OPsec Working Group M. Behringer
Internet-Draft E. Vyncke
Intended status: Informational Cisco
Expires: April 23, 2014 October 20, 2013
Using Only Link-Local Addressing Inside an IPv6 Network
draft-ietf-opsec-lla-only-04
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 help the decision
process for a given network.
Status of This Memo
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This Internet-Draft will expire on April 23, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Using Link-Local Address on Infrastructure Links . . . . . . 2
2.1. The Approach . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Advantages . . . . . . . . . . . . . . . . . . . . . . . 3
2.3. Caveats . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Internet Exchange Points . . . . . . . . . . . . . . . . 5
2.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. Informative References . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
An infrastructure link between a set of routers typically does not
require global or even unique local addressing [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 horizon.
This document discusses the advantages and caveats of this approach.
Note: [RFC6860] describes another approach for OPSFv2 and OSPFv3 by
modifying the existing protocols while this document does not modify
any protocol but works only for IPv6.
2. Using Link-Local Address on Infrastructure Links
This document discusses the approach of using only link-local
addresses (LLA) on all router interfaces on infrastructure links.
Routers typically need to receive packets neither from hosts, nor
from nodes outside the network. For an network operator there may be
reasons to use greater than link-local scope addresses on
infrastructure interfaces for certain operational tasks, for example
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
Neither global 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 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]. For the remainder of this
document we will refer to this interface as a "loopback interface".
[RFC6724] mandates that greater than link-local scope IPv6 addresses
must be used as the source IPv6 address for all generated ICMPv6
messages sent to a non link-local address.
The effect on specific traffic types is as follows:
o 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.
o Management plane traffic, such as SSH [RFC4251], Telnet [RFC0495],
SNMP [RFC1157], and ICMP echo request [RFC4443], can use as
destination address the address of the router loopback interface.
Router management can also be done over out-of-band channels.
o ICMP error message can be sourced from a loopback interface. They
must not be sourced from link-local addresses when the destination
is non link-local. See [RFC6724].
o Data plane traffic is forwarded independently of the link address
type.
o Neighbor discovery (neighbor solicitation and neighbor
advertisement) is done by using link-local unicast and multicast
addresses, therefore neighbor discovery is not affected.
We therefore conclude that it is possible to construct a working
network in this way.
2.2. Advantages
Smaller routing tables: Since the routing protocol only needs to
carry one global address (the loopback interface) per router, it is
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smaller than the traditional approach where every infrastructure link
addresses are carried in the routing protocol. This reduces memory
consumption, and increases the convergence speed in some routing
failover cases (notably because the Forwarding Information Base to be
downloaded to line cards is smaller but also because there are less
prefixes in the Routing Information Base hence accelerating the
routing algorithm). Note: smaller routing tables can also be
achieved by putting interfaces in passive mode for the IGP.
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]),
or can attempt SSH brute force password attacks. If a network only
uses the addresses of the router loopback interface(s), only those
need to be protected from outside the network. This may ease
protection measures, such as infrastructure access control lists.
Without using link-local addresses, it is still possible to achieve
the same result 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.
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.
2.3. Caveats
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 [RFC5837] suggests to include the
interface identifier of the interface a packet was received on in the
ICMP response; it must be noted that there are few implementions of
this ICMP 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, for
example to connect to the router via SSH and to check locally or use
SNMP.
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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 [RFC5837] provides a solution.
Hardware dependency: LLAs are usually 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. But, LLAs can
be statically configured such as fe80::1 and fe80::2 which can be
used to configure any required static routing neighborship. This
static configuration is similar in complexity to statically
configured greater than link-local addresses, however, it is only
required where routing peers are explicitly configured.
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 NMS functions, then it would no longer work, if the interface is
missing 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 the address of
the loopback interfaces for SYSLOG, IPfix, SNMP. 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] allows establishing 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 significant part of Internet traffic
pass through at least one IXP. An IXP with all the service provider
nodes 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
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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
[I-D.ietf-opsec-bgp-security]):
1. Not to announce the prefix in question, and
2. To drop all traffic destined to the IXP prefixes from traffic
from remote locations.
Not announcing the prefix of the IXP however would frequently result
in traceroute and similar packets (required for PMTUd) to be dropped
due to 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 on an IXP
routes does not filter correctly, then all IXP routers are reachable
from at least that service provider network.
As the prefix used in IXP is usually longer than a /48 it is
frequently dropped by route filters on the Internet having the same
net effect as not announced the prefix.
Using link-local addresses on the IXP may help in this scenario. In
this case, the generated ICMP packets would be generated from
loopback interfaces or from any other interfaces with globally
routable sources 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. Also all the
addresses of the loopback interfaces on the IXP can be discovered by
a potential attacker by 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.
If link-local addresses are used, these 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
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Using link-local addressing only on infrastructure links has a number
of advantages, such as a smaller routing table size and a reduced
attack surface. It also simplifies router configurations. However,
the way certain network management tasks are carried out today has to
be adapted to provide the same level of detail, for example interface
identifiers in traceroute.
3. Security Considerations
Using LLAs only on infrastructure links reduces the attack surface of
a router: addresses of 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. This 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).
As in the traditional approach, this approach relies on the
assumption that all routers can be trusted due to physical and
operational security.
4. IANA Considerations
There are no IANA considerations or implications that arise from this
document.
5. Acknowledgements
The authors would like to thank Salman Asadullah, Brian Carpenter,
Benoit Claise, Rama Darbha, Simon Eng, Wes George, Fernando Gont,
Harald Michl, Janos Mohacsi, Alvaro Retana and Ivan Pepelnjak for
their useful comments about this work.
6. Informative References
[I-D.ietf-opsec-bgp-security]
Durand, J., Pepelnjak, I., and G. Doering, "BGP operations
and security", draft-ietf-opsec-bgp-security-01 (work in
progress), July 2013.
[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
1992.
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[RFC0495] McKenzie, A., "Telnet Protocol specifications", RFC 495,
May 1973.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[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.
[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.
[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.
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[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.
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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