaan, 6A
   Diegem  1831

   Email: evyncke@cisco.com

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