Internet DRAFT - draft-ietf-opsec-lla-only
draft-ietf-opsec-lla-only
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
draft-ietf-opsec-lla-only-11
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 March 29, 2015.
Copyright Notice
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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
below.
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
channels.
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
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.
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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
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 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
[I-D.ietf-opsec-bgp-security]):
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 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
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 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
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
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
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
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as hop-limit expiration or packets containing a hop-by-hop extension
header).
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
document.
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
[I-D.ietf-opsec-bgp-security]
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
1992.
[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
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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