Dynamic Host Configuration L. Colitti
Internet-Draft J. Linkova
Intended status: Standards Track Google
Expires: June 11, 2020 M. Richardson
Sandelman
T. Mrugalski
ISC
December 9, 2019
IPv6-Only-Preferred Option for DHCP
draft-link-dhc-v6only-01
Abstract
This document specifies a DHCP option to indicate that a host
supports an IPv6-only mode and willing to forgo obtaining an IPv4
address if the network provides IPv6 connectivity.
Status of This Memo
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This Internet-Draft will expire on June 11, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets . . . . 4
3. IPv6-Only Preferred Option . . . . . . . . . . . . . . . . . 5
3.1. Option format . . . . . . . . . . . . . . . . . . . . . . 5
3.2. DHCPv4 Client Behaviour . . . . . . . . . . . . . . . . . 5
3.3. DHCPv4 Server Behaviour . . . . . . . . . . . . . . . . . 7
3.4. Configuration Variables . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
One of the biggest challenges of deploying IPv6-only LANs is that
such networks might contain rather heterogeneous collection of hosts.
Some of them are capable of operating in IPv6-only mode (either
because the OS and all applications are IPv6-only capable or because
the host has some form of 464XLAT [RFC6877] deployed). At the same
time some devices might still have IPv4 dependencies and need IPv4
connectivity to operate properly. To incrementally rollout
IPv6-only, network operators need to provide IPv4-as-a-service
whereby a host receives an IPv4 address if it needs it, while
IPv6-only capable devices (such as modern mobile devices) are not
allocated IPv4 addresses. Deploying separate LAN segments for
IPv6-only and for dual-stack hosts (such as two WiFi SSIDs or two
VLANs) is undesirable for a number of reasons, including but not
limited to:
o Doubling the number of network segments which leads to operational
complexity and performance impact, for instance due to TCAM
utilization increase from an increased number of ACL entries.
o Placing a host into the correct network segment is problematic.
For example, in the case of 802.11 Wi-Fi the user might select the
wrong SSID. In the case of wired 802.1x authentication the
authentication server might not have all the information required
to make the correct decision.
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It would be beneficial for IPv6 deployment if operators could
implement IPv6-mostly (or IPv4-as-a-Service) segments where IPv6-only
hosts co-exist with legacy dual-stack devices. The trivial solution
of disabling IPv4 stack on IPv6-only capable hosts is not feasible as
those clients must be able to operate on IPv4-only networks as well.
While IPv6-only capable devices might use a heuristic approach to
learning if the network provides IPv6-only functionality and stop
using IPv4 if it does, it might be practically undesirable. One
important reason is that when a host connects to a network, it does
not know if the network is IPv4-only, dual-stack or IPv6-only. To
ensure that the connectivity over whatever protocol is present
becomes available as soon as possible the host usually starts
configuring both IPv4 and IPv6 immediately. If hosts were to delay
requesting IPv4 until IPv6 reachability is confirmed, that would
penalize IPv4-only and dual-stack networks, which does not seem
practical. Requesting IPv4 and then releasing it later, after IPv6
reachability is confirmed, might cause user-visible errors as it
would be disruptive for applications which have started using the
assigned IPv4 address already. Instead it would be useful to have a
mechanism which would allow a host to indicate that IPv4 is optional
and a network to signal that IPv6-only functionality (such as NAT64)
is available. The proposed solution is to introduce a new DHCP
option which a client uses to indicate that it does not need IPv4 if
the network provides IPv6-only connectivity (as NAT64 and DNS64). If
the particular network segment provides IPv4-as-a-service such
clients would not be supplied with IPv4 addresses, while on IPv4-only
or dual-stack segments without NAT64 services IPv4 addresses will be
provided.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
IPv6-only capable host: a host which does not require IPv4 and can
operate on IPv6-only networks. Strictly speaking IPv6-only
capability is specific to a given interface of the host: if some
application on a host require IPv4 and 464XLAT CLAT [RFC6877] is only
enabled on one interface, the host is IPv6-only capable if connected
to a NAT64 network via that interface.
IPv4-as-a-Service: a deployment scenario when end hosts are expected
to operate in IPv6-only mode by default and IPv4 addresses can be
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assigned to some hosts if those hosts explicitly opt-in to receiving
IPv4 addresses.
IPv6-mostly network: a network which provides NAT64 (possibly with
DNS64) service as well as IPv4 connectivity. Such deployment
scenario allows operators to incrementally turn off IPv4 on end
hosts, while still providing IPv4 to devices which require IPv4 to
operate. But, IPv6-only capable devices need not be assigned IPv4
addresses.
IPv6-Only network: a network which does not provide routing
functionality for IPv4 packets. Such networks may or may not allow
intra-LAN IPv4 connectivity. IPv6-Only network usually provides
access to IPv4-only resources via NAT64 [RFC6147].
NAT64: Network Address and Protocol Translation from IPv6 Clients to
IPv4 Servers [RFC6146];
RA: Router Advertisement, a message used by IPv6 routers to advertise
their presence together with various link and Internet parameters
[RFC4861];
DNS64: a mechanism for synthesizing AAAA records from A records
[RFC6147];
2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets
For networks which contain both IPv6-capable and IPv4-requiring
devices and utilize DHCP for configuring IPv4 network stack on hosts,
it seems only natural to leverage the same protocol to signal that
IPv4 is discretional on a given segment. Such an approach limits the
attack surface to DHCP-related attacks without introducing new
vulnerable elements.
Another benefit of using DHCPv4 for signaling is that IPv4 will be
disabled only if both the client and the server indicate IPv6-only
capability. It allows IPv6-only capable hosts to turn off IPv4 only
upon receiving an explicit signal from the network and operate in
dual-stack or IPv4-only mode otherwise.
Coexistence of IPv6-only, dual-stack and even IPv4-only hosts on the
same LAN would not only allow network administrators to preserve
scarce IPv4 addresses but would also drastically simplify incremental
deployment of IPv6-only networks, positively impacting IPv6 adoption.
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3. IPv6-Only Preferred Option
3.1. Option format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (contd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IPv6-Only Preferred Option Format
Fields:
Type 8-bit identifier of the IPv6-Only Preferred option type as
assigned by IANA: TBD
Length 8-bit unsigned integer. The length of the option excluding
the Type and Length Fields. The server MUST set the length
field to 4. The receiver MUST ignore the IPv6-Only Preferred
option if the length field value is not 4.
Value 32-bit unsigned
integer. The number of
seconds the client should disable DHCPv4 for (V6ONLY_WAIT
configuration variable).
If the server pool is explicitly conifgured with V6ONLY_WAIT
timer the server MUST set the field to that configured value.
Otherwise the server MUST set it to zero.
The client MUST ignore V6ONLY_WAIT timer received from the
server if the value is less than 300 seconds.
3.2. DHCPv4 Client Behaviour
A DHCP client SHOULD allow a device administrator to configure
IPv6-only preferred mode either for a specific interface (to indicate
that the device is IPv6-only capable if connected to a NAT64 network
via that interface) or for all interfaces. If only a specific
interface is configured as IPv6-only capable the DHCP client MUST NOT
be considered as an IPv6-capable for the purpose of sending/receiving
DHCP packets over any other interfaces.
Clients not capable of operating in an IPv6-only NAT64 environment
MUST NOT include the IPv6-only Preferred option in the Parameter
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Request List of any DHCP packets and MUST ignore that option in
packets received from DHCP servers.
IPv6-only capable clients SHOULD include the IPv6-only Preferred
option in the Parameter Request List in DHCPDISCOVER and DHCPREQUEST
messages for interfaces so enabled and follow the processing as
described below on a per interface enabled basis.
If the client did not include the IPv6-only Preferred option in the
DHCPDISCOVER or DHCPREQUEST message it MUST ignore the IPv6-only
Preferred option in any messages received from the server.
If the client includes the IPv6-only Preferred option in the
Parameter Request List and the DHCPOFFER message from the server
contains a valid IPv6-only Preferred option, the client MUST NOT
configure the IPv4 address provided in the DHCPOFFER. If the
IPv6-only Preferred option returned by the server contains non-zero
value the client SHOULD set the V6ONLY_WAIT timer to that value. If
the server returns zero value the client MUST use its own
configuration for V6ONLY_WAIT timer. The client SHOULD stop the DHCP
configuration process for at least V6ONLY_WAIT seconds or until a
network attachment event happens. The host MAY disable IPv4 stack
completely for V6ONLY_WAIT seconds or until the network disconnection
event happens.
The client SHOULD include the IPv6-only Preferred option in
DHCPREQUEST messages (after receiving a DHCPOFFER without this
option, for a INIT-REBOOT, or when renewing or rebinding a leased
address). If the DHCP server responds with a DHCPACK that includes
the IPv6-only Preferred option, the client MAY send a DHCPRELEASE
message and MAY either stop the DHCP configuration process or disable
IPv4 stack completely for V6ONLY_WAIT seconds or until the network
disconnection event happens. Alternatively the client MAY continue
to use the assigned IPv4 address until further DHCP reconfiguration
events.
If the client includes the IPv6-only Preferred option in the
Parameter Request List and the server responds with DHCPOFFER message
without a valid IPv6-only Preferred option, the client MUST proceed
as normal with a DHCPREQUEST.
If the client waits for multiple DHCPOFFER responses and selects one
of them, it MUST follow the processing for the IPv6-only Preferred
option based on the selected response. A client MAY use the presence
of the IPv6-only Preferred option as a selection criteria.
When an IPv6-only capable client receives the IPv6-Only Preferred
option from the server, the client MAY configure IPv4 link-local
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address [RFC3927]. In that case IPv6-Only capable devices might
still be able to communicate over IPv4 to other devices on the link.
3.3. DHCPv4 Server Behaviour
The DHCP server SHOULD have a configuration option to configure the
given DHCP pool with an IPv6-only preferred option. The DHCP server
MAY have a configuration option to specify V6ONLY_WAIT timer for all
or individual IPv6-mostly pools.
The server MUST NOT include the IPv6-only Preferred option in the
DHCPOFFER or DHCPACK message if the YIADDR field in the message does
not belong to a pool configured as IPv6-mostly. The server MUST NOT
include the IPv6-only Preferred option in the DHCPOFFER or DHCPACK
message if the option was not present in the Parameter Request List
sent by the client.
If the IPv6-only Preferred option is present in the Parameter Request
List received from the client and the corresponding DHCP pool is
explicitly configured as belonging to an IPv6-mostly network segment,
the server MUST include the IPv6-only Preferred option when
responding with the DHCPOFFER or DHCPACK message. If the server
responds with the IPv6-only Preferred option and the V6ONLY_WAIT
timer is configured for the pool, the server MUST copy the configured
value to the IPv6-only Preferred option value field. Otherwise it
MUST set the field to zero. The server SHOULD include an available
IPv4 address from the pool into the DHCPOFFER as per recommendations
in [RFC2131] but SHOULD NOT reserve the address and SHOULD NOT verify
its uniqueness. The client is not expected to use that IPv4 address
so if the client responds with the DHCPREQUEST message for that
address the server SHOULD respond with DHCPNAK.
As an optional optimization an IPv6-mostly pool MAY be configured
with a dedicated IPv4 address to be returned to IPv6-only capable
clients. In that case the server SHOULD specify that address as the
client's network address and MUST NOT verify its uniqueness.
If a client includes both a Rapid-Commit option [RFC4039] and
IPv6-Only Preferred option in the DHCPDISCOVER message the server
SHOULD NOT honor the Rapid-Commit option if the response would
contain the IPv6-only Preferred option to the client. It SHOULD
instead respond with a DHCPOFFER so that the IP address does not need
to be reserved for the client until the lease expires.
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3.4. Configuration Variables
V6ONLY_WAIT The minimum time the client SHOULD stop the DHCP
configuration process for. MUST be no less than 300
seconds. Default: 1800 seconds
4. IANA Considerations
The IANA is requested to assign a new DHCP Option code for the
IPv6-Only Preferred option from the BOOTP Vendor Extensions and DHCP
Options registry, located at https://www.iana.org/assignments/bootp-
dhcp-parameters/bootp-dhcp-parameters.xhtml#options . If possible,
please assign option code 108.
+----------------------------+-------+
| Option Name | Type |
+----------------------------+-------+
| IPv6-only Preferred option | (TBD) |
+----------------------------+-------+
Table 1
5. Security Considerations
The proposed mechanism is not introducing any new security
implications. While clients using the IPv6-only Preferred option are
vulnerable to attacks related to a rogue DHCP server, enabling
IPv6-only Preferred option does not provide an attacker with any
additional mechanisms.
It should be noted that disabling IPv4 on a host upon receiving the
IPv6-only Preferred option from the DHCP server protects the host
from IPv4-related attacks and therefore could be considered a
security feature.
6. Acknowledgements
Thanks to the following people (in alphabetical order) for their
review and feedback: Mohamed Bboucadair, Bjorn Mork, Bernie Volz (AI:
add more names here). Authors would like to thank Bob Hinden and
Brian Carpenter for the initial idea of signaling IPv6-only
capability to hosts.
7. References
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7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
DOI 10.17487/RFC3927, May 2005,
<https://www.rfc-editor.org/info/rfc3927>.
[RFC4039] Park, S., Kim, P., and B. Volz, "Rapid Commit Option for
the Dynamic Host Configuration Protocol version 4
(DHCPv4)", RFC 4039, DOI 10.17487/RFC4039, March 2005,
<https://www.rfc-editor.org/info/rfc4039>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <https://www.rfc-editor.org/info/rfc6146>.
[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC6147, April 2011,
<https://www.rfc-editor.org/info/rfc6147>.
[RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
Combination of Stateful and Stateless Translation",
RFC 6877, DOI 10.17487/RFC6877, April 2013,
<https://www.rfc-editor.org/info/rfc6877>.
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Authors' Addresses
Lorenzo Colitti
Google
Shibuya 3-21-3
Shibuya, Tokyo 150-0002
JP
Email: lorenzo@google.com
Jen Linkova
Google
1 Darling Island Rd
Pyrmont, NSW 2009
AU
Email: furry@google.com
Michael C. Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
URI: http://www.sandelman.ca/
Tomek Mrugalski
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
USA
Email: tomasz.mrugalski@gmail.com
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