Network Time Protocol (NTP) Server Option for DHCPv6

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 .

This document defines a DHCPv6 option and associated suboptions to provide Network Time Protocol version 4 [draft-ntpv4] configuration information to DHCPv6 hosts.

(SNTP Configuration Option for DHCPv6) provides some degree of automatic time server configuration for IPv6, as it specifies how to transmit SNTP servers addresses through DHCPv6. However, this approach is not suitable for all NTP deployments. The use of the SNTP protocol is only authorized at the root and leafs, of an NTP tree (i.e. stratum 1 servers and highest stratum clients). But, NTP servers that receive time from an upstream source, and redistribute it to LAN workstations are not allowed to run SNTP. This is why SNTP is not the protocol of choice for general purpose NTP servers directly used by client workstations. Additionally the approach of only offering IPv6 addresses to specify server location does not meet NTP requirements that make uses of a FQDN (Fully Qualified Domain Name) as well. The NTP service is publicly offered on the Internet by a number of organizations. Those servers can be used but not abused, so any method which is tasked to disseminate locations of NTP servers must act responsibly in a manner that does not lead to public server overloading. When using DHCPv6 to offer NTP server location, and if there is a need to distribute a device with a hardcoded configuration, this configuration MUST NOT include server location that is not part of the organization that distribute this device. Typical usage of this option is to specify an NTP server that is part of the organization that operates the DHCPv6 server. The location of the NTP service, like any other Internet service, can be specified by an IP address or an FQDN. By design, DHCP offers information to multiple devices and is prone to amplification of mistakes, so great care must be taken to define its configuration. Specification of the NTP service by FQDN offers a level of indirection that works as a possible mitigation tool in case of misconfiguration. While the NTPv4 specification defines a comprehensive set of configuration parameters, modification of those parameters is best left to the decision of the client itself. The DHCPv6 option for NTP is then restricted to server location.

This option serves as a container for all the information related to one NTP server. This option can appear multiple times in a DHCPv6 message. Each instance of this option is to be considered by the NTP client as a server to include in its configuration. The option itself does not contain any value. Instead, it contains one or several suboptions that carry NTP server configuration information. The first suboption in this option MUST be either NTP_OPTION_SRV_ADDR or NTP_OPTION_SRV_FQDN. It carries the NTP server location, as an IPv6 address or as an NTP server FQDN (Fully Qualified Domain Name). If the NTP server location is an IPv6 multicast address, the client SHOULD use this address as an NTP multicast group address and listen to messages on this group in order to synchronize its clock. As an example, the format of this option when pointing to an NTP server located at address 2001:db8::1 is:

A client that receives this option SHOULD use the specified NTP server to synchronize its own clock. This document does not define any priority between the client's embedded configuration and the NTP servers discovered via this option. In particular, the client is allowed to simultaneously use its own configured NTP servers and the servers discovered via DHCP.

This sub-option is intended to appear inside the OPTION_NTP_SERVER option. It specifies the IPv6 address of an NTP server available to the client. An example of use is present in the previous section. This suboption can also be used to carry the IPv6 multicast address of a multicast group used on the local network to carry NTP multicast messages. As an example, the content of this Sub-Option to enable the client to join the FF05::101 NTP multicast group is:

This sub-option is intended to appear inside the OPTION_NTP_SERVER option. It specifies the FQDN of an NTP server available to the client. As an example, the content of this Sub-Option to enable the client to use the 'ntp.example.com' server is:

The OPTION_NTP_SERVER option can appear multiple times in a DHCPv6 message. The order in which these options appear is not significant. The client uses its usual algorithms to determine which server(s) or multicast group(s) should be prefered to synchronize its clock. The OPTION_NTP_SERVER option MUST NOT appear in messages other than the following: Solicit, Advertise, Request, Renew, Rebind, Information-Request, and Reply. If this option appears in messages other than those specified above, the receiver SHOULD ignore it. The option number for this option MAY appear in the "Option Request" option in the following messages: Solicit, Request, Renew, Rebind, Information-Request, and Reconfigure. If this option number appears in the "Option Request" option in messages other than those specified above, the receiver SHOULD ignore it.

This option could be used by an intruder to advertise the address of a malicious NTP server and adversely affect the clock of clients on the network. The consequences of such an attack can be critical, because many security protocols depend on time synchronization to run their algorithms. As an example, an attacker could break connectivity between SEND-enabled nodes , simply by affecting the clock on these nodes. To prevent these attacks, it is strongly advisable to secure the use of this option either by: using the NTPv4 Autokey public key authentication, as defined in or, using authenticated DHCP as described in section 21.

When this document is published, the IANA is requested to assign an option code from the "DHCPv6 Options Codes" registry for OPTION_NTP_SERVER.