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Internet Engineering Task Force (IETF) M. Behringer
Request for Comments: 7404 E. Vyncke
Category: Informational Cisco
ISSN: 2070-1721 November 2014
Using Only Link-Local Addressing inside an IPv6 Network
Abstract
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; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://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 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.
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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 . . . . . . . . . . . . . . . . . . . 8
4. Informative References . . . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 10
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, 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.
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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", 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 in Section 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.
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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 discussion
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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 the
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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.
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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 on
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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, RFC
1157, 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>.
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[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>.
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[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.com
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