Turning off IPv4 Using DHCPv6
draft-perreault-sunset4-noipv4-02

Abstract

This memo defines a new DHCPv6 option for indicating to a dual-stack host or router that IPv4 is to be turned off.

Status of This Memo

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This Internet-Draft will expire on October 13, 2013.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. The Problems We're Trying to Fix
• 3.1. Load on DHCPv4 Server
• 3.2. Bandwidth Consumption
• 3.3. Power Inefficiency
• 4. Design Considerations
• 4.1. DHCPv6 vs DHCPv4
• 5. The No-IPv4 Option
• 5.1. Wire Format
• 5.2. Semantics
• 5.3. Example
• 6. Security Considerations
• 7. IANA Considerations
• 8. Acknowledgements
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Test Results of Terminals Behavior
• Authors' Addresses

1. Introduction

When a dual-stack host makes a DHCPv4 request, it typically interprets the absence of a response as a failure condition. This makes it difficult to deploy such nodes in an IPv6-only network.

Take for example a home router that is dual-stack capable but provisioned with an IPv6-only WAN connection. When the router boots, it typically assigns an IPv4 address to its LAN interface, starts services on that interface, and starts handing out IPv4 addresses to clients on the LAN by answering DHCPv4 requests. This is done unconditionally, without taking the status of the IPv4 connectivity on the WAN interface into account. Hosts on the LAN, in turn, install a default route pointing to the router and start behaving as if IPv4 connectivity was available. IPv4 packets destined to the Internet get dropped at the router and timeouts happen. The end result is that IPv4 remains fully active on the LAN and on the router itself even when it is desired that it be turned off.

The other exmaple is about DHCPv4 server. In Dual-Stack LAN/WLAN network or intranet, the core router or AC often plays the role of DHCP server, and the clients are serval thousands PC or mobile phones. If the server is configured in IPv6-only, the dual-stack or IPv4-only clients will broadcast DHCPDISCOVER messages endlessly in the LAN or WLAN. The thousands clients will cause a DDOS-like attack to all the servers in the network. Scince there are not specific discriptions in any RFCs for client's behavior when it does not receive the DHCPOFFER in response to its DHCPDISCOVER message, verious OS deploy different backoff algorithms. We tested serval popuplar OS(es), the test results is listed in the appendix.

A new mechanism is needed to indicate the absence of IPv4 connectivity or service that the goal is turning off IPv4, this new signaling mechanism shall be transported over IPv6. Therefore, we introduce a new DHCPv6 [RFC3315] option for the purpose of explicitly indicating to the DHCPv6 client that IPv4 connectivity is unavailable.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

The following terms are also used in this document:

Upstream Interface:

An interface on which the No-IPv4 DHCPv6 option is received by a DHCPv6 client.

3. The Problems We're Trying to Fix

3.1. Load on DHCPv4 Server

When a DHCPv4 server is present but intentionally does not respond to a dual-stack node, the aggregated traffic generated by multiple such dual-stack nodes can represent a significant useless load. This scenario can be encountered for example with an ISP serving multiple types of subscribers where some will get IPv4 addresses and others not. It might not be feasible for operational reasons to block the useless requests before they reach the DHCPv4 server, e.g. if the DHCPv4 server itself is the one that has knowledge about which node should or should not get an IPv4 address.

3.2. Bandwidth Consumption

In addition to useless load on the DHCPv4 server, the above scenario could also consume a significant amount of bandwidth, particularly if the aggregated traffic from many clients goes through a low-bandwidth link.

3.3. Power Inefficiency

A dual-stack node that does not get a DHCPv4 response will usually continue retransmitting forever. Therefore, only providing IPv6 on a link will cause the node to needlessly wake up periodically and transmit a few packets. For example, the popular DHCPv4 client implementation by ISC wakes up every 5 minutes by default and tries to contact a DHCPv4 server for 60 seconds. With this configuration, a node will not be able to sleep 20% of the time.

4. Design Considerations

4.1. DHCPv6 vs DHCPv4

• NOTE: This section will be removed before publication as an RFC.

This document describes a new DHCPv6 option for turning off IPv4. An equivalent option could conceivably be created for DHCPv4. Here is a discussion of the pros and cons. Arguments with a + sign argue for a DHCPv4 option, arguments with a - sign argue against.

+

Devices that don't speak IPv6 won't be listening for a "turn off IPv4" code, and therefore won't stop trying to establish IPv4 connectivity.

-

Devices that haven't been updated to speak IPv6 likely won't recognize a new DHCPv4 code telling them that IPv4 isn't supported.

+

However, it's easier to implement something that turns off the IP stack than implement IPv6.

-

Devices that don't speak IPv6 that are still active on the network mean that either IPv4 can't/shouldn't be turned off yet, or IPv4 local connectivity should be maintained to retain local services, even if global IPv4 connectivity is not necessary (think local LAN DLNA streaming, etc).

-

When the goal is to turn off IPv4, having to maintain and operate an IPv4 infrastructure (routing, ACLs, etc.) just to be able to send negative responses to DHCPv4 requests is not productive. Having the option transported in IPv6 allows the ISP to focus on operating an IPv6-only network.

+

However, a full IPv4 infrastructure would not be necessary in many cases. The local router could contain a very restricted DHCPv4 server function whose only purpose would be to reply with the No-IPv4 option. No IPv4 traffic would have to be carried to a distant DHCPv4 server. Note however that this may not be operationally feasible in some situations.

-

Turning IPv4 off using an IPv4-transported signal means that there is no way to go back. Once the DHCPv4 option has been accepted by the DHCPv4 client, IPv4 can no longer be turned on remotely (rebooting the client still works). Configurations change, mistakes happen, and so it is necessary to have a way to turn IPv4 back on. With a DHCPv6 option, IPv4 can be turned back on as soon as the client makes a new DHCPv6 request, which can be the next scheduled one or can be triggered immediately with a Reconfigure message.

The authors conclude that a DHCPv6 option is clearly necessary, whereas it is not as clear for a DHCPv4 option. More feedback on this topic would be appreciated.

5. The No-IPv4 Option

5.1. Wire Format

The No-IPv4 DHCPv6 option is used to signal the unavailability of IPv4 connectivity. The format of the No-IPv4 option is:

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         OPTION_NO_IPV4        |          option-len           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    v4-level   |
+-+-+-+-+-+-+-+-+

  option-code     OPTION_NO_IPV4 (TBD).

  option-len      1.

  v4-level        Level of IPv4 functionality.
          

The DHCPv6 client MUST place the OPTION_NO_IPV4 option code in the Option Request Option ([RFC3315] section 22.7). Servers MAY include the option in responses (if they have been so configured). Servers MAY also place the OPTION_NO_IPV4 option code in an Option Request Option contained in a Reconfigure message.

5.2. Semantics

The option applies to the link on which it is received by the DHCPv6 client. It is used to indicate to the client that it should disable some or all of its IPv4 functionality. What should be disabled depends on the value of v4-level.

v4-level can take the following values:

0 - IPv4 fully enabled:

This is equivalent to the absence of the No-IPv4 option. It is included here so that a DHCPv6 server can explicitly re-enable IPv4 access by including it in a Reply message following a Reconfigure.

1 - No IPv4 upstream:

Any kind of IPv4 connectivity is unavailable on the link on which the option is received. Therefore, any attempts to provision IPv4 by the host or to use IPv4 in any fashion, on that link, will be useless. IPv4 MAY be dropped, blocked, or otherwise ignored on that link.

Upon reception of the No-IPv4 option with value 1, the following IPv4 functionality MUST be disabled on the Upstream Interface:

1. IPv4 addresses MUST NOT be assigned.
2. Currently-assigned IPv4 addresses MUST be unassigned.
3. Dynamic configuration of link-local IPv4 addresses [RFC3927] MUST be disabled.
4. IPv4, ICMPv4, or ARP packets MUST NOT be sent.
5. IPv4, ICMPv4, or ARP packets received MUST be ignored.
6. DNS A queries MUST NOT be sent, even transported over IPv6.

2 - No IPv4 upstream, local IPv4 restricted:

Same semantics as value 1, with the following additions:

If all DHCPv6-configured interfaces receive the No-IPv4 option with a mix of values 1, 2, and 3 (but not exclusively 3), and no other interface provides IPv4 connectivity to the Internet, IPv4 is partially shut down, leaving only local connectivity active. On the Upstream Interface, IPv4 MUST be shut down as listed above. On other interfaces, IPv4 addresses MUST NOT be assigned except for the following:

• Loopback (127.0.0.0/8)
• Link Local (169.254.0.0/16) [RFC3927]
• Private-Use (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) [RFC1918]

3 - No IPv4 at all:

This is intended to be a stricter version of the above.

The host or router running the DHCPv6 client that receives this option MUST disable IPv4 functionality on the Upstream Interface in the same way as for value 1 or 2.

If all DHCPv6-configured interfaces received the No-IPv4 option with exclusively value 3, and no other interface provides IPv4 connectivity to the Internet, IPv4 is completely shut down. In particular:

1. IPv4 address MUST NOT be assigned to any interface.
2. Currently-assigned IPv4 addresses MUST be unassigned.
3. Dynamic configuration of link-local IPv4 addresses [RFC3927] MUST be disabled.
4. IPv4, ICMPv4, or ARP packets MUST NOT be sent on any interface.
5. IPv4, ICMPv4, or ARP packets received on any interface MUST be ignored.
6. In the above, "any interface" includes loopback interfaces. In particular, the 127.0.0.1 special address MUST be removed.
7. Server programs listening on IPv4 addresses (e.g., a DHCPv4 server) MAY be shut down.
8. DNS A queries MUST NOT be sent, even transported over IPv6.
9. If the host or router also runs a DHCPv6 server, it SHOULD include the No-IPv4 option with value 2 in DHCPv6 responses it sends to clients that request it, unless prohibited by local policy. If it currently has active clients, it SHOULD send a Reconfigure to each of them with the OPTION_NO_IPV4 included in the Option Request Option.

The intent is to remove all traces of IPv4 activity. Once the No-IPv4 option with value 3 is activated, the network stack should behave as if IPv4 functionality had never been present. For example, a modular kernel implementation could accomplish the above by unloading the IPv4 kernel module at run time.

5.3. Example

A dual-stack home gateway is set up with a single WAN uplink and is configured to use DHCPv4 and DHCPv6 to automatically obtain IPv4 and IPv6 connectivity. On the LAN side, it has one link with multiple hosts.

When it boots, the router assigns 192.168.1.1/24 to its LAN interfaces and starts a DHCPv4 server listening on it. It hands out addresses 191.168.1.100-199 to clients. It also starts an IPv6 Router Advertisement daemon as well as a stateless DHCPv6 server, also listening on the LAN interfaces.

On the WAN side, it starts two provisioning procedures in parallel: one for IPv4 and one for IPv6.

At this point, the ISP does not know if the router supports IPv6-only operation. Therefore, by default, the ISP responds to DHCPv4 requests as usual.

As part of the IPv6 provisioning procedure, the router sends a DHCPv6 request containing OPTION_NO_IPV4 in an Option Request Option. The ISP's DHCPv6 server's reply includes the No-IPv4 option with value 3. When this procedure finishes, the ISP has determined that this customer will run in IPv6-only mode and starts dropping all IPv4 packets at the first hop. If an IPv4 address was assigned, it is reclaimed, and possibly reassigned to another subscriber.

The home router aborts the IPv4 provisioning procedure (if it is still running) and deactivates all IPv4 functionality. It shuts down its DHCPv4 server. It also configures its own stateless DHCPv6 server to send the No-IPv4 option to clients that request it.

As an optimization, the router could delay setting up IPv4 by a few seconds (10 seconds seems reasonable). If the IPv6 procedure completes with the No-IPv4 option during that time, IPv4 will never have been set up and the router will operate in pure IPv6-only mode from the start.

6. Security Considerations

One security concern is that an attacker could use the No-IPv4 option to deny IPv4 access to a victim. However, unprotected vanilla DHCP can already be exploited to cause such a denial of service ([RFC2131] section 7).

TO BE COMPLETED

7. IANA Considerations

IANA is requested to assign value TBD with description OPTION_NO_IPV4 in the "DHCP Option Codes" table which is part of the dhcpv6-parameters registry.

8. Acknowledgements

Thanks in particular to Marc Blanchet who was the driving force behind this work.

9. References

9.1. Normative References

9.2. Informative References

Appendix A. Test Results of Terminals Behavior

In RFC3315 [RFC3315, DHCPv6], SOL_MAX_RT is defined in DHCPv6 to prevent the frequently requesting of clients, which reduces the aggregated traffic. But in RFC2131 [RFC2131, DHCPv4], there are not corresponding IPv4 definitions or options for client's behavior if the server does not respond for the Discover messages.

In fact, most of the terminals creat backoff algorithms to help them retransmit DHCPDISCOVER message in different frequency according to their state machine. The same point of almost all the verious Operating Systems is that they could not stop DHCPDISCOVER requests to the server. And that will cause DDoS-Like attack to the server and bandwidth consumption in the link.

We test some of the most popular terminals' OS in WLAN, the results are illuminated as below.

------------------------------------------------------------------------
|                    DHCP Discovery Packages Time Table                |
|----------------------------------------------------------------------|
|  | Windows7   |Windows XP   |  IOS_5.0.1 |Android_2.3.7|Symbian_S60  |
|No|Time | Time | Time | Time |Time | Time |Time | Time  | Time | Time |
|  |     |offset|      |offset|     |offset|     |offset |       offset|
|--|-----|------|------|------|-----|------|-----|-------|------|------|
|1 |0    |      |0     |      |0.1  |      |7.8  |       |  0   |      |
|2 |3.9  |3.9   |0.1   | 0.1  |1.4  |  1.3 |10.3 |  2.5  |  2   |  2   |
|3 |13.3 |9.4   |4.1   | 4    |3.8  |  2.4 |17.9 |  7.6  |  6   |  4   |
|4 |30.5 |17.2  |12.1  | 8    |7.9  |  4.1 |33.9 |  16   |  8   |  2   |
|5 |62.8 |32.3  |29.1  | 17   |16.3 |  8.4 |36.5 |  2.6  |  12  |  4   |
|6 |65.9 |3.1   |64.9  | 35.8 |24.9 |  8.6 |  reconnect  |  14  |  2   |
|7 |74.9 |9     |68.9  | 4    |33.4 |  8.5 |56.6 |  20.1 |  18  |  4   |
|8 |92.1 |17.2  |77.9  | 9    |42.2 |  8.8 |60.2 |  3.6  |  20  |  2   |
|9 |395.2|303.1 |93.9  | 16   |50.8 |  8.6 |68.4 |  8.2  |  24  |  4   |
|10|399.1|3.9   |433.9 | 340  |59.1 |  8.3 |84.8 |  16.4 |  26  |  2   |
|11|407.1|8     |438.9 | 5    |127.3|  68.2|86.7 |  1.9  |  30.1|  4.1 |
|12|423.4|16.3  |447.9 | 9    |128.9|  1.6 |  reconnect  |  32.1|  2   |
|13|455.4|32    |464.9 | 17   |131.1|  2.2 |106.7|  20   |  36.1|  4   |
|14|460.4|5     |794.9 | 330  |135.1|  4   |111.4|  4.7  |  38.1|  2   |
|15|467.4|7     |799.9 | 5    |143.4|  8.3 |120.6|  9.2  |  42.1|  4   |
|16|483.4|16    |808.9 | 9    |151.7|  8.3 |134.9|  14.3 |  44.1|  2   |
|17|842.9|359.5 |824.9 | 16   |160.4|  8.7 |136.8|  1.9  |  48.2|  4.1 |
|18|846.9|4     |1141.9| 317  |168.8|  8.4 |  reconnect  |  50.2|  2   |
------------------------------------------------------------------------

 					

Figure:Terminals DHCPDISCOVER requests when Server's DHCPv4 module is down

In this figure:

For Windows7, it seems to initiate 8 times DHCPDISCOVER requests in about 300s interval.

For WindowsXP, firstly it launches 9 times DHCPDISCOVER messages, but after that it cannot get any response from the server, then it initiates 5 times requests in one cycle in around 330s intervals, and never stop.

For IOS5.0.1, it seems like WindowsXP. There are 10 times attempts in one cycle, and the interval is about 68s.

Symbian_S60 uses the simplest backoff method, it launches DISCOVER in every 2 or 4 seconds.

Android2.3.7 is the only Operating System which can stop DISCOVER request by disconnect its wireless connection. It reboot wireless and dhcp connection every 20 seconds.

Authors' Addresses