NTP configuration statements

Below is a list of the configuration statements you can use in the NTP configuration file and an explanation of each statement. The term ``local host'' in the explanations refers to the host that you are configuring with these statements.

peer host_address [ key key_ID ] [ version version_number] [ minpoll ]

Specifies that the host with IP address host_address is to be polled in symmetric active mode. The key option indicates that all packets sent to the host at host_address are to include authentication fields encrypted using the specified key ID. The default is not to include authentication.

The version option allows you to specify the version number to use for outgoing NTP packets. The allowed values for version_number are 1 and 2. If this option is omitted, version 3 is assumed. The minpoll option specifies that the polling interval should be kept at the minimum value allowed. This maximizes the frequency with which the host is polled and should, therefore, be used only for debugging purposes.

server host_address [ key key_ID ] [ version version_number] [ minpoll ]

Specifies that the host with IP address host_address is to be polled in client mode. See the peer statement for an explanation of the remaining arguments.

broadcast IP_address [ key key_ID ] [ minpoll ]

Specifies that xntpd broadcast NTP packets to the address IP_address. This should be the broadcast address of your local network. See the peer statement for an explanation of the remaining arguments.

precision integer

Indicates the precision of the local clock as 2 raised to the integer power. The value of integer should be negative to indicate a fraction. See ``Testing and tuning NTP'' for more information.

driftfile filename

Specifies the name of the file used to record the drift that xntpd has computed. Drift is also known as ``frequency error''.

monitor flag

Indicates whether or not to enable the xntpd monitoring function. The allowed values for flag are yes and no. A valid requestkey statement must be included in the configuration file for monitoring to function. The default value for flag is no.

broadcastclient flag

Indicates whether or not the local host should listen for, and attempt to synchronize with, NTP packets broadcast by time servers on the local network. The allowed values for flag are yes and no. The default is no.

broadcastdelay seconds

Specifies the roundtrip delay in seconds between the local host and the time server that is broadcasting NTP packets. Note that you specify a roundtrip delay, even though NTP cares only about one-way delay when it actually computes the correct time. The default value for seconds is 0.008.

authenticate flag

Indicates whether or not the local server operates in authenticate mode. The allowed values for flag are yes and no. The default is no.

trustedkey key_ID

Specifies which key IDs are trusted.

keys pathname

Specifies the pathname of the keys file. At installation, the value of pathname is /etc/ntp.keys. This file must exist for the trustedkey, requestkey, and controlkey configuration statements to be valid. See ``The NTP keys file'' for more information.

requestkey key_ID . . .

Specifies the key IDs used to authenticate runtime reconfiguration requests made by xntpdc and xntpres using mode 7 control messages. The keys file must exist and properly define all of the listed key IDs for this statement to be valid. To disable runtime configuration of xntpd, omit this statement from the configuration file.

controlkey key_ID . . .

Specifies the key ID used to authenticate runtime reconfiguration requests made by ntpq and by trap receivers using mode 6 control messages. The keys file must exist and properly define all of the listed key IDs for this statement to be valid. To cause mode 6 control messages to be ignored, omit this statement from the configuration file. Doing so has the effect of disabling ntpq and trap receivers on your local host.

restrict host_address mask address_mask flag . . .

Specifies various restrictions on how certain hosts and networks can interact with the local host. See ``NTP address and mask facility'' for more information.

trap host_address port port_number interface interface_address

Configures a trap receiver at address host address and port port number. The value of interface_address must be the IP address of the local host. The host at host_address must be listening for NTP mode 6 control messages or it will not respond to the trap signal sent by the local host. This means that the host at host_address must have the controlkey statement in its configuration file and both that host and the local host must share at least one of the keys listed in that controlkey configuration statement. See ``The NTP keys file'' for more information.

resolver pathname

Indicates the full pathname of the xntpres program, which resolves network names to IP addresses. When NTP is installed, the pathname of the xntpres program is /etc/xntpres. If for some reason you move xntpres to another directory, use the pathname corresponding to its new location. You must also include the requestkey statement in the configuration file for xntpres to operate.

Choose an Enforcement Method

Updated: October 6, 2008

Applies To: Windows 7, Windows Server 2008, Windows Server 2008 R2, Windows Vista

To choose the right NAP enforcement method for your network, review the following sections of this guide to understand the details of how each enforcement method works and determine which method best meets your deployment goals:

You might also consider the following when choosing an enforcement method:

Infrastructure. How well does your current network support the enforcement method?
Cost. Which aspect of your NAP deployment is more important: cost or flexibility?
Complexity. Do you have the expertise to implement and maintain your deployment?
Security. How secure is the enforcement method?

Infrastructure

All NAP enforcement methods can be implemented using a minimum of one computer running Windows Server 2008 R2 or Windows Server 2008. All enforcement methods require that NPS is installed on this computer and configured to evaluate the health of NAP clients. Additional required services depend on the enforcement method. For example, IPsec, VPN, and DHCP enforcement methods require a NAP enforcement server running Windows Server 2008 R2 or Windows Server 2008. The 802.1X enforcement method requires network hardware that supports the 802.1X authentication method and is capable of controlling port characteristics using RADIUS tunnel attributes. For more information, see NAP Configuration Overview.

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Cost

Because support for it is built into the Windows operating system, the cost of deploying NAP is typically low, but depends on how well your existing network hardware and software support your design. If you have deployed Windows Server 2008 R2 or Windows Server 2008 on your network, then you already have some or all of the components required to deploy NAP. Some NAP enforcement methods might require an upgrade of the components of your network infrastructure. For example, deploying NAP with 802.1X enforcement can be costly if you do not already have hardware that supports 802.1X authentication. If you already have hardware that supports 802.1X, then it is likely that you can deploy NAP with 802.1X enforcement at minimal cost. If your hardware does not support NAP, in some cases adding support can be as simple as downloading and installing new firmware. To determine the costs associated with a NAP deployment, review hardware and software requirements for the enforcement methods you have chosen and determine whether server and network hardware upgrades are required. Additional costs that might also be associated with a NAP deployment include planning and design costs and the training of support personnel.

Complexity

NAP with IPsec enforcement is generally the most complex enforcement method to deploy because it requires a public key infrastructure (PKI) and IPsec policies to create logical networks. However, Windows Server 2008 R2 and Windows Server 2008 include tools for managing and monitoring IPsec that eliminates much of this complexity. The 802.1X enforcement method is next in terms of complexity, followed by VPN enforcement. DHCP enforcement is the least complex enforcement method to deploy.

Security

IPsec enforcement is a powerful method for protecting compliant computers from any others; it can be combined with server and domain isolation to ensure that after a computer has demonstrated it is compliant, it will still be restricted to communicating only with authorized hosts. IPsec provides other benefits, too. Network packets are authenticated, which reduces the risk of man-in-the-middle and replay attacks, and traffic can be encrypted with IPsec, providing a high degree of protection from eavesdropping attacks.

Like IPsec, 802.1X offers a high degree of protection. Several enhancements to 802.1X are also available that provide improved security. Until a client has demonstrated that it complies with the organization�s health requirements, its access to the network will be restricted by the network switches and wireless access points. These restrictions can be very difficult to bypass, even by a determined malicious user.

VPN enforcement is a good choice for protecting remote access to the corporate network. Clients that connect to a NAP-enabled VPN server can be granted restricted access to the network at the point of entry using IP packet filters. The health of remote client computers is evaluated when the client connects and it is monitored during the VPN session. If a client computer becomes noncompliant, then access is restricted until the computer is compliant.

DHCP enforcement restricts noncompliant computers by providing a limited IPv4 address configuration. It is the least secure NAP enforcement method. It is easily bypassed by an advanced user who has administrative privileges on the computer.

Combining NAP enforcement methods

It is possible to use more than one enforcement method simultaneously. An organization might invest additional resources into combining these enforcement technologies because they have complementary strengths and weaknesses. NAP with VPN enforcement can be used to enforce organizational compliance policies on remote clients, while NAP with IPsec enforcement can be used for local clients. 802.1X and IPsec offer a particularly robust combination because together they can restrict network connectivity at multiple layers of the network protocol stack. The following table shows which enforcement methods can be combined. Keep in mind that the complexity of your NAP deployment will increase when you combine enforcement methods.

	IPsec	802.1X	VPN	DHCP
IPsec
802.1X			X
VPN		X		X
DHCP

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