Network Working Group W. Kumari
Internet-Draft Google
Intended status: Informational I. Gashinsky
Expires: July 11, 2012 Yahoo!
J. Jaeggli
Zynga
January 8, 2012
Neighbor Discovery Enhancements for DOS mititgation
draft-gashinsky-6man-v6nd-enhance-00
Abstract
In IPv4, subnets are generally small, made just large enough to cover
the actual number of machines on the subnet. In contrast, the
default IPv6 subnet size is a /64, a number so large it covers
trillions of addresses, the overwhelming number of which will be
unassigned. Consequently, simplistic implementations of Neighbor
Discovery can be vulnerable to denial of service attacks whereby they
attempt to perform address resolution for large numbers of unassigned
addresses. Such denial of attacks can be launched intentionally (by
an attacker), or result from legitimate operational tools that scan
networks for inventory and other purposes. As a result of these
vulnerabilities, new devices may not be able to "join" a network, it
may be impossible to establish new IPv6 flows, and existing ipv6
transported flows may be interrupted.
This document describes possible modifications to the traditional
[RFC4861] neighbor discovery protocol for improving the resilience of
the neighbor discovery process as well as an alternative method for
maintaining ND caches.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 11, 2012.
Copyright Notice
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document authors. All rights reserved.
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
2. The Problem . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Neighbor Discovery Overview . . . . . . . . . . . . . . . . . 7
6. Recommendations for Implementors. . . . . . . . . . . . . . . 7
6.1. Priortize NDP Activities . . . . . . . . . . . . . . . . . 8
6.2. Queue Tuning. . . . . . . . . . . . . . . . . . . . . . . 9
6.3. ND cache priming and refresh . . . . . . . . . . . . . . . 9
6.4. NDP Protocol Gratuitous NA . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Text goes here. . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
This document describes modifications to the IPv6 Neighbor Discovery
protocol [RFC4861] in order to reduce exposure to vulnerabilities
when a network is scanned, either by an intruder, as part of a
deliberate dos attempt, or through the use of scanning tools that
peform network inventory, security audits, etc. (e.g., "nmap").
1.1. Applicability
This document is primarily intended for implementors of [RFC4861].
2. The Problem
In IPv4, subnets are generally small, made just large enough to cover
the actual number of machines on the subnet. For example, an IPv4
/20 contains only 4096 address. In contrast, the default IPv6 subnet
size is a /64, a number so large it covers literally billions of
billions of addresses, the overwhelming number of which will be
unassigned. Consequently, simplistic implementations of Neighbor
Discovery can be vulnerable to denial of service attacks whereby they
perform address resolution for large numbers of unassigned addresses.
Such denial of attacks can be launched intentionally (by an
attacker), or result from legitimate operational tools that scan
networks for inventory and other purposes. As a result of these
vulnerabilities, new devices may not be able to "join" a network, it
may be impossible to establish new IPv6 flows, and existing ipv6
transport flows may be interrupted.
Network scans attempt to find and probe devices on a network.
Typically, scans are performed on a range of target addresses, or all
the addresses on a particular subnet. When such probes are directed
via a router, and the target addresses are on a directly attached
network, the router will to attempt to perform address resolution on
a large number of destinations (i.e., some fraction of the 2^64
addresses on the subnet). The process of testing for the
(non)existance of neighbors can induce a denial of service condition,
where the number of Neighbor Discovery requests overwhelms the
implementation's capacity to process them, exhausts available memory,
replaces existing in-use mappings with incomplete entries that will
never be completed, etc. The result can be network disruption, where
existing traffic may be impacted, and devices that join the net find
that address resolutions fails.
In order to alleviate risk associated with this DOS threat, some
router implementations have taken steps to rate-limit the processing
rate of Neighbor Solicitations (NS). While these mitigations do
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help, they do not fully address the issue and may introduce their own
set of potential liabilities to the neighbor discovery process.
3. Terminology
Address Resolution Address resolution is the process through which a
node determines the link-layer address of a neighbor given only
its IP address. In IPv6, address resolution is performed as part
of Neighbor Discovery [RFC4861], p60
Forwarding Plane That part of a router responsible for forwarding
packets. In higher-end routers, the forwarding plane is typically
implemented in specialized hardware optimized for performance.
Forwarding steps include determining the correct outgoing
interface for a packet, decrementing its Time To Live (TTL),
verifying and updating the checksum, placing the correct link-
layer header on the packet, and forwarding it.
Control Plane That part of the router implementation that maintains
the data structures that determine where packets should be
forwarded. The control plane is typically implemented as a
"slower" software process running on a general purpose processor
and is responsible for such functions as the routing protocols,
performing management and resolving the correct link-layer address
for adjacent neighbors. The control plane "controls" the
forwarding plane by programming it with the information needed for
packet forwarding.
Neighbor Cache As described in [RFC4861], the data structure that
holds the cache of (amongst other things) IP address to link-layer
address mappings for connected nodes. The forwarding plane
accesses the Neighbor Cache on every forwarded packet. Thus it is
usually implemented in an ASIC .
Neighbor Discovery Process The Neighbor Discovery Process (NDP) is
that part of the control plane that implements the Neighbor
Discovery protocol. NDP is responsible for performing address
resolution and maintaining the Neighbor Cache. When forwarding
packets, the forwarding plane accesses entries within the Neighbor
Cache. Whenever the forwarding plane processes a packet for which
the corresponding Neighbor Cache Entry is missing or incomplete,
it notifies NDP to take appropriate action (typically via a shared
queue). NDP picks up requests from the shared queue and performs
any necessary actions. In many implementations it is also
responsible for responding to router solicitation messages,
Neighbor Unreachability Detection (NUD), etc.
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4. Background
Modern router architectures separate the forwarding of packets
(forwarding plane) from the decisions needed to decide where the
packets should go (control plane). In order to deal with the high
number of packets per second the forwarding plane is generally
implemented in hardware and is highly optimized for the task of
forwarding packets. In contrast, the NDP control plane is mostly
implemented in software processes running on a general purpose
processor.
When a router needs to forward an IP packet, the forwarding plane
logic performs the longest match lookup to determine where to send
the packet and what outgoing interface to use. To deliver the packet
to an adjacent node, It encapsulates the packet in a link-layer frame
(which contains a header with the link-layer destination address).
The forwarding plane logic checks the Neighbor Cache to see if it
already has a suitable link-layer destination, and if not, places the
request for the required information into a queue, and signals the
control plane (i.e., NDP) that it needs the link-layer address
resolved.
In order to protect NDP specifically and the control plane generally
from being overwhelmed with these requests, appropriate steps must be
taken. For example, the size and rate of the queue might be limited.
NDP running in the control plane of the router dequeues requests and
performs the address resolution function (by performing a neighbor
solicitation and listening for a neighbor advertisement). This
process is usually also responsible for other activities needed to
maintain link-layer information, such as Neighbor Unreachability
Detection (NUD).
An attacker sending the appropriate packets to addresses on a given
subnet can cause the router to queue attempts to resolve so many
addresses that it crowds out attempts to resolve "legitimate"
addresses (and in many cases becomes unable to perform maintenance of
existing entries in the neighbor cache, and unable to answer Neighbor
Solicitiation). This condition can result the inability to resolve
new neighbors and loss of reachability to neighbors with existing ND-
Cache entries. During testing it was concluded that 4 simultaneous
nmap sessions from a low-end computer was sufficient to make a
router's neighbor discovery process unhappy and therefore forwarding
unusable.
This behavior has been observed across multiple platforms and
implementations.
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5. Neighbor Discovery Overview
When a packet arrives at (or is generated by) a router for a
destination on an attached link, the router needs to determine the
correct link-layer address to send the packet to. The router checks
the Neighbor Cache for an existing Neighbor Cache Entry for the
neighbor, and if none exists, invokes the address resolution portions
of the IPv6 Neighbor Discovery [RFC4861] protocol to determine the
link-layer address.
RFC4861 Section 5.2 (Conceptual Sending Algorithm) outlines how this
process works. A very high level summary is that the device creates
a new Neighbor Cache Entry for the neighbor, sets the state to
INCOMPLETE, queues the packet and initiates the actual address
resolution process. The device then sends out one or more Neighbor
Solicitiations, and when it receives a correpsonding Neighbor
Advertisement, completes the Neighbor Cache Entry and sends the
queued packet.
6. Recommendations for Implementors.
The section provides some recommendations to implementors of IPv4
Neighbor Discovery.
At a high-level, implementors should program defensively. That is,
they should assume that intruders will attempt to exploit
implementation weaknesses, and should ensure that implementations are
robust to various attacks. In the case of Neighbor Discovery, the
following general considerations apply:
Manage Resources Explicitely - Resources such as processor cycles,
memory, etc. are never infinite, yet with IPv6's large subnets it
is easy to cause NDP to generate large numbers of address
resolution requests for non-existant destinations.
Implementations need to limit resources devoted to processing
Neighbor Discovery requests in a thoughtful manner.
Prioritize - Some NDP requests are more important than others. For
example, when resources are limited, responding to Neighbor
Solicitations for one's own address is more important than
initiating address resolution requests that create new entries.
Likewise, performing Neighbor Unreachability Detection, which by
definition is only invoked on destinations that are actively being
used, is more important than creating new entries for possibly
non-existant neighbors.
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6.1. Priortize NDP Activities
Not all Neighbor Discovery activies are equally important.
Specifically, requests to perform large numbers of address
resolutions on non-existant Neighbor Cache Entries should not come at
the expense of servicing requests related to keeping existing, in-use
entries properly up-to-date. Thus, implementations should divide
work activities into categories having different priorities. The
following gives examples of different activities and their importance
in rough priority order.
1. It is critical to respond to Neighbor Solicitations for one's own
address, especially when a router. Whether for address resolution or
Neighbor Unreachability Detection, failure to respond to Neighbor
Solicitations results in immediate problems. Failure to respond to
NS requests that are part of NUD can cause neighbors to delete the
NCE for that address, and will result in followup NS messages using
multicast. Once an entry has been flushed, existing traffic for
destinations using that entry can no longer be forwarded until
address resolution completes succesfully. In other words, not
responding to NS messages further increases the NDP load, and causes
on-going communication to fail.
2. It is critical to revalidate one's own existing NCEs in need of
refresh. As part of NUD, ND is required to frequently revalidate
existing, in-use entries. Failure to do so can result in the entry
being discarded. For in-use entries, discarding the entry will
almost certainly result in a subsquent request to perform address
resolution on the entry, but this time using multicast. As above,
once the entry has been flushed, existing traffic for destinations
using that entry can no longer be forwarded until address resolution
completes succesfully.
3. To maintin the stability of the control plane, Neighbor Discovery
activity related to traffic sourced by the router (as opposed to
traffic being forwarded by the router) should be given high priority.
Whenever network problems occur, debugging and making other
operational changes requires being able to query and access the
router. In addition, routing protocols may begin to react
(negatively) to perceived connectivity problems, causing addition
undesirable ripple effects.
4. Activities related to the sending and recieving of Router
Advertisements also impact address resolutions. [XXX say more?]
5. Traffic to unknown addresses should be given lowest priority.
Indeed, it may be useful to distinguish between "never seen"
addresses and those that have been seen before, but that do not have
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a corresponding NCE. Specifically, the conceptual processing
algorithm in IPv6 Neighbor Discovery [RFC4861] calls for deleting
NCEs under certain conditions. Rather than delete them completely,
however, it might be useful to at least keep track of the fact that
an entry at one time existed, in order to prioritize address
resolution requests for such neighbors compared with neighbors that
have never been seen before.
6.2. Queue Tuning.
On implementations in which requests to NDP are submitted via a
single queue, router vendors SHOULD provide operators with means to
control both the rate of link-layer address resolution requests
placed into the queue and the size of the queue. This will allow
operators to tune Neighbour Discovery for their specific environment.
The ability to set or have per interface or subnet queue limits at a
rate below that of the global queue limit might limit the damage to
the neighbor discovery process to the taret network.
Setting those values must be a very careful balancing act - the lower
the rate of entry into the queue, the less load there will be on the
ND process, however, it also means that it will take the router
longer to learn legitimate destinations. In a datacenter with 6,000
hosts attached to a single router, setting that value to be under
1000 would mean that resolving all of the addresses from an initial
state (or something that invalidates the address cache, such as a STP
TCN) may take over 6 seconds. Similarly, the lower the size of the
queue, the higher the likelihood of an attack being able to knock out
legitimate traffic (but less memory utilization on the router).
6.3. ND cache priming and refresh
With all of the above recommendations implemented, it should be
possible to survive a "scan attack" with very little impact to the
network, however, adding new hosts to the network (and the sending of
traffic to them) may still be negatively impacted. Traffic to those
new hosts would have to go through the unknown Neighbor Resolution
queue, which is where the attack traffic would end up as well. A
solution to this would be that any new host that joins the network
would "announce" itself, and be added to the cache, therefore not
requiring packets destined to it to go through the unknown NDP queue.
This could be done by sending a ping packet to the all-routers
multicast address, which would then trigger the router's own neighbor
resolution process, which should be in a different queue then other
packets.
All attempts should be made to keep these addresses in cache, since
any eviction of legitimate hosts from the cache could potentially
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place resolutions for them into the same queue as the attack traffic.
At present, [RFC4861] states that there should be MAX_UNICAST_SOLICIT
(3) attempts, RETRANS_TIMER 1 second apart, so if there is an
interruption in the network or control plane processing for longer
then 3 seconds during the refresh, the entry would be evicted from
the ND Cache. Any network event which takes longer then 3 seconds to
converge (UDLD, STP, etc may take 30+ seconds) while under an attack,
would result in ND cache eviction. If an entry is evicted during a
scan, connectivity could be lost for an extended period of time.
NDP refresh timers could be revised as suggested in
draft-nordmark-6man-impatient-nud-00 [1] and SHOULD have a
configurable value for MAX_UNICAST_SOLICIT and RETRANS_TIMER, and
include capabilities for binary/exponential backoff.
A suggested algorithm, which retains backward compatiblity with
[RFC4861] is: operator configurable values for MAX_UNICAST_SOLICIT,
RETRANS_TIMER, and a way to set adaptive back-of multiple, simmilar
to ipv4 -- call it BACKOFF_MULTIPLE), so that we could implement:
next_retrans =
($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RETRANS_TIMER + jittered
value.
The recommended behavior is to have 5 attempts, with timing spacing
of 0 (initial request), 1 second later, 3 seconds later, then 9, and
then 27, which represents:
MAX_UNICAST_SOLICIT=5
RETRANS_TIMER=1 (default)
BACKOFF_MULTIPLE=3
If BACKOFF_MULTIPLE=1 (which should be the default value), and
MAX_UNICAST_SOLICIT=3, you would get the backwards-compatible RFC
behavior, but operators should be able to adjust the values as
necessary to insure that they are sufficiently aggressive about
retaining ND entries in cache.
An Implementation following this algorithm would if the request was
not answered at first due for example to a transitory condition,
retry immediately, and then back off for progressively longer
periods. This would allow for a reasonably fast resolution time when
the transitory condition clears.
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6.4. NDP Protocol Gratuitous NA
Per RFC 4861, section 7.2.5 and 7.2.6 [RFC4861] requires that
unsolicited neighbor advertisements result in the receiver setting
it's neighbor cache entry to STALE, kicking off the resolution of the
neighbor using neighbor solicitation. If the link layer address in
an unsolicited neighbor advertisement matches that of the existing ND
cache entry, routers SHOULD retain the existing entry updating it's
status with regards to LRU retention policy.
Hosts MAY be configured to send unsolicited Neighbor advertisement at
a rate set at the discretion of the operators. The rate SHOULD be
appropriate to the sizing of ND cache parameters and the host count
on the subnet. An unsolicited NA rate parameter MUST NOT be enabled
by default. The unsolicted rate interval as interpreted by hosts
must jitter the value for the interval between transmissions. Hosts
receiving a neighbor solicitation requests from a router following
each of three subsequent gratuitous NA intervals MUST revert to RFC
4861 behavior.
Implementation of new behavior for unsolicited neighbor advertisement
would make it possible under appropriate circumstances to greatly
reduce the dependence on the neighbor solicitation process for
retaining existing ND cache entries.
This may impact the detection of one-way reachability.
It is understood that this section may need to be moved into a
separate document -- it is (currently) provided here for discussion
purposes.
7. IANA Considerations
No IANA resources or consideration are requested in this draft.
8. Security Considerations
This document outlines mitigation options that operators can use to
protect themselves from Denial of Service attacks. Implementation
advice to router vendors aimed at ameliorating known problems carries
the risk of previously unforeseen consequences. It is not believed
that these techniques create additional security or DOS exposure
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9. Acknowledgements
The authors would like to thank Ron Bonica, Troy Bonin, John Jason
Brzozowski, Randy Bush, Vint Cerf, Jason Fesler Erik Kline, Jared
Mauch, Chris Morrow and Suran De Silva. Special thanks to Thomas
Narten for detailed review and (even more so) for providing text!
Apologies for anyone we may have missed; it was not intentional.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4398] Josefsson, S., "Storing Certificates in the Domain Name
System (DNS)", RFC 4398, March 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.
[RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
Router Links", RFC 6164, April 2011.
10.2. Informative References
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
January 2006.
URIs
[1] <http://tools.ietf.org/html/
draft-nordmark-6man-impatient-nud-00>
Appendix A. Text goes here.
TBD
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Authors' Addresses
Warren Kumari
Google
Email: warren@kumari.net
Igor
Yahoo!
45 W 18th St
New York, NY
USA
Email: igor@yahoo-inc.com
Joel
Zynga
111 Evelyn
Sunnyvale, CA
USA
Email: jjaeggli@zynga.com
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