draft-ietf-v6ops-v6nd-problems-05.txt   rfc6583.txt 
v6ops I. Gashinsky Internet Engineering Task Force (IETF) I. Gashinsky
Internet-Draft Yahoo! Request for Comments: 6583 Yahoo!
Intended status: Informational J. Jaeggli Category: Informational J. Jaeggli
Expires: September 4, 2012 Zynga ISSN: 2070-1721 Zynga
W. Kumari W. Kumari
Google Inc Google, Inc.
March 03, 2012 March 2012
Operational Neighbor Discovery Problems Operational Neighbor Discovery Problems
draft-ietf-v6ops-v6nd-problems-05
Abstract Abstract
In IPv4, subnets are generally small, made just large enough to cover In IPv4, subnets are generally small, made just large enough to cover
the actual number of machines on the subnet. In contrast, the the actual number of machines on the subnet. In contrast, the
default IPv6 subnet size is a /64, a number so large it covers default IPv6 subnet size is a /64, a number so large it covers
trillions of addresses, the overwhelming number of which will be trillions of addresses, the overwhelming number of which will be
unassigned. Consequently, simplistic implementations of Neighbor unassigned. Consequently, simplistic implementations of Neighbor
Discovery (ND) can be vulnerable to deliberate or accidental denial Discovery (ND) can be vulnerable to deliberate or accidental denial
of service, whereby they attempt to perform address resolution for of service (DoS), whereby they attempt to perform address resolution
large numbers of unassigned addresses. Such denial of attacks can be for large numbers of unassigned addresses. Such denial-of-service
launched intentionally (by an attacker), or result from legitimate attacks can be launched intentionally (by an attacker) or result from
operational tools or accident conditions. As a result of these legitimate operational tools or accident conditions. As a result of
vulnerabilities, new devices may not be able to "join" a network, it these vulnerabilities, new devices may not be able to "join" a
may be impossible to establish new IPv6 flows, and existing IPv6 network, it may be impossible to establish new IPv6 flows, and
transported flows may be interrupted. existing IPv6 transported flows may be interrupted.
This document describes the potential for DOS in detail and suggests This document describes the potential for DoS in detail and suggests
possible implementation improvements as well as operational possible implementation improvements as well as operational
mitigation techniques that can in some cases be used to protect mitigation techniques that can, in some cases, be used to protect
against or at least alleviate the impact of such attacks. against or at least alleviate the impact of such attacks.
Status of this Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
Internet-Drafts are working documents of the Internet Engineering This document is a product of the Internet Engineering Task Force
Task Force (IETF). Note that other groups may also distribute (IETF). It represents the consensus of the IETF community. It has
working documents as Internet-Drafts. The list of current Internet- received public review and has been approved for publication by the
Drafts is at http://datatracker.ietf.org/drafts/current/. Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Internet-Drafts are draft documents valid for a maximum of six months Information about the current status of this document, any errata,
and may be updated, replaced, or obsoleted by other documents at any and how to provide feedback on it may be obtained at
time. It is inappropriate to use Internet-Drafts as reference http://www.rfc-editor.org/info/rfc6583.
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 4, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction ....................................................3
1.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Applicability ..............................................3
2. The Problem . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. The Problem .....................................................3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology .....................................................4
4. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Background ......................................................5
5. Neighbor Discovery Overview . . . . . . . . . . . . . . . . . 7 5. Neighbor Discovery Overview .....................................6
6. Operational Mitigation Options . . . . . . . . . . . . . . . . 8 6. Operational Mitigation Options ..................................7
6.1. Filtering of unused address space. . . . . . . . . . . . . 8 6.1. Filtering of Unused Address Space ..........................7
6.2. Minimal Subnet Sizing. . . . . . . . . . . . . . . . . . . 8 6.2. Minimal Subnet Sizing ......................................7
6.3. Routing Mitigation. . . . . . . . . . . . . . . . . . . . 9 6.3. Routing Mitigation .........................................8
6.4. Tuning of the NDP Queue Rate Limit. . . . . . . . . . . . 9 6.4. Tuning of the NDP Queue Rate Limit .........................8
7. Recommendations for Implementors. . . . . . . . . . . . . . . 9 7. Recommendations for Implementors ................................8
7.1. Prioritize NDP Activities . . . . . . . . . . . . . . . . 10 7.1. Prioritize NDP Activities ..................................9
7.2. Queue Tuning. . . . . . . . . . . . . . . . . . . . . . . 11 7.2. Queue Tuning ..............................................10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 8. Security Considerations ........................................11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12 9. Acknowledgements ...............................................11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 10. References ....................................................11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 10.1. Normative References .....................................11
11.1. Normative References . . . . . . . . . . . . . . . . . . . 12 10.2. Informative References ...................................11
11.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
This document describes implementation issues with IPv6's Neighbor This document describes implementation issues with IPv6's Neighbor
Discovery protocol that can result in vulnerabilities when a network Discovery protocol that can result in vulnerabilities when a network
is scanned, either by an intruder or through the use of scanning is scanned, either by an intruder or through the use of scanning
tools that perform network inventory, security audits, etc. (e.g. tools that perform network inventory, security audits, etc. (e.g.,
"nmap"). "nmap").
This document describes the problem in detail, suggests possible This document describes the problem in detail, suggests possible
implementation improvements, as well as operational mitigation implementation improvements, as well as operational mitigation
techniques, that can in some cases protect against such attacks. techniques, that can, in some cases, protect against such attacks.
The RFC series documents generally describe the behavior of The RFCs generally describe the behavior of protocols, that is,
protocols, that is, "what" is to be done by a protocol, but not "what" is to be done by a protocol, but not exactly "how" it is to be
exactly "how" it is to be implemented. The exact details of how best implemented. The exact details of how best to implement a protocol
to implement a protocol will depend on the overall hardware and will depend on the overall hardware and software architecture of a
software architecture of a particular device. The actual "how" particular device. The actual "how" decisions are (correctly) left
decisions are (correctly) left in the hands of implementers, so long in the hands of implementors, so long as implementation differences
as implementations differences will generally produce proper on-the- will generally produce proper on-the-wire behavior.
wire behavior.
While reading this document, it is important to keep in mind that While reading this document, it is important to keep in mind that
discussions of how things have been implemented beyond basic discussions of how things have been implemented beyond basic
compliance with the specification is not within the scope of the compliance with the specification is not within the scope of the
neighbor discovery RFCs. Neighbor Discovery RFCs.
1.1. Applicability 1.1. Applicability
This document is primarily intended for operators of IPV6 networks This document is primarily intended for operators of IPV6 networks
and implementors of [RFC4861]. The Document provides some and implementors of [RFC4861]. The document provides some
operational considerations as well as recommendations to increase the operational considerations as well as recommendations to increase the
resilience of the Neighbor Discovery protocol. resilience of the Neighbor Discovery protocol.
2. The Problem 2. The Problem
In IPv4, subnets are generally small, made just large enough to cover In IPv4, subnets are generally small, made just large enough to cover
the actual number of machines on the subnet. For example, an IPv4 the actual number of machines on the subnet. For example, an IPv4
/20 contains only 4096 address. In contrast, the default IPv6 subnet /20 contains only 4096 address. In contrast, the default IPv6 subnet
size is a /64, a number so large it covers literally billions of size is a /64, a number so large it covers literally billions of
billions of addresses, the overwhelming majority of which will be billions of addresses, the overwhelming majority of which will be
unassigned. Consequently, simplistic implementations of Neighbor unassigned. Consequently, simplistic implementations of Neighbor
Discovery may fail to perform as desired when they perform address Discovery may fail to perform as desired when they perform address
resolution of large numbers of unassigned addresses. Such failures resolution of large numbers of unassigned addresses. Such failures
can be triggered either intentionally by an attacker launching a can be triggered either intentionally by an attacker launching a
Denial of Service attack (DoS)[RFC4732] to exploit this denial-of-service attack (DoS) [RFC4732] to exploit this
vulnerability, or unintentionally due to the use of legitimate vulnerability or unintentionally due to the use of legitimate
operational tools that scan networks for inventory and other operational tools that scan networks for inventory and other
purposes. As a result of these failures, new devices may not be able purposes. As a result of these failures, new devices may not be able
to "join" a network, it may be impossible to establish new IPv6 to "join" a network, it may be impossible to establish new IPv6
flows, and existing IPv6 transport flows may be interrupted. flows, and existing IPv6 transport flows may be interrupted.
Network scans attempt to find and probe devices on a network. Network scans attempt to find and probe devices on a network.
Typically, scans are performed on a range of target addresses, or all Typically, scans are performed on a range of target addresses, or all
the addresses on a particular subnet. When such probes are directed the addresses on a particular subnet. When such probes are directed
via a router, and the target addresses are on a directly attached via a router, and the target addresses are on a directly attached
network, the router will attempt to perform address resolution on a network, the router will attempt to perform address resolution on a
large number of destinations (i.e., some fraction of the 2^64 large number of destinations (i.e., some fraction of the 2^64
addresses on the subnet). The router's process of testing for the addresses on the subnet). The router's process of testing for the
(non)existence of neighbors can induce a denial of service condition, (non)existence of neighbors can induce a denial-of-service condition,
where the number of necessary Neighbor Discovery requests overwhelms where the number of necessary Neighbor Discovery requests overwhelms
the implementation's capacity to process them, exhausts available the implementation's capacity to process them, exhausts available
memoryand replaces existing in-use mappings with incomplete entries memory and replaces existing in-use mappings with incomplete entries
that will never be completed. A directed DoS attack may seek to that will never be completed. A directed DoS attack may seek to
intentionally create similar conditions to that created intentionally create similar conditions to those created
unintentionally by a network scan. The resulting network disruption unintentionally by a network scan. The resulting network disruption
may impact existing traffic, and devices that join the network may may impact existing traffic, and devices that join the network may
find that address resolution attempts fail. The DOS as a consequence find that address resolution attempts fail. The DoS as a consequence
of network scanning was previously described in [RFC5157] of network scanning was previously described in [RFC5157].
In order to mitigate risk associated with this DoS threat, some In order to mitigate risk associated with this DoS threat, some
router implementations have taken steps to rate-limit the processing router implementations have taken steps to rate-limit the processing
rate of Neighbor Solicitations (NS). While these mitigations do rate of Neighbor Solicitations (NS). While these mitigations do
help, they do not fully address the issue and may introduce their own help, they do not fully address the issue and may introduce their own
set of issues to the neighbor discovery process. set of issues to the Neighbor Discovery process.
3. Terminology 3. Terminology
Address Resolution Address resolution is the process through which a Address Resolution: Address resolution is the process through which
node determines the link-layer address of a neighbor given only a node determines the link-layer address of a neighbor given only
its IP address. In IPv6, address resolution is performed as part its IP address. In IPv6, address resolution is performed as part
of Neighbor Discovery [RFC4861], p60 of Neighbor Discovery [RFC4861], Section 7.2.
Forwarding Plane That part of a router responsible for forwarding Forwarding Plane: The part of a router responsible for forwarding
packets. In higher-end routers, the forwarding plane is typically packets. In higher-end routers, the forwarding plane is typically
implemented in specialized hardware optimized for performance. implemented in specialized hardware optimized for performance.
Steps in the forwarding process include determining the correct Steps in the forwarding process include determining the correct
outgoing interface for a packet, decrementing its Time To Live outgoing interface for a packet, decrementing its Time To Live
(TTL), verifying and updating the checksum, placing the correct (TTL), verifying and updating the checksum, placing the correct
link-layer header on the packet, and forwarding it. link-layer header on the packet, and forwarding it.
Control Plane That part of the router implementation that maintains Control Plane: The part of the router implementation that maintains
the data structures that determine where packets should be the data structures that determine where packets should be
forwarded. The control plane is typically implemented as a forwarded. The control plane is typically implemented as a
"slower" software process running on a general purpose processor "slower" software process running on a general purpose processor
and is responsible for such functions as communicating network and is responsible for such functions as communicating network
status changes via routing protocols, maintaining the forwarding status changes via routing protocols, maintaining the forwarding
table, performing management, and resolving the correct link-layer table, performing management, and resolving the correct link-layer
address for adjacent neighbors. The control plane "controls" the address for adjacent neighbors. The control plane "controls" the
forwarding plane by programming it with the information needed for forwarding plane by programming it with the information needed for
packet forwarding. packet forwarding.
Neighbor Cache As described in [RFC4861], the data structure that Neighbor Cache: As described in [RFC4861], the data structure that
holds the cache of (amongst other things) IP address to link-layer holds the cache of (amongst other things) IP address to link-layer
address mappings for connected nodes. As the information in the address mappings for connected nodes. As the information in the
Neighbor Cache is needed by the forwarding plane every time it Neighbor Cache is needed by the forwarding plane every time it
forwards a packet, it is usually implemented in an ASIC. forwards a packet, it is usually implemented in an Application-
specific Integrated Circuit (ASIC).
Neighbor Discovery Process The Neighbor Discovery Process (NDP) is Neighbor Discovery Process: The Neighbor Discovery Process (NDP) is
that part of the control plane that implements the Neighbor that part of the control plane that implements the Neighbor
Discovery protocol. NDP is responsible for performing address Discovery protocol. NDP is responsible for performing address
resolution and maintaining the Neighbor Cache. When forwarding resolution and maintaining the Neighbor Cache. When forwarding
packets, the forwarding plane accesses entries within the Neighbor packets, the forwarding plane accesses entries within the Neighbor
Cache. When the forwarding plane processes a packet for which the Cache. When the forwarding plane processes a packet for which the
corresponding Neighbor Cache Entry is missing or incomplete, it corresponding Neighbor Cache Entry (NCE) is missing or incomplete,
notifies NDP to take appropriate action (typically via a shared it notifies NDP to take appropriate action (typically via a shared
queue). NDP picks up requests from the shared queue and performs queue). NDP picks up requests from the shared queue and performs
any necessary discovery action. In many implementations the NDP any necessary discovery action. In many implementations, the NDP
is also responsible for responding to router solicitation is also responsible for responding to router solicitation
messages, Neighbor Unreachability Detection (NUD), etc. messages, Neighbor Unreachability Detection (NUD), etc.
4. Background 4. Background
Modern router architectures separate the forwarding of packets Modern router architectures separate the forwarding of packets
(forwarding plane) from the decisions needed to decide where the (forwarding plane) from the decisions needed to decide where the
packets should go (control plane). In order to deal with the high packets should go (control plane). In order to deal with the high
number of packets per second, the forwarding plane is generally number of packets per second, the forwarding plane is generally
implemented in hardware and is highly optimized for the task of implemented in hardware and is highly optimized for the task of
skipping to change at page 7, line 19 skipping to change at page 6, line 19
requests and performs the address resolution function (by performing requests and performs the address resolution function (by performing
a neighbor solicitation and listening for a neighbor advertisement). a neighbor solicitation and listening for a neighbor advertisement).
This process is usually also responsible for other activities needed This process is usually also responsible for other activities needed
to maintain link-layer information, such as Neighbor Unreachability to maintain link-layer information, such as Neighbor Unreachability
Detection (NUD). Detection (NUD).
By sending appropriate packets to addresses on a given subnet, an By sending appropriate packets to addresses on a given subnet, an
attacker can cause the router to queue attempts to resolve so many attacker can cause the router to queue attempts to resolve so many
addresses that it crowds out attempts to resolve "legitimate" addresses that it crowds out attempts to resolve "legitimate"
addresses (and in many cases becomes unable to perform maintenance of addresses (and in many cases becomes unable to perform maintenance of
existing entries in the neighbor cache, and unable to answer Neighbor existing entries in the Neighbor Cache, and unable to answer Neighbor
Solicitation). This condition can result in the inability to resolve Solicitation). This condition can result in the inability to resolve
new neighbors and loss of reachability to neighbors with existing ND- new neighbors and loss of reachability to neighbors with existing
Cache entries. During testing it was concluded that 4 simultaneous NCEs. During testing, it was concluded that four simultaneous nmap
nmap sessions from a low-end computer was sufficient to make a sessions from a low-end computer were sufficient to make a router's
router's neighbor discovery process unusable and therefore forwarding Neighbor Discovery process unusable; therefore, forwarding became
became unavailable to the destination subnets. unavailable to the destination subnets.
The failure to maintain proper NDP behavior whilst under attack has The failure to maintain proper NDP behavior whilst under attack has
been observed across multiple platforms and implementations, been observed across multiple platforms and implementations,
including the largest modern router platforms available (at the including the largest modern router platforms available (at the
inception of work on this document). inception of work on this document).
5. Neighbor Discovery Overview 5. Neighbor Discovery Overview
When a packet arrives at (or is generated by) a router for a When a packet arrives at (or is generated by) a router for a
destination on an attached link, the router needs to determine the destination on an attached link, the router needs to determine the
correct link-layer address to use in the destination field of the correct link-layer address to use in the destination field of the
layer 2 encapsulation. The router checks the Neighbor Cache for an Layer 2 encapsulation. The router checks the Neighbor Cache for an
existing Neighbor Cache Entry for the neighbor, and if none exists, existing Neighbor Cache Entry for the neighbor, and if none exists,
invokes the address resolution portions of the IPv6 Neighbor invokes the address resolution portions of the IPv6 Neighbor
Discovery [RFC4861] protocol to determine the link-layer address of Discovery [RFC4861] protocol to determine the link-layer address of
the neighbor. the neighbor.
[RFC4861] Section 5.2 (Conceptual Sending Algorithm) outlines how [RFC4861], Section 5.2, outlines how this process works. A very
this process works. A very high level summary is that the device high-level summary is that the device creates a new Neighbor Cache
creates a new Neighbor Cache Entry for the neighbor, sets the state Entry for the neighbor, sets the state to INCOMPLETE, queues the
to INCOMPLETE, queues the packet and initiates the actual address packet, and initiates the actual address resolution process. The
resolution process. The device then sends out one or more Neighbor device then sends out one or more Neighbor Solicitations, and when it
Solicitations, and when it receives a corresponding Neighbor receives a corresponding Neighbor Advertisement, completes the
Advertisement, completes the Neighbor Cache Entry and sends the Neighbor Cache Entry and sends the queued packet.
queued packet.
6. Operational Mitigation Options 6. Operational Mitigation Options
This section provides some feasible mitigation options that can be This section provides some feasible mitigation options that can be
employed today by network operators in order to protect network employed today by network operators in order to protect network
availability while vendors implement more effective protection availability while vendors implement more effective protection
measures. It can be stated that some of these options are "kludges", measures. It can be stated that some of these options are "kludges",
and can be operationally difficult to manage. They are presented, as and can be operationally difficult to manage. They are presented, as
they represent options we currently have. It is each operator's they represent options we currently have. It is each operator's
responsibility to evaluate and understand the impact of changes to responsibility to evaluate and understand the impact of changes to
their network due to these measures. their network due to these measures.
6.1. Filtering of unused address space. 6.1. Filtering of Unused Address Space
The DoS condition is induced by making a router try to resolve The DoS condition is induced by making a router try to resolve
addresses on the subnet at a high rate. By carefully addressing addresses on the subnet at a high rate. By carefully addressing
machines into a small portion of a subnet (such as the lowest machines into a small portion of a subnet (such as the lowest
numbered addresses), it is possible to filter access to addresses not numbered addresses), it is possible to filter access to addresses not
in that assigned portion of address space using Access Control Lists in that assigned portion of address space using Access Control Lists
(ACLs), or by null routing, features which are available on most (ACLs), or by null routing, features which are available on most
existing platforms. This will prevent the attacker from making the existing platforms. This will prevent the attacker from making the
router attempt to resolve unused addresses. For example if there are router attempt to resolve unused addresses. For example, if there
only 50 hosts connected to an interface, you may be able to filter are only 50 hosts connected to an interface, you may be able to
any address above the first 64 addresses of that subnet by null- filter any address above the first 64 addresses of that subnet by
routing the subnet carrying a more specific /122 route or by applying null-routing the subnet carrying a more specific /122 route or by
ACLs on the WAN link to prevent the attack traffic reaching the applying ACLs on the WAN link to prevent the attack traffic reaching
vulnerable device. the vulnerable device.
As mentioned at the beginning of this section, it is fully understood As mentioned at the beginning of this section, it is fully understood
that this is ugly (and difficult to manage); but failing other that this is ugly (and difficult to manage); but failing other
options, it may be a useful technique especially when responding to options, it may be a useful technique especially when responding to
an attack. an attack.
This solution requires that the hosts be statically or statefully This solution requires that the hosts be statically or statefully
addressed (as is often done in a datacenter) and may not interact addressed (as is often done in a datacenter), and they may not
well with networks using [RFC4862] interact well with networks using [RFC4862].
6.2. Minimal Subnet Sizing. 6.2. Minimal Subnet Sizing
By sizing subnets to reflect the number of addresses actually in use, By sizing subnets to reflect the number of addresses actually in use,
the problem can be avoided. For example, [RFC6164] recommends sizing the problem can be avoided. For example, [RFC6164] recommends sizing
the subnets for inter-router links to only have 2 addresses (a /127). the subnets for inter-router links so they only have two addresses (a
It is worth noting that this practice is common in IPv4 networks, in /127). It is worth noting that this practice is common in IPv4
part to protect against the harmful effects of ARP request flooding. networks, in part to protect against the harmful effects of Address
Resolution Protocol (ARP) request flooding.
Subnet prefixes longer than a /64 are not able to use stateless auto- Subnet prefixes longer than a /64 are not able to use stateless auto-
configuration [RFC4862] so this approach is not suitable for use with configuration [RFC4862], so this approach is not suitable for use
hosts that are not statically configured. with hosts that are not statically configured.
6.3. Routing Mitigation. 6.3. Routing Mitigation
One very effective technique is to route the subnet to a discard One very effective technique is to route the subnet to a discard
interface (most modern router platforms can discard traffic in interface (most modern router platforms can discard traffic in
hardware / the forwarding plane) and then have individual hosts hardware / the forwarding plane) and then have individual hosts
announce routes for their IP addresses into the network (or use some announce routes for their IP addresses into the network (or use some
method to inject much more specific addresses into the local routing method to inject much more specific addresses into the local routing
domain). For example the network 2001:db8:1:2:3::/64 could be routed domain). For example, the network 2001:db8:1:2:3::/64 could be
to a discard interface on "border" routers, and then individual hosts routed to a discard interface on "border" routers, and then
could announce 2001:db8:1:2:3::10/128, 2001:db8:1:2:3::66/128 into individual hosts could announce 2001:db8:1:2:3::10/128, 2001:db8:1:2:
the IGP. This is typically done by having the IP address bound to a 3::66/128 into the IGP. This is typically done by having the IP
virtual interface on the host (for example the loopback interface), address bound to a virtual interface on the host (for example, the
enabling IP forwarding on the host and having it run a routing loopback interface), enabling IP forwarding on the host and having it
daemon. For obvious reasons, host participation in the IGP makes run a routing daemon. For obvious reasons, host participation in the
many operators uncomfortable, but can be a very powerful technique if IGP makes many operators uncomfortable, but it can be a very powerful
used in a disciplined and controlled manner. One method to help technique if used in a disciplined and controlled manner. One method
address these concerns is to have the hosts participate in a to help address these concerns is to have the hosts participate in a
different IGP (or difference instance of the same IGP) and carefully different IGP (or difference instance of the same IGP) and carefully
redistribute into the main IGP. redistribute into the main IGP.
6.4. Tuning of the NDP Queue Rate Limit. 6.4. Tuning of the NDP Queue Rate Limit
Many implementations provide a means to control the rate of Many implementations provide a means to control the rate of
resolution of unknown addresses. By tuning this rate, it may be resolution of unknown addresses. By tuning this rate, it may be
possible to ameliorate the issue, as with most tuning knobs possible to ameliorate the issue, as with most tuning knobs
(especially those that deal with rate limiting), the attack may be (especially those that deal with rate-limiting), the attack may be
completed more quickly due to the lower threshold. By excessively completed more quickly due to the lower threshold. By excessively
lowering this rate you may negatively impact how long the device lowering this rate, you may negatively impact how long the device
takes to learn new addresses under normal conditions (for example, takes to learn new addresses under normal conditions (for example,
after clearing the neighbor cache or when the router first boots). after clearing the Neighbor Cache or when the router first boots).
Under attack conditions you may be unable to resolve "legitimate" Under attack conditions, you may be unable to resolve "legitimate"
addresses sooner than if you had just left the parameter untouched. addresses sooner than if you had just left the parameter untouched.
It is worth noting that this technique is worth investigating only if It is worth noting that this technique is worth investigating only if
the device has separate queues for resolution of unknown addresses the device has separate queues for resolution of unknown addresses
and the maintenance of existing entries. and the maintenance of existing entries.
7. Recommendations for Implementors. 7. Recommendations for Implementors
The section provides some recommendations to implementors of IPv6 This section provides some recommendations to implementors of IPv6
Neighbor Discovery. Neighbor Discovery.
At a high-level, implementors should program defensively. That is, At a high-level, implementors should program defensively. That is,
they should assume that attackers will attempt to exploit they should assume that attackers will attempt to exploit
implementation weaknesses, and should ensure that implementations are implementation weaknesses, and they should ensure that
robust to various attacks. In the case of Neighbor Discovery, the implementations are robust to various attacks. In the case of
following general considerations apply: Neighbor Discovery, the following general considerations apply:
Manage Resources Explicitly Resources such as processor cycles, Manage Resources Explicitly: Resources such as processor cycles,
memory, etc. are never infinite, yet with IPv6's large subnets it memory, etc., are never infinite, yet with IPv6's large subnets,
is easy to cause NDP to generate large numbers of address it is easy to cause NDP to generate large numbers of address
resolution requests for non-existent destinations. resolution requests for nonexistent destinations. Implementations
Implementations need to limit resources devoted to processing need to limit resources devoted to processing Neighbor Discovery
Neighbor Discovery requests in a thoughtful manner. requests in a thoughtful manner.
Prioritize Some NDP requests are more important than others. For Prioritize: Some NDP requests are more important than others. For
example, when resources are limited, responding to Neighbor example, when resources are limited, responding to Neighbor
Solicitations for one's own address is more important than Solicitations for one's own address is more important than
initiating address resolution requests that create new entries. initiating address resolution requests that create new entries.
Likewise, performing Neighbor Unreachability Detection, which by Likewise, performing Neighbor Unreachability Detection, which by
definition is only invoked on destinations that are actively being definition is only invoked on destinations that are actively being
used, is more important than creating new entries for possibly used, is more important than creating new entries for possibly
non-existent neighbors. nonexistent neighbors.
7.1. Prioritize NDP Activities 7.1. Prioritize NDP Activities
Not all Neighbor Discovery activities are equally important. Not all Neighbor Discovery activities are equally important.
Specifically, requests to perform large numbers of address Specifically, requests to perform large numbers of address
resolutions on non-existent Neighbor Cache Entries should not come at resolutions on non-existent Neighbor Cache Entries should not come at
the expense of servicing requests related to keeping existing, in-use the expense of servicing requests related to keeping existing, in-use
entries properly up-to-date. Thus, implementations should divide entries properly up to date. Thus, implementations should divide
work activities into categories having different priorities. The work activities into categories having different priorities. The
following gives examples of different activities and their importance following gives examples of different activities and their importance
in rough priority order. If implmented, the operation and priority in rough priority order. If implemented, the operation and priority
of these should be configurable by the operator. of these should be configurable by the operator.
1. It is critical to respond to Neighbor Solicitations for one's own 1. It is critical to respond to Neighbor Solicitations for one's own
address, especially for a router. Whether for address resolution or address, especially for a router. Whether for address resolution
Neighbor Unreachability Detection, failure to respond to Neighbor or Neighbor Unreachability Detection, failure to respond to
Solicitations results in immediate problems. Failure to respond to Neighbor Solicitations results in immediate problems. Failure to
NS requests that are part of NUD can cause neighbors to delete the respond to NS requests that are part of NUD can cause neighbors
NCE for that address, and will result in followup NS messages using to delete the NCE for that address and will result in follow-up
multicast. Once an entry has been flushed, existing traffic for NS messages using multicast. Once an entry has been flushed,
destinations using that entry can no longer be forwarded until existing traffic for destinations using that entry can no longer
address resolution completes successfully. In other words, not be forwarded until address resolution completes successfully. In
responding to NS messages further increases the NDP load, and causes other words, not responding to NS messages further increases the
on-going communication to fail. NDP load and causes ongoing communication to fail.
2. It is critical to revalidate one's own existing NCEs in need of 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 refresh. As part of NUD, ND is required to frequently revalidate
existing, in-use entries. Failure to do so can result in the entry existing, in-use entries. Failure to do so can result in the
being discarded. For in-use entries, discarding the entry will entry being discarded. For in-use entries, discarding the entry
almost certainly result in a subsequent request to perform address will almost certainly result in a subsequent request to perform
resolution on the entry, but this time using multicast. As above, address resolution on the entry, but this time using multicast.
once the entry has been flushed, existing traffic for destinations
using that entry can no longer be forwarded until address resolution As above, once the entry has been flushed, existing traffic for
completes successfully. destinations using that entry can no longer be forwarded until
address resolution completes successfully.
3. To maintain the stability of the control plane, Neighbor 3. To maintain the stability of the control plane, Neighbor
Discovery activity related to traffic sourced by the router (as Discovery activity related to traffic sourced by the router (as
opposed to traffic being forwarded by the router) should be given opposed to traffic being forwarded by the router) should be given
high priority. Whenever network problems occur, debugging and making high priority. Whenever network problems occur, debugging and
other operational changes requires being able to query and access the making other operational changes requires being able to query and
router. In addition, routing protocols dependent on Neighbor access the router. In addition, routing protocols dependent on
Discovery for connectivity may begin to react (negatively) to Neighbor Discovery for connectivity may begin to react
perceived connectivity problems, causing additional undesirable (negatively) to perceived connectivity problems, causing
ripple effects. additional undesirable ripple effects.
4. Traffic to unknown addresses should be given lowest priority. 4. Traffic to unknown addresses should be given lowest priority.
Indeed, it may be useful to distinguish between "never seen" Indeed, it may be useful to distinguish between "never seen"
addresses and those that have been seen before, but that do not have addresses and those that have been seen before, but that do not
a corresponding NCE. Specifically, the conceptual processing have a corresponding NCE. Specifically, the conceptual
algorithm in IPv6 Neighbor Discovery [RFC4861] calls for deleting processing algorithm in IPv6 Neighbor Discovery [RFC4861] calls
NCEs under certain conditions. Rather than delete them completely, for deleting NCEs under certain conditions. Rather than delete
however, it might be useful to at least keep track of the fact that them completely, however, it might be useful to at least keep
an entry at one time existed, in order to prioritize address track of the fact that an entry at one time existed, in order to
resolution requests for such neighbors compared with neighbors that prioritize address resolution requests for such neighbors
have never been seen before. compared with neighbors that have never been seen before.
7.2. Queue Tuning. 7.2. Queue Tuning
On implementations in which requests to NDP are submitted via a On implementations in which requests to NDP are submitted via a
single queue, router vendors should provide operators with means to single queue, router vendors should provide operators with means to
control both the rate of link-layer address resolution requests control both the rate of link-layer address resolution requests
placed into the queue and the size of the queue. This will allow placed into the queue and the size of the queue. This will allow
operators to tune Neighbour Discovery for their specific environment. operators to tune Neighbor Discovery for their specific environment.
The ability to set, or have per interface or per prefix queue limits The ability to set, or have per-interface or per-prefix queue limits
at a rate below that of the global queue limit might limit the damage at a rate below that of the global queue limit might restrict the
to the neighbor discovery processing to the network targeted by the damage to the Neighbor Discovery processing to the network targeted
attack. by the attack.
Setting those values must be a very careful balancing act - the lower Setting those values must be a very careful balancing act -- the
the rate of entry into the queue, the less load there will be on the lower the rate of entry into the queue, the less load there will be
ND process, however, it will take the router longer to learn on the ND process; however, it will take the router longer to learn
legitimate destinations as a result. In a datacenter with 6,000 legitimate destinations as a result. In a datacenter with 6,000
hosts attached to a single router, setting that value to be under hosts attached to a single router, setting that value to be under
1000 would mean that resolving all of the addresses from an initial 1000 would mean that resolving all of the addresses from an initial
state (or something that invalidates the address cache, such as a STP state (or something that invalidates the address cache, such as a
TCN) may take over 6 seconds. Similarly, the lower the size of the Spanning Tree Protocol (STP) Topology Change Notification (TCN)) may
queue, the higher the likelihood of an attack being able to knock out take over 6 seconds. Similarly, the lower the size of the queue, the
legitimate traffic (but less memory utilization on the router). higher the likelihood of an attack being able to knock out legitimate
traffic (but less memory utilization on the router).
8. IANA Considerations
No IANA resources or consideration are requested in this draft.
9. Security Considerations 8. Security Considerations
This document outlines mitigation options that operators can use to This document outlines mitigation options that operators can use to
protect themselves from Denial of Service attacks. Implementation protect themselves from denial-of-service attacks. Implementation
advice to router vendors aimed at ameliorating known problems carries advice to router vendors aimed at ameliorating known problems carries
the risk of previously unforeseen consequences. It is not believed the risk of previously unforeseen consequences. It is not believed
that these mitigation techniques or the implementation of finer- that these mitigation techniques or the implementation of finer-
grained queuing of NDP activity create additional security risks or grained queuing of NDP activity create additional security risks or
DOS exposure. DoS exposure.
10. Acknowledgements 9. Acknowledgements
The authors would like to thank Ron Bonica, Troy Bonin, John Jason The authors would like to thank Ron Bonica, Troy Bonin, John Jason
Brzozowski, Randy Bush, Vint Cerf, Tassos Chatzithomaoglou, Jason Brzozowski, Randy Bush, Vint Cerf, Tassos Chatzithomaoglou, Jason
Fesler, Wes George, Erik Kline, Jared Mauch, Chris Morrow and Suran Fesler, Wes George, Erik Kline, Jared Mauch, Chris Morrow, and Suran
De Silva. Special thanks to Thomas Narten and Ray Hunter for De Silva. Special thanks to Thomas Narten and Ray Hunter for
detailed review and (even more so) for providing text! detailed review and (even more so) for providing text!
Apologies for anyone we may have missed; it was not intentional. Apologies for anyone we may have missed; it was not intentional.
11. References 10. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 10.1. Normative References
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007. September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007. Address Autoconfiguration", RFC 4862, September 2007.
[RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti, [RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter- L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
Router Links", RFC 6164, April 2011. Router Links", RFC 6164, April 2011.
11.2. Informative References 10.2. Informative References
[RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of- [RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006. Service Considerations", RFC 4732, December 2006.
[RFC5157] Chown, T., "IPv6 Implications for Network Scanning", [RFC5157] Chown, T., "IPv6 Implications for Network Scanning",
RFC 5157, March 2008. RFC 5157, March 2008.
Authors' Addresses Authors' Addresses
Igor Gashinsky Igor Gashinsky
Yahoo! Yahoo!
45 W 18th St 45 W 18th St
New York, NY New York, NY
USA USA
Email: igor@yahoo-inc.com EMail: igor@yahoo-inc.com
Joel Jaeggli Joel Jaeggli
Zynga Zynga
111 Evelyn 111 Evelyn
Sunnyvale, CA Sunnyvale, CA
USA USA
Email: jjaeggli@zynga.com EMail: jjaeggli@zynga.com
Warren Kumari Warren Kumari
Google Inc Google, Inc.
1600 Amphitheatre Parkway 1600 Amphitheatre Parkway
Mountain View, CA Mountain View, CA
USA USA
Email: warren@kumari.net EMail: warren@kumari.net
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