Operational Security Capabilities for M. Behringer
IP Network Infrastructure E. Vyncke
Internet-Draft Cisco
Intended status: Informational September 21, 2012
Expires: March 25, 2013
Using Only Link-Local Addressing Inside an IPv6 Network
draft-ietf-opsec-lla-only-01
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. Using Link-Local Address on Infrastructure Links . . . . . . . 3
2.1. The Approach . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Advantages . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Caveats and Possible Workarounds . . . . . . . . . . . . . 5
2.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
An infrastructure link between a set of routers typically does not
require global or even unique local addressing [RFC4193]. Using
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.
We propose to configure neither globally routable IPv6 addresses nor
unique local addresses on infrastructure links of routers, wherever
possible. We recommend to use exclusively link-local addresses on
such links.
This document discusses the advantages and caveats of this approach.
Note: [I-D.ietf-ospf-prefix-hiding] 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.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119] when
they appear in ALL CAPS. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.
2. Using Link-Local Address on Infrastructure Links
This document proposes to use only link-local addresses (LLA) on all
router interfaces on infrastructure links. Routers typically do not
need to be reached from nodes of the network, nor from outside the
network. For an network operator there may be reasons to send
packets to an infrastructure link for certain monitoring tasks; many
of those tasks could also be handled differently, not requiring
routable address space on infrastructure links.
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.
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These link-local addresses SHOULD be hard-coded to prevent the change
of EUI-64 addresses when changing of MAC address (such as after
changing a network interface card).
ICMPv6 [RFC4443] error messages (packet-too-big, time-exceeded...)
are required for routers, therefore a loopback interface MUST be
configured with an IPv6 address with a greater scope than link-local
(this will usually be a global scope). This greater-than-link scope
IPv6 address MUST be used as the source IPv6 address for all
generated ICMPv6 messages sent to a non-link-local address and MUST
belong to the operator to avoid being dropped by other routers
implementing [RFC3704].
The effect on specific traffic types is as follows:
o Control plane protocols, such as BGP, ISIS, OSPFv3, RIPng, PIM
work by default or can be configured to work with link-local
addresses.
o Management plane traffic, such as SSH, Telnet, SNMP, ICMP echo
request ... can be addressed to loopback addresses of routers with
a greater than link-local scope address. Router management can
also be done over out-of-band channels.
o ICMP error message can also be sourced from the loopback 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.
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 loopback address per router, it is smaller than in 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 are 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.
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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, or he can
intent SSH brute force password attacks. If a network only uses
loopback addresses for the routers, only those loopback addresses
need to be protected from outside the network. This significantly
eases protection measures, such as infrastructure access control
lists. See also [I-D.ietf-grow-private-ip-sp-cores] for further
discussion on this topic.
Lower configuration complexity: LLAs require no specific
configuration, thereby lowering the complexity and size of router
configurations. This also reduces the likelihood of configuration
mistakes.
Simpler DNS: Less greater-than-link-local address space in use also
means less DNS mappings to maintain because DNS is not really
suitable to contain link-local addresses as DNS has no clue to the
link scope.
2.3. Caveats and Possible Workarounds
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 little implemention of
this ICMP extension. With this approach it would be possible to ping
a router on the loopback address, 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.
Traceroute: Similar to the ping case, a reply to a traceroute packet
would come from a loopback address with a greater than link-local
address. Today this does 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
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used to configure any required static routing neighborship.
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 loopback address of the router instead.
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 FRR. However, if an interface uses only a
link-local address, then such LSPs can not be established. A
possible workaround is to use loose sequence of IP prefixes or
addresses (each pertaining to a router) to identify an explicit path
along with shared-risk-link-group (to not use a set of common
interfaces).
2.4. Summary
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: loopback addresses 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 proposal does not impact the security of
the data plane. This proposal 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.
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5. Acknowledgements
The authors would like to thank Salman Asadullah, Brian Carpenter,
Benoit Claise, Simon Eng, Wes George, Janos Mohacsi, Alvaro Retana
for their useful comments about this work.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2. Informative References
[I-D.ietf-grow-private-ip-sp-cores]
Kirkham, A., "Issues with Private IP Addressing in the
Internet", draft-ietf-grow-private-ip-sp-cores-07 (work in
progress), July 2012.
[I-D.ietf-ospf-prefix-hiding]
Yang, Y., Retana, A., and A. Roy, "Hiding Transit-only
Networks in OSPF", draft-ietf-ospf-prefix-hiding-05 (work
in progress), July 2012.
[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.
[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.
[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.
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Authors' Addresses
Michael Behringer
Cisco
400 Avenue Roumanille, Bat 3
Biot, 06410
France
Email: mbehring@cisco.com
Eric Vyncke
Cisco
De Kleetlaan, 6A
Diegem, 1831
Belgium
Email: evyncke@cisco.com
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