IPv6 Address Accountability Considerations
draft-chown-v6ops-address-accountability-01

Abstract

Hosts in IPv4 networks typically acquire addresses by use of DHCP, and retain that address and only that address while the DHCP lease remains valid. In IPv6 networks, hosts may use DHCPv6, but may instead autoconfigure their own global addresses, and potentially use many privacy addresses over time. This behaviour places an additional burden on network operators who require address accountability for their users and devices. There has been some discussion of this issue on various mail lists; this text attempts to capture the issues to encourage further discussion.

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 January 13, 2012.

Copyright Notice

Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Accountability Approaches
• 2.1. Switch-router polling
• 2.2. Record all ND traffic
• 2.3. Force use of DHCPv6 only
• 2.4. Use SAVI mechanisms
• 3. Privacy Considerations
• 4. Conclusions
• 5. Security Considerations
• 6. IANA Considerations
• 7. Acknowledgments
• 8. References
• Author's Address

1. Introduction

Administrators of IPv4 networks are used to an address accountability model where devices acquire a single global address using DHCP and then use that address while the DHCP lease is valid. The model allows an administrator to track back an IP address to a user or device, in the event of some incident or fault requiring investigation. While by no means foolproof, this model, which may include use of DHCP option 82, is one that IPv4 network administrators are generally comfortable with.

There are many reasons why address stability is desirable, e.g. DNS mappings, ACLs using IP addresses, and logging. However, such stability may not typically exist in IPv6 client networks, particularly where clients are user managed.

In IPv6 networks, where hosts may use SLAAC [RFC4862] and Privacy Addresses [RFC4941], it is quite possible that a host may use multiple IPv6 addresses over time, possibly changing addresses used frequently, or using multiple addresses concurrently. Where privacy addresses are used, a host may choose to generate and start using a new privacy address at any time, and will also typically generate a new privacy address after rebooting. Clients may use different IPv6 addresses per application, while servers may have multiple addresses configured, one per service offered.

It is also worth noting that in an IPv4 network, it is more difficult for a user to pick and use an address manually without clashing with an existing device on the network, while in IPv6 networks, picking an unused address is simple to do without an address clash. Thus picking an unused IP address becomes as simple as picking an unused Layer 2 address. Continuing that comparison, some virtualised OSes may pick randomly generated link layer addresses, and may change these upon virtual host reboot.

2. Accountability Approaches

There are various approaches to address accountability, which have different costs, benefits and trade-offs.

2.1. Switch-router polling

By polling network switch and router devices for IPv4 ARP tables and IPv6 ND tables, and correlating the results with switch port MAC tables, it should be possible to determine which IP addresses are in use at any specific point in time and which addresses are being used on which switch ports (and thus users or devices).

This is the approach adopted by tools such as NAV and Netdot, but there is some concern expressed at the load that may be placed on devices by frequent SNMP or other polling. The polling frequency needs to be rapid enough to ensure that cached ND/ARP data on devices is not expired between polling intervals, i.e. the ND/ARP data should not be expired more frequently than the device is polled.

2.2. Record all ND traffic

If all ND traffic observed on a link can be captured, it should be possible for IPv6 address usage to be recorded. This would require appropriate capability on a device on any given subnet, e.g. as is currently achieved for RAmond or NDPmon, or a reporting mechanism for the subnet router. There may also be mechanisms such as a (filtered) RSPAN that may be suitable; at least one implementation of this has been published.

A benefit of this approach is that collecting all ND traffic would allow additional accounting and fault detection to be undertaken, e.g. rogue RA detection, or DAD DoS detection.

2.3. Force use of DHCPv6 only

One approach to accountability is to attempt to force devices to only use DHCPv6, which would in principle give the same address accountability model as exists for IPv4 today. [RFC4649] for DHCPv6 appears to give at least some of the functionality of DHCP option 82.

While it is possible to craft IPv6 Router Advertisements that give 'hints' to hosts that DHCPv6 should be used ('M' bit set), there is no obligation on the host to honour that hint. However, if the Autonomous (A) flag in the Prefix Information option is unset (as discussed in section 5.5.3 of RFC 4862), the Preifx Information option should be ignored. A user running the device will need to determine the on-link prefix if they wish to manually configure their own address.

2.4. Use SAVI mechanisms

Discussion of appropriateness of SAVI mechanisms to be added here. (In principle, SAVI mechanisms work by observing NDP and DHCP messages, allowing bindings to be set up and recorded.)

3. Privacy Considerations

This draft discusses mechanisms for a site or organisation to manage address accountability where IPv6 has been deployed. In most networks there is a requirement to be able to identify which users have been using which addresses or devices at a given point in time. This draft was written in response to requests for improved accountability for IPv6 traffic in (mainly) UK academic sites, but the same rationale is likely to apply elsewhere.

While the sources of data that may be used for such purposes (e.g. state on routers or switches) is generally not available to general users of the network, it is available to administrators of the network. The use of privacy mechanisms, e.g. RFC 4941, gives the greatest benefit when the addresses are being observed by external third parties.

4. Conclusions

This text is an initial draft attempting to capture the issues related to IPv6 address accountability models. If an all-DHCPv6 model is not viable, IPv6 network administrators will need to deploy management and monitoring tools to allow them to account for hosts that will have multiple IPv6 addresses that may also change rapidly over time.

Some of the approaches described do not depend on a specific type of address management being used, and will thus work with other addressing methods if they emerge in the future.

Feedback on the issues discussed here is welcomed.

5. Security Considerations

There are no extra security consideration for this document.

6. IANA Considerations

There are no extra IANA consideration for this document.

7. Acknowledgments

The author would like to thank the following people for comments on this text: Mark Smith, and James Woodyatt.

8. References

Author's Address