IPv6 Operations T. Chown
Internet-Draft University of Southampton
Intended status: Informational S. Venaas
Expires: May 7, 2009 UNINETT
November 3, 2008
Rogue IPv6 Router Advertisement Problem Statement
draft-chown-v6ops-rogue-ra-02
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Abstract
When deploying IPv6, whether IPv6-only or dual-stack, routers are
configured to send IPv6 Router Advertisements to convey information
to nodes that enable them to autoconfigure on the network. This
information includes the implied default router address taken from
the observed source address of the Router Advertisement (RA) message.
However, unintended misconfigurations or possibly malicious attacks
on the network may lead to bogus RAs being present, which in turn can
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cause operational problems for hosts on the network. In this draft
we summarise the scenarios in which rogue RAs may be observed and
present a list of possible solutions to the problem. The goal of
this text is to be Informational, and as such to present a framework
around which solutions can be proposed and discussed.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Bogus RA Scenarios . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Administrator misconfiguration . . . . . . . . . . . . . . 3
2.2. User misconfiguration . . . . . . . . . . . . . . . . . . 4
2.3. Malicious misconfiguration . . . . . . . . . . . . . . . . 4
3. Methods to Mitigate against Rogue RAs . . . . . . . . . . . . 4
3.1. Manual configuration . . . . . . . . . . . . . . . . . . . 4
3.2. Secure Neighbor Discovery . . . . . . . . . . . . . . . . 4
3.3. Introduce RA snooping . . . . . . . . . . . . . . . . . . 5
3.4. Use ACLs on Managed Switches . . . . . . . . . . . . . . . 5
3.5. Use the Router Preference Option . . . . . . . . . . . . . 5
3.6. Rely on Layer 2 admission control . . . . . . . . . . . . 5
3.7. Use host-based packet filters . . . . . . . . . . . . . . 6
3.8. Use an 'intelligent' deprecation tool . . . . . . . . . . 6
3.9. Wait before using new advertisements . . . . . . . . . . . 6
3.10. Use Layer 2 Partitioning . . . . . . . . . . . . . . . . . 6
3.11. Add a Default Gateway Option to DHCPv6 . . . . . . . . . . 7
4. Scenarios and mitigations . . . . . . . . . . . . . . . . . . 7
5. Other related considerations . . . . . . . . . . . . . . . . . 8
5.1. Unicast RAs . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. The DHCP vs RA threat model . . . . . . . . . . . . . . . 9
5.3. IPv4-only networks . . . . . . . . . . . . . . . . . . . . 9
5.4. Network monitoring tools . . . . . . . . . . . . . . . . . 10
5.5. Recovering from bad configuration state . . . . . . . . . 10
5.6. Behaviour of a node after receiving a rogue RA . . . . . . 10
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
10. Informative References . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
The Neighbor Discovery protocol [7] describes the operation of IPv6
Router Advertisements (RAs), which are used during the IPv6
autoconfiguration process, whether a node's configuration is stateful
(via DHCPv6 [2]) or stateless (as per RFC4862 [8], possibly in
combination with DHCPv6 Light [3]).
In observing the operation of deployed IPv6 networks, it is apparent
that there is a problem with undesired or 'bogus' IPv6 Router
Advertisements (RAs) appearing on network links or subnets. By
'bogus' we mean RAs that were not the intended configured RAs, rather
RAs that have appeared for some other reason.
The problem with rogue RAs is that they can cause partial or complete
failure of operation of hosts on an IPv6 link. For example, the
default router address is drawn directly from the source address of
the RA message. As such, rogue RAs are an operational issue for
which solution(s) are required, and for which best practice needs to
be conveyed. This not only includes preventing or detecting rogue
RAs, but also where necessary the ability to quickly recover from a
state where host configuration is incorrect as a result of processing
such an RA.
In the next section, we discuss the scenarios that may give rise to
rogue RAs being present. In the following section we present some
candidate solutions for the problem, some of which may be more
practical to deploy than others.
2. Bogus RA Scenarios
There are three broad classes of scenario in which bogus RAs may be
introduced to an IPv6 network.
2.1. Administrator misconfiguration
Here an administrator incorrectly configures RAs on a router
interface, causing incorrect RAs to appear on links and hosts to
generate incorrect or unintended IPv6 address, gateway or other
information. In such a case the default gateway may be correct, but
a host might for example become multi-addressed, possibly with a
correct and incorrect address based on a correct and incorrect
prefix. There is also the possibility of other configuration
information being misconfigured, such as the lifetime option.
In the case of a Layer 2 VLAN misconfiguration, RAs may 'flood' to
unintended links, causing hosts or more than one link to potentially
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become incorrectly multiaddressed, with possibly two different
default routers available.
2.2. User misconfiguration
In this case a user's device 'accidentally' transmits RAs onto the
local link, adding an additional default gateway and associated
prefix information.
This seems to typically be seen on wireless (though sometimes wired)
networks where a laptop has enabled the Windows Internet Connection
Sharing service (ICS) which turns a host into a 6to4 [1] gateway;
this can be a useful feature, unless of course it is run when not
intended. We have had reports that hosts may not see the genuine RAs
on link due to host firewalls, and then turning on a connection
sharing service and 6to4 as a result. In some cases more technical
users may also use a laptop as a home gateway (e.g. again a 6to4
gateway) and then connect to another network with the gateway
configuration still active.
There are also reported incidents in enterprise networks of users
physically plugging Ethernet cables into the wrong sockets and
bridging two subnets together, causing an problem similar to VLAN
flooding.
2.3. Malicious misconfiguration
Here an attacker is deliberately generating RAs on the local network
in an attempt to perform some form of denial of service or man-in-
the-middle attack.
3. Methods to Mitigate against Rogue RAs
In this section we present a summary of methods suggested to date for
reducing or removing the possibility of rogue RAs being seen on a
network.
3.1. Manual configuration
The default gateway and host address can usually be manually
configured on a device. This is of course a resource intensive
solution, and also prone to administrative mistakes in itself.
3.2. Secure Neighbor Discovery
The SeND [4] protocol provides a method for hosts and routers to
perform secure Neighbor Discovery. At present there are very few
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SeND implementations available, and SeND is perceived as a complex
protocol to deploy. It is also likely that not all scenarios will be
able to use SeND, for various reasons. In particular SeND may be
best suited for fully managed enterprise networks, rather than those
where administrative control is not present.
3.3. Introduce RA snooping
It should be possible to implement 'RA snooping' in Layer 2 switches
in a similar way to DHCP snooping, such that RAs observed from
incorrect sources are blocked or dropped, and not propagated through
a subnet. One candidate solution in this space called RA-Guard [9]
has recently been proposed. This type of solution has appeal because
it is a familiar model for enterprise network managers, but it can
also be used to complement SeND.
This type of solution may not be applicable everywhere, e.g. in
environments where there are not centrally controlled switches.
3.4. Use ACLs on Managed Switches
Certain switch platforms can already implement some level of rogue RA
filtering by the administrator configuring Access Control Lists
(ACLs) that block RA ICMP messages that might be inbound on 'user'
ports. Again this type of 'solution' depends on the presence of such
configurable switches.
3.5. Use the Router Preference Option
RFC4191 [5] introduced router preference options, such that an RA
could carry one of three router preference values: High, Medium
(default) or Low. Thus an administrator could use High settings for
managed RAs, and hope that 'accidental' RAs would be medium priority.
This of course would only work in some scenarios - an attacker would
certainly seek to use High priority anyway - and would also rely on
clients having implementations of the protocol.
3.6. Rely on Layer 2 admission control
In principle, if a technology such as IEEE 802.1x is used, devices
would first need to authenticate to the network before being able to
send or receive IPv6 traffic. Ideally authentication would be
mutual.
Improving Layer 2 security may help to mitigate against a malicious
attacker's capability to join the network to send RAs, but it doesn't
prevent misconfiguration issues.
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3.7. Use host-based packet filters
In a managed environment hosts could be configured via their
'personal firewall' to only accept RAs from trusted sources. Hosts
could also potentially be configured to discard 6to4-based RAs in a
managed enterprise environment.
However, the problem is then pushed to keeping this configuration
maintained and correct. If a router fails and is replaced, possibly
with a new Layer 2 interface address, the link local source address
in the filter may become incorrect and no thus no method would be
available to push the new information to the host over the network.
3.8. Use an 'intelligent' deprecation tool
It could be possible to run a daemon on a link (perhaps on the router
on the link) to watch for incorrect RAs and to send a deprecating RA
with router lifetime of zero when such an RA is observed. The KAME
rafixd is an example of such a tool, which has been used at IETF
meetings with some success. A slightly enhanced ramond has since
been developed from this code. As with host based firewalling, the
daemon would need to somehow know what 'good' and 'bad' RAs are, from
some combination of known good sources and/or link prefixes.
In contrast, an attacker might use such a tool to learn about
prefixes being advertised on a link and to deprecate the 'good' RA(s)
in favour of their bogus RA(s).
Whether or not use of such a tool is the preferred method, monitoring
a link for observed RAs seems prudent from a network management
perspective. Some such tools exist already, e.g. ndpmon.
3.9. Wait before using new advertisements
It might be possible, in networks where configurations are very
static and systems generally remain up, to configure an option such
that any new RAs that are seen are not acted upon for a certain
period, e.g. 2 hours. This might allow time for a misconfiguration
or accidental RA to be detected and stopped, before hosts use the
data in the RA. Of course this would add delays where genuine new
RAs are required, while new hosts appearing on a network would still
be vulnerable (or be unable to configure at all).
3.10. Use Layer 2 Partitioning
If each system or user on a network is partitioned into a different
Layer 2 medium, then the impact of rogue RAs can be limited. In
broadband networks RFC1483 bridging may be available, for example.
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The benefit may be scenario-specific, e.g. whether a given user or
customer has their own network prefix or whether the provisioning is
in a shared subnet or link. It is certainly desirable that any given
user or customer's system(s) are unable to see RAs that may be
generated by other users or customers.
However, such partitioning would certainly increase address space
consumption significantly if applied in enterprise networks, and in
many cases hardware costs and software licensing costs to enable
routing to the edge can be quite significant.
3.11. Add a Default Gateway Option to DHCPv6
It may be possible to define a new Default Gateway Option for DHCPv6
that would allow network administrators to only have hosts use DHCPv6
for default gateway configuration in managed networks. While such an
option could be defined, its ramifications remain unclear, and such a
fundamental change to the IPv6 model of autoconfiguration would need
very careful consideration.
In the absence of RAs, other configuration information would also be
missing, e.g. on-link prefix information. Of course, it may be that
an RA is still required to inform the host to use DHCPv6, and that
may introduce a Catch-22 unless hosts are configured directly to only
use DHCPv6.
An advantage of DHCPv6 is that should an error be introduced, only
hosts that have refreshed their DHCP information since that time are
affected, while a multicast rogue RA will most likely affect all
hosts immediately. DHCPv6 also allows different answers to be given
to different hosts.
4. Scenarios and mitigations
In this section we summarise the scenarios and mitigations described
above in a summary matrix. We consider, for the case of a rogue
multicast RA, which of the mitigation methods protects against each
cause:
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+------------------------+-------------+-------------+-------------+
| Scenario | Admin | User | Malicious |
| Mitigation | Error | Error | Attack |
+------------------------+-------------+-------------+-------------+
| Manual configuration | Y | Y | Y |
+------------------------+-------------+-------------+-------------+
| SeND | Partly | Y | Y |
+------------------------+-------------+-------------+-------------+
| RA snooping | Y | Y | Y |
+------------------------+-------------+-------------+-------------+
| Use switch ACLs | Y | Y | Y |
+------------------------+-------------+-------------+-------------+
| Router preference | N | Y | N |
+------------------------+-------------+-------------+-------------+
| Layer 2 admission | N | N | Y |
+------------------------+-------------+-------------+-------------+
| Host firewall | Y | Y | N |
+------------------------+-------------+-------------+-------------+
| Deprecation daemon | Y | Y | Partly |
+------------------------+-------------+-------------+-------------+
| New prefix delay | Partly | Partly | Partly |
+------------------------+-------------+-------------+-------------+
| Layer 2 partition | N | Y | N |
+------------------------+-------------+-------------+-------------+
| DHCPv6 gateway option | If Auth | If Auth | If Auth |
+------------------------+-------------+-------------+-------------+
What the above summary does not consider is the practicality of
deploying the measure. An easy-to-deploy method that buys improved
resilience to rogue RAs without significant administrative overhead
is attractive. On that basis the RA snooping proposal, e.g. RA
Guard, has merit, while approaches like manual configuration and
perhaps SeND may be less appealing. However RA Guard is not yet
fully defined or available, while only certain managed switch
equipment may support the required ACLs.
Methods that involve changes to IPv6 protocols such as delays in
processing RAs or a new DHCPv6 option have rather reduced appeal,
regardless of their merits or complications.
5. Other related considerations
There are a number of related issues that have come out of
discussions on the rogue RA topic, which the authors believe are
worth capturing in this document.
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5.1. Unicast RAs
The above discussion was initially held on the assumption that rogue
multicast RAs were the cause of problems on a shared network subnet.
However, the specifications for Router Advertisements allow them to
be sent unicast to a host, as per Section 6.2.6 of RFC4861. It is
thus possible for an attacker to target one or more hosts on a shared
medium without (potentially) a rogue multicast RA being observed
elsewhere on the network (e.g. by a monitoring daemon).
5.2. The DHCP vs RA threat model
Comparing the threat model for rogue RAs and rogue DHCPv6 servers is
an interesting exercise. In the case of Windows ICS causing rogue
6to4-based RAs to appear on a network, it is very likely that the
same host is also acting as a rogue IPv4 DHCP server. The rogue
DHCPv4 server can allocate a default gateway and an address to hosts,
just as a rogue RA can lead hosts to learning of a new (additional)
default gateway and address. In the case of multicast rogue RAs
however, the impact is potentially immediate to all hosts, while the
rogue DHCP server's impact will depend on lease timers for hosts.
In principle Authenticated DHCP can be used to protect against rogue
DHCPv4 servers, just as SeND could be used to protect against rogue
IPv6 RAs. However use of Authenticated DHCP is currently minimal,
while SeND is in its infancy in implementation status.
Were a new DHCPv6 default gateway option to be defined as described
above, then without Authenticated DHCP the (lack of) security is just
pushed to another place, albeit one that site administrators are more
familiar and (rightly or wrongly) comfortable with.
The RA Guard approach is essentially using a similar model to DHCP
message snooping to protect against rogue RAs in network (switch)
equipment. It is worth noting that DHCPv6 message snooping would
also be very desirable in IPv6 networks.
5.3. IPv4-only networks
The rogue RA problem should also be considered by administrators and
operators of IPv4-only networks, where IPv6 monitoring, firewalling
and other related mechanisms may not be in place.
For example a comment has been made that in the case of 6to4 being
run by a host on a subnet that is not administratively configured
with IPv6, some OSes or applications may begin using IPv6 to the 6to4
host (router) rather than IPv4 to the intended default IPv4 router,
because they have IPv6 enabled by default and some applications
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prefer IPv6 by default. Technically aware users may also
deliberately choose to use IPv6, possibly for subversive reasons.
Mitigating against this condition can also be seen to be important.
5.4. Network monitoring tools
It would generally be prudent for network monitoring or management
platforms to be able to observe and report on observed RAs, and
whether unintended RAs (possibly from unintended sources) are present
on a network. Further, it may be useful for individual hosts to be
able to report their address status (assuming their configuration
status allowed it of course), e.g. this could be useful during an
IPv6 renumbering phased process as described in RFC4192 [6].
5.5. Recovering from bad configuration state
An important issue is how readily a host can recover from bad
configuration information, e.g. considering the '2 hour rule' of
Section 5.5.3 of RFC4862 (though this applies to the prefix lifetime
not the router lifetime). We should ensure that methods exist for a
network administrator to correct bad configuration information on a
link or subnet, and that OS platforms support these methods. At
least if the problem can be detected, and corrected promptly, the
impact is minimised.
5.6. Behaviour of a node after receiving a rogue RA
After a host receives and processes a rogue RA, it may have multiple
default gateways, global addresses, and potentially clashing RA
options (e.g. M/O bits). The host's behaviour may then be
unpredictable, in terms of the default router that is used, and the
(source) address(es) used in communications. A host that is aware of
protocols such as shim6 may believe it is genuinely multihomed.
6. Conclusions
In this text we have described scenarios via which rogue Router
Advertisements (RAs) may appear on a network, and some measures that
could be used to mitigate against these. We have also noted some
related issues that have arisen in the rogue RA discussions.
While SeND perhaps offers the most robust solution, implementations
are not widely available, and the solution is perceived as complex
(parallels can possibly be drawn with Authenticated DHCP in terms of
likely deployment). Adding a new DHCPv6 Default Gateway Option would
allow configuration by DHCP, and be a method that IPv4 administrators
are comfortable with (for better or worse), but such an option would
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have significant impacts elsewhere, and in any event one must
recognise that the security risk is then simply shifted elsewhere.
Further work on the solutions is certainly welcome. In the meantime,
the simplest initial step would be for RA snooping to be defined and
deployed for Layer 2 devices, in such a way that can address (shared)
wireless as well as wired networks. One draft proposal in this
space, RA-Guard, has recently been published [9]. Alternatively,
certain switch platforms allow configuration of Access Control Lists
(ACLs) that can achieve a similar function.
In the longer term wider experience of SeND will be beneficial, while
the use of RA snooping (and DHCPv6 snooping) will remain useful
either to complement SeND or to assist in scenarios that do not lend
themselves to SeND deployment.
7. Security Considerations
There are no extra Security consideration for this document.
8. IANA Considerations
There are no extra IANA consideration for this document.
9. Acknowledgments
Thanks are due to members of the IETF IPv6 Operations and DHCP WGs
for their inputs on this topic, as well as some comments from various
operational mailing lists, including but not limited to: Iljitsch van
Beijnum, Dale Carder, Remi Denis-Courmant, Tony Hain, Christian
Huitema, Tatuya Jinmei, David Malone, Chip Popoviciu, Dave Thaler,
Goeran Weinholt and Dan White.
10. Informative References
[1] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4
Clouds", RFC 3056, February 2001.
[2] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003.
[3] Droms, R., "Stateless Dynamic Host Configuration Protocol (DHCP)
Service for IPv6", RFC 3736, April 2004.
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[4] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[5] Draves, R. and D. Thaler, "Default Router Preferences and More-
Specific Routes", RFC 4191, November 2005.
[6] Baker, F., Lear, E., and R. Droms, "Procedures for Renumbering
an IPv6 Network without a Flag Day", RFC 4192, September 2005.
[7] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor
Discovery for IP version 6 (IPv6)", RFC 4861, September 2007.
[8] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address
Autoconfiguration", RFC 4862, September 2007.
[9] Van de Velde, G., Levy-Abegnoli, E., Popoviciu, C., and J.
Mohacsi, "IPv6 RA-Guard (draft-ietf-v6ops-ra-guard-00)",
July 2008.
Authors' Addresses
Tim Chown
University of Southampton
Southampton, Hampshire SO17 1BJ
United Kingdom
Email: tjc@ecs.soton.ac.uk
Stig Venaas
UNINETT
Trondheim NO 7465
Norway
Email: venaas@uninett.no
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