Network Working Group R. Gayraud
Internet-Draft B. Lourdelet
Intended status: Standards Track Cisco Systems, Inc.
Expires: January 12, 2009 July 11, 2008
Network Time Protocol (NTP) Server Option for DHCPv6
draft-ietf-ntp-dhcpv6-ntp-opt-02.txt
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Abstract
The NTP Server Option for DHCPv6 provides NTP (Network Time Protocol
version 4) configuration information to DHCPv6 hosts.
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Related Work and Usage Model . . . . . . . . . . . . . . . 3
3. NTP Server Option for DHCPv6 . . . . . . . . . . . . . . . . . 4
3.1. NTP Server Address Suboption . . . . . . . . . . . . . . . 5
3.2. NTP Multicast Address Suboption . . . . . . . . . . . . . 6
3.3. NTP Server FQDN Suboption . . . . . . . . . . . . . . . . 6
4. Examples of use . . . . . . . . . . . . . . . . . . . . . . . 7
5. Appearance of this Option . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Requirements notation
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].
2. Introduction
This document defines a DHCPv6 option and associated suboptions to
provide Network Time Protocol version 4 [draft-ntpv4] or greater
configuration information to DHCPv6 hosts.
2.1. Related Work and Usage Model
[RFC4075] (SNTP Configuration Option for DHCPv6) provides some degree
of automatic time server configuration for IPv6, as it specifies how
to transmit SNTP [RFC4330] 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. DNS can be used to redirect misconfigured clients
to an unexisting IPv6 address instead of having to change the address
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of the NTP Server itself.
While the NTP 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.
3. NTP Server Option for DHCPv6
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. This option MUST include one, and only one, time source
suboption. The currently defined time source suboptions are:
NTP_OPTION_SRV_ADDR, NTP_OPTION_SRV_MC_ADDR, NTP_OPTION_SRV_FQDN. It
carries the NTP server location, as a unicast or multicast IPv6
address or as an NTP server FQDN. More time source suboptions may be
defined in the future. While the FQDN option offers the most
deployment flexibility, resiliency as well as security, the IP
address options are defined to cover cases where a dependancy to DNS
is not desirable.
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 sent to this group in order to synchronize its clock.
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The format of the NTP Server 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_NTP_SERVER | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| suboption-1 |
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| suboption-2 |
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| suboption-n |
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_NTP_SERVER (TBD_IANA),
option-len: Total length of the included suboptions.
A client that receives this option SHOULD use the specified NTP
server to synchronize its 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.
3.1. NTP Server Address Suboption
This suboption is intended to appear inside the OPTION_NTP_SERVER
option. It specifies the IPv6 unicast address of an NTP server
available to the client. An example of use is present in Section 4.
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The format of the NTP Server Address Suboption 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP_SUBOPTION_SRV_ADDR | suboption-len = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| IPv6 address of NTP server |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
suboption-code: NTP_SUBOPTION_SRV_ADDR (1),
suboption-len: 16.
3.2. NTP Multicast Address Suboption
This suboption is intended to appear inside the OPTION_NTP_SERVER
option. It specifies the IPv6 address of the IPv6 multicast group
address used by NTP on the local network. An example of use is
present in Section 4.
The format of the NTP Multicast Address Suboption 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP_SUBOPTION_MC_ADDR | suboption-len = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| Multicast IPv6 address |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
suboption-code: NTP_SUBOPTION_MC_ADDR (2),
suboption-len: 16.
3.3. NTP Server FQDN Suboption
This suboption is intended to appear inside the OPTION_NTP_SERVER
option. It specifies the FQDN of an NTP server available to the
client.
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The format of the NTP Server FQDN Suboption 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP_SUBOPTION_SRV_FQDN | suboption-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| FQDN of NTP server |
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
suboption-code: NTP_SUBOPTION_SRV_FQDN (3),
suboption-len: Length of the included FQDN field,
FQDN: Fully Qualidied Domain Name of the NTP server. This
field MUST be encoded as described in [RFC3315],
section 8.
4. Examples of use
To instruct a client to use an NTP server located at address 2001:
db8::1, a DHCP server shall use the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_NTP_SERVER | option-len = 20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP_SUBOPTION_SRV_ADDR | suboption-len = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IPv6 address = 2001:db8::1 |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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To to enable a client to use the 'ntp.example.com' server, a DHCP
server shall use the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_NTP_SERVER | option-len = 21 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP_SUBOPTION_SRV_FQDN | suboption-len = 17 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 'n' | 't' | 'p' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 7 | 'e' | 'x' | 'a' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 'm' | 'p' | 'l' | 'e' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 'c' | 'o' | 'm' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
To instruct a client to join the FF05::101 NTP multicast group, a
DHCP server shall use the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_NTP_SERVER | option-len = 20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP_SUBOPTION_MC_ADDR | suboption-len = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IPv6 address = FF05::101 |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5. Appearance of this Option
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
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other than those specified above, the receiver SHOULD ignore it.
The option number for this option MAY appear in the "Option Request"
option [RFC3315] 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.
6. Security Considerations
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 [RFC3971], 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 [draft-autokey] or,
- using authenticated DHCP as described in [RFC3315] section 21.
7. IANA Considerations
When this document is published, the IANA is requested to assign an
option code from the "DHCPv6 Options Codes" registry for
OPTION_NTP_SERVER.
8. References
8.1. Normative References
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[draft-ntpv4]
Burbank, J., Kasch, W., Martin, J., and D. Mills, "Network
Time Protocol Version 4 Protocol And Algorithms
Specification", draft-ietf-ntp-ntpv4-proto-08 (work in
progress), November 2007.
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[draft-autokey]
Haberman, B. and D. Mills, "Network Time Protocol Version
4 Autokey Specification", draft-ietf-ntp-autokey-00 (work
in progress), September 2007.
8.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4075] Kalusivalingam, V., "Simple Network Time Protocol (SNTP)
Configuration Option for DHCPv6", RFC 4075, May 2005.
[RFC4330] Mills, D., "Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI", RFC 4330, January 2006.
Authors' Addresses
Richard Gayraud
Cisco Systems, Inc.
Village ent. GreenSide, Bat T3,
400, Av de Roumanille,
06410 BIOT - Sophia-Antipolis Cedex
France
Phone: +33 4 97 23 26 49
Email: rgayraud@cisco.com
Benoit Lourdelet
Cisco Systems, Inc.
Village ent. GreenSide, Bat T3,
400, Av de Roumanille,
06410 BIOT - Sophia-Antipolis Cedex
France
Phone: +33 4 97 23 26 23
Email: blourdel@cisco.com
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