Neighbor Discovery Enhancement for DOS mititgation
draft-gashinsky-6man-v6nd-enhance-02

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 a modification to the [RFC4861] neighbor discovery protocol aimed at improving the resilience of the neighbor discovery process. We call this process Gratuitous neighbor discovery and it derives inspiration in part from analogous IPv4 gratuitous ARP implementation.

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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http:/⁠/⁠datatracker.ietf.org/⁠drafts/⁠current/⁠.

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This Internet-Draft will expire on April 23, 2013.

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Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Applicability
• 2. The Problem
• 2.1. Scenario 1 - DoS condition induced by default router failure
• 3. Terminology
• 4. Background
• 5. Neighbor Discovery Overview
• 6. Proposed Solutions
• 6.1. NDP Protocol Gratuitous NA
• 6.2. User Configurable DELAY_FIRST_PROBE_TIME
• 7. IANA Considerations
• 8. Security Considerations
• 9. Acknowledgements
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

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 perform network inventory, security audits, etc. (e.g., "nmap"). In some cases, DOS-like conditions can also be induced by legitimate traffic in heavy traffic networks such as campuses or datacenters.

1.1. Applicability

This document is primarily intended for implementors of [RFC4861].

This document is a companion to two additional documents. The first document was [RFC6583] Operational Neighbor Discovery Problems which addressed the problem in detail and described operational and implementation mitigation within the framework of the Existing protocol. The second related document [I-D.ietf-6man-impatient-nud] Neighbor Unreachability Detection is too impatient proposes to alter the Neighbor unreachability Detection by relaxing rules in an attempt to keep devices in the cache.

In this document we propose alterations that allow the update or installation of neighbor entries without the instigation of a full [RFC4861] neighbor solicitation.

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)existence 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 help, they do not fully address the issue and may introduce their own set of potential liabilities to the neighbor discovery process.

In some network environments, legitimate Neighbor Discovery traffic from a large number of connected hosts could induce a DoS condition even without the use of any scanning tools.

2.1. Scenario 1 - DoS condition induced by default router failure

Consider the following scenario - You have a pair of routers, R1 and R2, acting as default routers for a campus wifi network that serves thousands of clients. These clients range from traditional laptops with common OSes such as Windows, MAC OS X, etc., to smart phones and tablets running a slew of mobile OSes. R1, R2 and all clients are configured with default ND parameters.

Under normal operating conditions, R1 acts as a default gateway for all client traffic and R2 is mostly acting as a standby. R1 and R2 routinely send out Router Advertisements and all nodes perform Neighbor Discovery as per the default timers configured. Clients that are actively transmitting and receiving data will likely have a Neighbor Cache entry for R1 as REACHABLE and R2 as STALE.

Now imagine that for some reason (power outage, hardware failure, etc.) R1 goes down. When this happens, R2 begins various housekeeping tasks such as reconverging its routing protocols (OSPF, BGP, etc.), recalculating layer 2 topologies such as in STP and so on. Typically, such reconvergence incidents are quite CPU intensive depending on the size of the topology and are generally aggravated in dual stack environments. Once clients determine that R1 is no longer reachable, they would start using R2 as their default router.

At this point, the Neighbor Cache Entry for R2 is still marked as STALE. As per RFC4861, a node will start sending packets to R2, mark the neighbor cache entry for R2 as DELAY and set a timer to expire in DELAY_FIRST_PROBE_TIME seconds. DELAY_FIRST_PROBE_TIME is a fixed node constant with a value of 5 seconds. If the entry is still in the DELAY state when the timer expires, the entry's state changes to PROBE. Upon entering the PROBE state, a node sends a unicast Neighbor Solicitation message to R2 using the cached link-layer address.

Ordinarily, it is highly likely that the client will receive reachability confirmation within the 5 seconds of DELAY_FIRST_PROBE_TIME by virtue of hints from upper layer protocols. However, in this scenario, given that R2 is busy doing other things, it is possible that it will take a longer time for the client to receive said reachability confirmation, forcing it to enter the PROBE state and send out a unicast NS message.

With thousands of clients now sending out unicast NS messages to R2 in a short period of time, while it is busy dealing with other reconvergence related calculations, you effectively end up in a DoS situation entirely with legitimate traffic.

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.

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.

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 Solicitations, and when it receives a corresponding Neighbor Advertisement, completes the Neighbor Cache Entry and sends the queued packet.

6. Proposed Solutions

Let us examine a few possible solutions that could alleviate the issues discussed in 'The Problem' section

6.1. NDP Protocol Gratuitous NA

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 unsolicited 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.

6.2. User Configurable DELAY_FIRST_PROBE_TIME

A very simple solution for Scenario 1 could be to have a user configurable DELAY_FIRST_PROBE_TIME that could be set to a higher value than the current constant of 5 seconds. This would allow clients to keep sending traffic in the DELAY state, while giving more time for R2 to stabilize before it has to process the barrage of ND messages. It will be up to Network administrators to determine what this value should be based upon unique characteristics of their setup. Having a longer DELAY_FIRST_PROBE_TIME does run the risk of clients sending traffic without ever knowing that they have forward reachability. However, in most cases, the router's forwarding plane remains unaffected during high CPU events and therefore the likelihood of the traffic making it to the destination is high.

7. IANA Considerations

No IANA resources or consideration are requested in this draft.

8. Security Considerations

This technique has potential impact on neighbor detection and in particular the discovery of unidirectional forwarding problems.

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

10.2. Informative References

Authors' Addresses