Internet Engineering Task Force M. Smith
Internet-Draft IMOT
Updates: 4861 (if approved) October 13, 2012
Intended status: Standards Track
Expires: April 16, 2013
Mitigating IPv6 Router Neighbor Cache DoS Using Stateless Neighbor
Discovery
draft-smith-6man-mitigate-nd-cache-dos-slnd-01
Abstract
The IPv6 neighbor discovery cache is vulnerable to a Denial of
Service attack that purposely exhausts the state used during the
neighbor discovery address resolution process. This attack can be
very disruptive when the target is a router. This memo proposes a
stateless form of neighbor discovery to be used by routers to
mitigate this attack. It does not require any changes to hosts.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 16, 2013.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Stateless Neighbor Discovery . . . . . . . . . . . . . . . . . 4
3.1. SLND Variables . . . . . . . . . . . . . . . . . . . . . . 4
3.2. SLND Process . . . . . . . . . . . . . . . . . . . . . . . 5
4. Consequences of Stateless Neighbor Discovery . . . . . . . . . 6
4.1. Neighbor Advertisement Validation . . . . . . . . . . . . 6
4.2. Optimisation Functions . . . . . . . . . . . . . . . . . . 7
5. Trusted/Untrusted Source Prefix List . . . . . . . . . . . . . 8
5.1. Configured Trusted and Untrusted Prefixes . . . . . . . . 8
5.2. Routing Information . . . . . . . . . . . . . . . . . . . 8
5.3. Default to Untrusted . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Change Log [RFC Editor please remove] . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
The IPv6 neighbor discovery cache [RFC4861] is vulnerable to a Denial
of Service attack that purposely exhausts the state used during the
neighbor discovery address resolution process [RFC3756].
When a router is the target of this attack, an off-link attacker
sends traffic towards many non-existent addresses within a prefix
attached to the router. This causes the router to create neighbor
cache state for neighbor solicitations for these non-existent
addresses. The denial of service occurs when the router's neighbor
cache state capacity is exhausted due to too many outstanding neighor
solicitations.
Sizing a prefix proportional to the number of attached hosts, rather
than using the standard /64 prefix size [RFC4291], would mitigate
this attack. However, operational conveniences and benefits such as
universal fixed length prefixes and interface identifiers, Stateless
Address Autoconfiguration (SLAAC) [RFC4862] and privacy addresses
[RFC4941], and never having to resize the prefix or add secondary
prefixes to attach more hosts to the link would be lost.
This memo proposes a stateless form of neighbor discovery to prevent
this type of DoS attack on a router. It does not require any changes
to the operation of neighbor discovery on hosts. It takes advantage
of hosts' ability to recover from packet loss in the network,
necessary due to IPv6's best effort nature. This method would be
used for unknown or untrusted packet sources, when the router's
neighbor cache's state capacity reaches a medium to high threshold of
use, suggesting a neighbor cache DoS attack is occuring. Trusted
packet sources would continue to be provided with traditional
stateful neighbor discovery.
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 RFC 2119 [RFC2119].
2. Terminology
Stateful Neighbor Discovery (SFND): Traditional neighbor discovery,
as specified in [RFC4861]. This form of neighbor discovery maintains
per packet destination state for all unresolved destinations during
the neighbor discovery process. The neighbor cache's state capacity
is intentionally exhausted to cause the neighbor cache Denial of
Service attack.
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Stateless Neighbor Discovery (SLND): The form of neighbor discovery
described in this memo. This form of neighbor discovery does not
maintain per packet destination state for unresolved destinations
during the neighbor discovery process.
3. Stateless Neighbor Discovery
3.1. SLND Variables
To perform stateless neighbor discovery, four variables are
maintained:
SLND Flag - This flag indicates whether or not the interface will
perform SLND if necessary.
SLDN Activate Threshold - This variable specifies the threshold when
stateless neighbor discovery is activated. The threshold specifies a
neighbor cache utilisation level. It is expressed as a percentage,
with a default value of 80%. It may be either a per-interface or
router global variable depending on whether the router implementation
has per-interface neighbor caches or a global neighbor cache used by
all interfaces.
SLND Active Flag - This flag indicates whether or not the interface
is performing SLND for untrusted packet sources. It is maintained
for each interface on the router.
Trusted/Untrusted Sources Prefix List ("TUSP List") - This variable
specifies a list of trusted and/or untrusted packet source address
prefixes. It is a per-interface variable, as different interfaces on
the router may have different sets of trusted and/or untrusted packet
sources. A router may also maintain a single global TUSP List, used
by interfaces that don't have an interface specific TUSP List.
SLND Neighbor Solicitation Rate Limit ("SLND NS Rate Limit") - This
variable specifies a threshold for multicast Neighbor Solictiations
when the interface is performing SLND, specified in packets per
second. It is a per-interface attribute, as different interfaces may
have different thresholds. The rate value should be an appropriate
portion of the multicast packet per second capabilities of the
interface link technology, such as 10%. A router may maintain a
global threshold that is applied to interfaces that do not have an
interface specific rate limit.
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3.2. SLND Process
The stateless neighbor discovery process may occur once a router has
determined the outgoing interface for a packet, and that the packet's
destination is on-link.
If the packet's destination address is present in the neighbor cache,
and the link-layer address has been resolved, the packet is forwarded
to it's destination.
If the packet's destination address is not present in the neighbor
cache, and the SLND Flag is off, traditional stateful neighbor
discovery is performed for the packet's destination.
If the packet's destination address is not present in the neighbor
cache, and the SLND Flag is on, the packet's source address is
compared to the TUSP List.
If the packet's source address is determined to be trusted,
traditional stateful neighbor discovery is performed.
If the packet's source address is determined to be untrusted,
stateless neighbor discovery is performed. The stateless neighbor
discovery process is as follows:
1. The router determines if sending a multicast neighbor
solicitation would exceed the SLND NS Rate Limit for the outgoing
interface. If the SLND NS Rate Limit would be exceeded, drop the
packet and do not proceed any further.
2. A multicast neighbor solicitation is sent by the router for the
destination address in the packet. The packet is then dropped.
3. As some later point in time, the router is likely to receive a
unicast neighbor advertisement, for a previously sent neighbor
solicitation.
4. If the SLND Active Flag is off, the router ignores the neighbor
advertisement.
5. If the SLND Active Flag is on, the router creates an entry in
it's neighbor cache using the information received in the unicast
neighbor advertisement. Stateless neighbor discovery is now
complete.
The utilisation of the neighbor cache has to be measured to determine
if it crosses the SLDN Activate Threshold. If the utilisation
increases above the SLDN Activate Threshold, the SLND Active Flag is
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switched on, and if it decreases below the SLDN Activate Threshold,
the SLND Active Flag is switched off. Neighbor cache utlisation
should be measured and compared to the SLDN Activate Threshold when:
o entries are added to the neighbor cache, during either stateful or
stateless neighbor discovery
o entries are removed from the neighbor cache when NUD discovers the
neighbor has become unreachable
4. Consequences of Stateless Neighbor Discovery
During traditional stateful neighbor discovery, state is used to
perform the following:
o ensure a received neighbor advertisement corresponds to a
previously sent neighbor solicitation
o to retransmit a limited number of neighbor solicitations if
previous solicitations remain unanswered
o to store a small number of packets that triggered the neighbor
discovery process, so that they can be transmitted if neighbor
discovery completes successfully
o to generate an ICMPv6 destination unreachable, address unreachble
messages back to the packet source, should the neighbor discovery
process fail
Stateless neighbor discovery sacrifices these functions and the
related state to mitigate the neighbor cache DoS attack.
4.1. Neighbor Advertisement Validation
Ensuring received neighbor advertisements correspond to previously
sent neighbor solicitations prevents on-link nodes from sending
unsolicited neighbor advertisements to the router, and then having
them added to the router's neighbor cache without validation. This
would allow on-link hosts to perform a neighbor cache DoS attack, as
they could send many neighbor advertisements for non-existent
addresses within the link assigned prefixes, exhausting the neighbor
cache capacity.
If neighbor advertisement validation occurs, then the router is
vulnerable to an off-link sourced neighbor cache DoS attack, but is
not vulnerable to an on-link sourced neighbor cache DoS attack. If
neighbor advertisement validation does not occur, then the router is
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vulnerable to an on-link sourced neighbore cache DoS attack, but is
now not vulnerable to an off-link sourced neighbor cache DoS attack.
Considering that on-link nodes will usually have a vested interest in
the router continuing to operate, that there will be a much smaller
set of on-link sources, and that they'll be far better known and
possibly access controlled, the likelihood of an on-link sourced
neighbor cache DoS is much lower than an off-link sourced neighbor
cache DoS. It is therefore beneficial to sacrifice on-link neighbor
cache DoS protection to gain off-link neighbor cache DoS protection.
Also note that during the stateless neighbor discovery process
proposed in this memo, neighbor advertisement validation is only
sacrificed when an off-link sourced neighbor cache DoS appears to be
taking place. Under normal circumstances on-link sourced neighbor
advertisement validation continues to occur.
4.2. Optimisation Functions
The nature of IPv6 is best effort, meaning that there is a
possibility that packets may be lost as they transit the network, and
that IPv6 will not make any attempt to recover lost packets. If an
application residing on an IPv6 node requires reliable packet
delivery, it will need to utilise locally implemented reliable upper
layer protocols such as TCP and SCTP, or implement it's own
reliability mechanisms. These reliability mechanisms involve
retransmitting packets. Alternatively, the application needs to
accept the possibility of packet loss.
The remaining uses of stateful neighbor discovery state are not
assured of success. The limited number of neighbor solicitation
retransmissions may not be enough, causing neighbor discovery to fail
even though the target node exists. There may be more packets sent
that trigger neighbor discovery than are stored for transmission when
neighbor discovery completes successfully, causing them to be
dropped. The ICMPv6 destination unreachable message may be dropped
on the way back to the traffic originating node, perhaps
intentionally by a network located firewall.
This means that these functions are useful but not essential
optimisations. If necessary, they do not need to be performed, as
the packet source will retransmit it's packets, reinitiating the
neighbor discovery process, or accept that packet loss has occured.
This provides the opportunity to perform a stateless form of neighbor
discovery if there is evidence that a neighbor cache DoS attack is
occuring, mitigating the off-link sourced neighbor cache DoS attack.
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5. Trusted/Untrusted Source Prefix List
As previously described, the Trusted/Untrused Source Prefix list
(TUSP List) is used to determine whether a packet source is trusted
or untrusted, with trusted sources continuing to receive traditional
stateful neighbor discovery services, and untrusted hosts receiving
stateless neighbor discovery services.
For routers where it may not be operationally convenient or possible
to implement comprehensive trusted and untrusted packet source
selection, such as on low-end or embedded devices, it would be
acceptable to consider all packet sources untrusted when stateless
neighbor discovery is active.
For routers that can support more comprehensive trusted and untrusted
packet source selection, the following information sources can be
used to construct the trusted/untrusted source prefix list (TUSP
List).
5.1. Configured Trusted and Untrusted Prefixes
The first TUSP List source is an operator configured list of prefixes
and their lengths, each with a flag indicating whether traffic with
source addresses that falls within the specified prefix is from a
trusted or untrusted source.
How this list is evaluated would be implementation dependent, however
it is likely to be either sequential from first to last entry, or
using a longest match algorithm.
This list should have a default entry of the ULA prefix (fc00::/7)
[RFC4193], flagged as a trusted source. An implementation must allow
this entry to be removed.
5.2. Routing Information
The second TUSP List source is the network's routing information.
The network's routing information can be used to distinguish trusted
and untrusted packet sources. An advantage of using routing
information for this purpose is that it will typically be dynamically
and automatically distributed to all routers within the network, when
dynamic routing protocols are used. This avoids individual routers
in the network having to be manually reconfigured wht trusted
prefixes when subnets are added or removed from the network.
The contents of a stub network's route table is typically all the
internal routes for the network, and then a default route used to
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reach the Internet. The list of internal routes can be used to
distinguish between trusted and untrusted sources, with packet
sources matching internal routes being trusted, and all other packet
sources being untrusted.
In more complex routing environments, such as those using one or more
IGPs and an EGP such as BGP, there may be other methods available to
distinguish between trusted and untrusted sources. For example,
routes carried in an IGP could be considered trusted, while routes
carried in BGP are untrusted. For a network using BGP to carry all
reachability information, except network transit and loopback
interface routes, routes may be tagged with one or more BGP
communities which indicate internal and therefore trusted prefixes.
A default route should never be used to define a trusted packet
source prefix. If a router's operator wishes to trust all packet
sources, they should specify ::/0 as a configured trusted prefix.
Implementations should provide convenient methods to use the
network's routing information to distinguish between trusted and
untrusted packet source prefixes.
5.3. Default to Untrusted
Finally, should none of the previous trusted or untrusted source
prefix information sources match the source address of traffic that
would trigger neighbor discovery, the packet source should be
considered untrusted.
6. Acknowledgements
Review and comments were provided by Ray Hunter and Matthew Moyle-
Croft.
This memo was prepared using the xml2rfc tool.
7. Security Considerations
This memo proposes a security mitigation for an off-link sourced
neighbor cache Denial of Service attack, aimed at a router.
As discussed in Section 4.1, the method proposed creates an
opportunity for an on-link sourced neighbor cache DoS attack, when
mitigating the off-link sourced neighbor cache DoS. This is
considered to be an acceptable security trade-off.
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8. Change Log [RFC Editor please remove]
draft-smith-6man-mitigate-nd-cache-dos-slnd-00, initial version,
2012-09-04
draft-smith-6man-mitigate-nd-cache-dos-slnd-01, more clarity, 2012-
10-13
o more comprehensive introduction (problem definition) text
o make it more obvious that hosts don't need to be changed
o low-end/embedded hosts can consider all packet sources untrusted
o misc. minor text updates
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
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Author's Address
Mark Smith
In My Own Time
PO BOX 521
HEIDELBERG, VIC 3084
AU
Email: markzzzsmith@yahoo.com.au
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