Internet-Draft Registering SLAAC Addresses using DHCPv6 August 2023
Kumari, et al. Expires 17 February 2024 [Page]
Dynamic Host Configuration
Intended Status:
Standards Track
W. Kumari
Google, LLC
S. Krishnan
Cisco Systems, Inc.
R. Asati
Cisco Systems, Inc.
L. Colitti
Google, LLC
J. Linkova
Google, LLC
S. Jiang
Beijing University of Posts and Telecommunications

Registering Self-generated IPv6 Addresses using DHCPv6


This document defines a method to inform a DHCPv6 server that a device has a self-generated or statically configured address.

About This Document

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This Internet-Draft will expire on 17 February 2024.

Table of Contents

1. Introduction

It is very common operational practice, especially in enterprise networks, to use IPv4 DHCP logs for troubleshooting or security purposes. Examples of this include a help desk dealing with a ticket such as "The CEO's laptop cannot connect to the printer"; if the MAC address of the printer is known (for example from an inventory system), the IPv4 address can be retrieved from the DHCP logs and the printer pinged to determine if it is reachable. Another common example is a Security Operations team discovering suspicious events in outbound firewall logs and then consulting DHCP logs to determine which employee's laptop had that IPv4 address at that time so that they can quarantine it and remove the malware.

This operational practice relies on the DHCP server knowing the IP address assignments. Therefore, the practice does not work if static IP addresses are manually configured on devices or self-assigned addresses (such as when self-configuring an IPv6 address using SLAAC [RFC4862]) are used.

The lack of this parity with IPv4 is one of the reasons which may be hindering IPv6 deployment, especially in enterprise networks.

This document provides a mechanism for a device to inform the DHCPv6 server that it has a self-configured IPv6 address (or has a statically configured address), and thus provides parity with IPv4 in this aspect.

2. Conventions and Definitions

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.

3. Registration Mechanism Overview

The DHCPv6 protocol is used as the address registration protocol when a DHCPv6 server performs the role of an address registration server. The DHCPv6 IA Address option [RFC8415] is used to specify the address to be registered.

After successfully assigning a self-generated IPv6 address on one of its interfaces, a client implementing this specification SHOULD multicast an ADDR-REG-INFORM message in order to inform the DHCPv6 server that this self-generated address is in use (as shown in Fig.1).

+----+   +----------------+                  +---------------+
|Host|   |First-hop router|                  |Addr-Reg Server|
+----+   +----------------+                  +---------------+
|   SLAAC   |                                      |
|<--------->|                                      |
|           |                                      |
|           |        ADDR-REG-INFORM               |
|           |                                      |Register / log
|           |        ADDR-REG-REPLY                |address

Figure 1: Address Registration Procedure

4. DHCPv6 Address Registration Procedure

4.1. DHCPv6 Address Registration Request

The DHCPv6 client sends an ADDR-REG-INFORM message to inform that an IPv6 address is in use. The format of the ADDR-REG-INFORM message is described as follows:

  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
 |    msg-type   |               transaction-id                  |
 |                                                               |
 .                            options                            .
 .                           (variable)                          .
 |                                                               |
  msg-type             Identifies the DHCPv6 message type;
                       Set to ADDR-REG-INFORM (TBA1).

  transaction-id       The transaction ID for this message exchange.

  options              Options carried in this message.
Figure 2: DHCPv6 ADDR-REG-INFORM message

The client MUST generate a transaction ID and insert this value in the "transaction-id" field.

The client MUST include a Client Identifier option in the ADDR-REG-INFORM message.

The ADDR-REG-INFORM message MUST NOT contain the Server Identifier option and MUST contain exactly one IA Address option containing the address being registered. The valid-lifetime and preferred-lifetime fields in the option MUST match the current Valid Lifetime and Preferred Lifetime of the address being registered.

The ADDR-REG-INFORM message is dedicated for clients to initiate an address registration request toward an address registration server. Consequently, clients MUST NOT put any Option Request Option(s) in the ADDR-REG-INFORM message. Clients MAY include other options, such as the Client FQDN Option [RFC4704].

The client sends the DHCPv6 ADDR-REG-INFORM message to the All_DHCP_Relay_Agents_and_Servers multicast address (ff02::1:2). The client MUST send separate messages for each address being registered.

Unlike other types of messages, which are sent from the link-local address of the client, the ADDR-REG-INFORM message MUST be sent from the address being registered. This is primarily for "fate sharing" purposes - for example, if the network implements some form of L2 security to prevent a client from spoofing other clients' addresses this prevents an attacker from spoofing ADDR-REG-INFORM messages.

On clients with multiple interfaces, the client MUST only send the packet on the network interface that has the address being registered, even if it has multiple interfaces with different addresses. If the same address is configured on multiple interfaces, then the client MUST send ADDR-REG-INFORM each time the address is configured on an interface that did not previously have it, and refresh each registration independently from the others.

The client MUST only send the ADDR-REG-INFORM message for valid ([RFC4862]) addresses of global scope ([RFC4007]). This includes ULA addresses, which are defined in [RFC4193] to have global scope. The client MUST NOT send the ADDR-REG-INFORM message for addresses configured by DHCPv6.

The client SHOULD NOT send the ADDR-REG-INFORM message if it has not received any Router Advertisement message with either M or O flags set to 1.

Clients MUST discard any received ADDR-REG-INFORM messages.

4.1.1. Server message processing

Servers MUST discard any ADDR-REG-INFORM messages that meet any of the following conditions:

  • the message does not include a Client Identifier option;
  • the message includes a Server Identifier option;
  • the message does not include the IA Address option, or the IP address in the IA Address option does not match the source address of the original ADDR-REG-INFORM message sent by the client. The source address of the original message is the source IP address of the packet if it is not relayed, or the Peer-Address field of the innermost Relay-Forward message if it is relayed.
  • the message includes an Option Request Option.

After receiving this ADDR-REG-INFORM message, the address registration server SHOULD verify that the address being registered is "appropriate to the link" as defined by [RFC8415]. If the server believes that the address being registered is not appropriate to the link [RFC8415], it MUST drop the message, and SHOULD log this fact. Otherwise, the server:

  • SHOULD register or update a binding between the provided Client Identifier and IPv6 address in its database. The lifetime of the binding is equal to the Valid Lifetime of the address reported by the client. If there is already a binding between the registered address and another another client, the server SHOULD log the fact and update the binding.
  • SHOULD log the address registration information (as is done normally for clients which have requested an address), unless configured not to do so.
  • SHOULD mark the address as unavailable for use and not include it in future ADVERTISE messages.
  • SHOULD send back an ADDR-REG-REPLY message.

DHCPv6 relay agents and switches that relay address registration messages directly from clients SHOULD include the client's link-layer address in the relayed message using the Client Link-Layer Address option ([RFC6939]).

4.2. DHCPv6 Address Registration Acknowledgement

The server SHOULD acknowledge receipt of a valid ADDR-REG-INFORM message by sending a ADDR-REG-REPLY message back. The format of the ADDR-REG-REPLY message is described as follows:

  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
 |    msg-type   |               transaction-id                  |
 |                                                               |
 .                            options                            .
 .                           (variable)                          .
 |                                                               |
  msg-type             Identifies the DHCPv6 message type;
                       Set to ADDR-REG-REPLY (TBA2).

  transaction-id       The transaction ID for this message exchange.

  options              Options carried in this message.
Figure 3: DHCPv6 ADDR-REG-REPLY message

If the ADDR-REG-INFORM message that the server is replying to was not relayed, then the IPv6 destination address of the message MUST be the address being registered. If the ADDR-REG-INFORM message was relayed, then the server MUST construct the Relay-reply message as specified in [RFC8415] section 19.3.

The server MUST copy the transaction-id from the ADDR-REG-INFORM message to the transaction-id field of the ADDR-REG-REPLY.

The ADDR-REG-REPLY message MUST contain an IA Address option for the address being registered. The option MUST be identical to the one in the ADDR-REG-INFORM message that the server is replying to.

Servers MUST ignore any received ADDR-REG-REPLY messages.

Clients MUST discard any ADDR-REG-REPLY messages that meet any of the following conditions:

  • The IPv6 destination address does not match the address being registered.
  • The IA-Address option does not match the address being registered.
  • The address being registered is not assigned to the interface receiving the message.
  • The transaction-id does not match the transaction-id the client used in the corresponding ADDR-REG-INFORM message.

The ADDR-REG-REPLY message only indicates that the ADDR-REG-INFORM message has been received. The ADDR-REG-REPLY message MUST NOT be considered as any indication of the address validity and MUST NOT be required for the address to be usable. DHCPv6 relays, or other devices that snoop ADDR-REG-REPLY messages, MUST NOT add or alter any forwarding or security state based on the ADDR-REG-REPLY message.

4.3. Registration Expiry and Refresh

The client MUST refresh addresses as described below. Each refresh is scheduled after AddrRegRefresh seconds, where AddrRegRefresh is min(4 hours, 80% of the address's current Valid Lifetime). Refreshes SHOULD be jittered by +/- 10% to avoid synchronization causing a large number of registrations to expire at the same time.

Whenever the client creates an address or receives a PIO which changes the Valid Lifetime of an existing address by more than 1%, then:

  1. If no refresh is currently scheduled, it MUST register immediately and schedule a refresh.
  2. If a refresh is currently scheduled, it MUST reschedule the existing refresh if this would result in the refresh being sooner than currently scheduled.

Discussion: this algorithm ensures that refreshes are not sent too frequently, while ensuring that the server never believes that the address has expired when it has not. Specifically: - If the network never changes the lifetime, or stops refreshing the lifetime, then only one refresh ever occurs. The address expires. - #1 ensures that any time the network changes the lifetime when no refresh is scheduled, the server will be informed of the correct lifetime. If the network does not change the address's lifetime, then the server already knows the correct lifetime and no refresh needs to be sent. - #2 ensures that if the network reduces the lifetime of the address, then the server will be informed of the new lifetime. If the network increases the lifetime of the address, the refresh will be sent at the previously scheduled time, and the server will be informed of the correct lifetime. From this point on, either the address expires (and the server is informed of when this will happen) or an RA increases the lifetime, in which case a refresh will be sent. - The 1% tolerance ensures that the client will not refresh or reschedule refreshes if the Valid Lifetime experiences minor changes due to transmission delays or clock skew between the client and the router(s) sending the Router Advertisement.

Registration refresh packets SHOULD be retransmitted using the same logic as described in the 'Retransmission' section below.

The client SHOULD generate a new transaction ID when refreshing the registration.

The client MAY choose to notify the server when an address is no longer being used (e.g., if the client is disconnecting from the network, the address lifetime expired, or the address is being removed from the interface). To indicate that the address is not being used anymore the client MUST set the preferred-lifetime and valid-lifetime fields of the IA Address option to zero.

When the Client-Identifier-to-IPv6-address binding has expired, the server SHOULD remove remove it and consider the address as available for use. If the server receives a message with a valid-lifetime of zero, it SHOULD act as if the address has expired.

4.4. Retransmission

To reduce the effects of packet loss on registration, the client SHOULD retransmit the registration message. Retransmissions SHOULD follow the standard retransmission logic specified by section 15 of [RFC8415] with the following default parameters:

  • IRT 1 sec
  • MRC 3

The client SHOULD allow these parameters to be configured by the administrator.

To comply with section 16.1 of [RFC8415], the client MUST leave the transaction ID unchanged in retransmissions of an ADDR-REG-INFORM message. When the client retranmits the registration message, the lifetimes in the packet MUST be updated so that they match the current lifetimes of the address.

If an ADDR-REG-REPLY message is received for the address being registered, the client MUST stop retransmission. However, the client cannot rely on the server acknowledging receipt of the registration message, because the server might not support address registration.

5. Host configuration

DHCP clients SHOULD allow the administrator to disable sending ADDR-REG-INFORM messages. This could be used, for example, to reduce network traffic on networks where the servers are known not to support the message type. Sending the messages SHOULD be enabled by default.

6. Security Considerations

An attacker may attempt to register a large number of addresses in quick succession in order to overwhelm the address registration server and / or fill up log files. Similar attack vectors exist today, e.g. an attacker can DoS the server with messages contained spoofed DUIDs.

If a network is using FCFS SAVI [RFC6620], then the DHCPv6 server can trust that the ADDR-REG-INFORM message was sent by the legitimate holder of the address. This prevents a host from registering an address owned by another host.

One of the use-cases for the mechanism described in this document is to identify sources of malicious traffic after the fact. Note, however, that as the device itself is responsible for informing the DHCPv6 server that it is using an address, a malicious or compromised device can simply not send the ADDR-REG-INFORM message. This is an informational, optional mechanism, and is designed to aid in troubleshooting and forensics. On its own, it is not intended to be a strong security access mechanism. In particular, the ADDR-REG-INFORM message MUST not be used for authentication and authorization purposes, because in addition to the reasons above, the packets containing the message may be dropped.

7. IANA Considerations

This document defines two new DHCPv6 messages, ADDR-REG-INFORM message (TBA1) described in Section 4.1, and ADDR-REG-REPLY (TBA2) described in Section 4.2, that requires an allocation out of the registry of Message Types defined at

8. References

8.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and B. Zill, "IPv6 Scoped Address Architecture", RFC 4007, DOI 10.17487/RFC4007, , <>.
Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, DOI 10.17487/RFC4193, , <>.
Volz, B., "The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN) Option", RFC 4704, DOI 10.17487/RFC4704, , <>.
Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, , <>.
Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer Address Option in DHCPv6", RFC 6939, DOI 10.17487/RFC6939, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, , <>.

8.2. Informative References

Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS SAVI: First-Come, First-Served Source Address Validation Improvement for Locally Assigned IPv6 Addresses", RFC 6620, DOI 10.17487/RFC6620, , <>.


Much thanks to Bernie Volz for significant review and feedback, as well as Hermin Anggawijaya, Stuart Cheshire, Alan DeKok, Ryan Globus, Erik Kline, David Lamparter, Ted Lemon, Eric Levy-Abegnoli, Jim Reid, Michael Richardson, Mark Smith, Eric Vynke, Timothy Winter for their feedback, comments and guidance.

This document borrows heavily from a previous document, draft-ietf-dhc-addr-registration, which defined "a mechanism to register self-generated and statically configured addresses in DNS through a DHCPv6 server". That document was written Sheng Jiang, Gang Chen, Suresh Krishnan, and Rajiv Asati.


Gang Chen
China Mobile
53A, Xibianmennei Ave.
Xuanwu District
P.R. China

Authors' Addresses

Warren Kumari
Google, LLC
Suresh Krishnan
Cisco Systems, Inc.
Rajiv Asati
Cisco Systems, Inc.
7025 Kit Creek road
Research Triangle Park, 27709-4987
United States of America
Lorenzo Colitti
Google, LLC
Shibuya 3-21-3,
Jen Linkova
Google, LLC
1 Darling Island Rd
Pyrmont 2009
Sheng Jiang
Beijing University of Posts and Telecommunications
No. 10 Xitucheng Road
Haidian District, 100083