IPv6 Operations T. Chown
Internet-Draft University of Southampton
Intended status: Informational July 12, 2011
Expires: January 13, 2012
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
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This Internet-Draft will expire on January 13, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Accountability Approaches . . . . . . . . . . . . . . . . . . . 3
2.1. Switch-router polling . . . . . . . . . . . . . . . . . . . 3
2.2. Record all ND traffic . . . . . . . . . . . . . . . . . . . 4
2.3. Force use of DHCPv6 only . . . . . . . . . . . . . . . . . 4
2.4. Use SAVI mechanisms . . . . . . . . . . . . . . . . . . . . 4
3. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 4
4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
8. Informative References . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
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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).
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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
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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.
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8. Informative References
[RFC4649] Volz, B., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6) Relay Agent Remote-ID Option", RFC 4649,
August 2006.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
Author's Address
Tim Chown
University of Southampton
Highfield
Southampton, Hampshire SO17 1BJ
United Kingdom
Email: tjc@ecs.soton.ac.uk
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