28.11. Clock Synchronization with NTP

Over time, a computer's clock is prone to drift. This is problematic as many network services require the computers on a network to share the same accurate time. Accurate time is also needed to ensure that file timestamps stay consistent. The Network Time Protocol (NTP) is one way to provide clock accuracy in a network.

FreeBSD includes ntpd(8) which can be configured to query other NTP servers in order to synchronize the clock on that machine or to provide time services to other computers in the network. The servers which are queried can be local to the network or provided by an ISP. In addition, an online list of publicly accessible NTP servers is available. When choosing a public NTP server, select one that is geographically close and review its usage policy.

Choosing several NTP servers is recommended in case one of the servers becomes unreachable or its clock proves unreliable. As ntpd receives responses, it favors reliable servers over the less reliable ones.

This section describes how to configure ntpd on FreeBSD. Further documentation can be found in /usr/share/doc/ntp/ in HTML format.

28.11.1. NTP Configuration

On FreeBSD, the built-in ntpd can be used to synchronize a system's clock. To enable ntpd at boot time, add ntpd_enable="YES" to /etc/rc.conf. Additional variables can be specified in /etc/rc.conf. Refer to rc.conf(5) and ntpd(8) for details.

This application reads /etc/ntp.conf to determine which NTP servers to query. Here is a simple example of an /etc/ntp.conf:

Example 28.3.  Sample /etc/ntp.conf
server ntplocal.example.com prefer
server timeserver.example.org
server ntp2a.example.net

driftfile /var/db/ntp.drift

The format of this file is described in ntp.conf(5). The server option specifies which servers to query, with one server listed on each line. If a server entry includes prefer, that server is preferred over other servers. A response from a preferred server will be discarded if it differs significantly from other servers' responses; otherwise it will be used. The prefer argument should only be used for NTP servers that are known to be highly accurate, such as those with special time monitoring hardware.

The driftfile entry specifies which file is used to store the system clock's frequency offset. ntpd uses this to automatically compensate for the clock's natural drift, allowing it to maintain a reasonably correct setting even if it is cut off from all external time sources for a period of time. This file also stores information about previous responses from NTP servers. Since this file contains internal information for NTP, it should not be modified.

By default, an NTP server is accessible to any network host. The restrict option in /etc/ntp.conf can be used to control which systems can access the server. For example, to deny all machines from accessing the NTP server, add the following line to /etc/ntp.conf:

restrict default ignore

Note:

This will also prevent access from other NTP servers. If there is a need to synchronize with an external NTP server, allow only that specific server. Refer to ntp.conf(5) for more information.

To allow machines within the network to synchronize their clocks with the server, but ensure they are not allowed to configure the server or be used as peers to synchronize against, instead use:

restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

where 192.168.1.0 is the local network address and 255.255.255.0 is the network's subnet mask.

Multiple restrict entries are supported. For more details, refer to the Access Control Support subsection of ntp.conf(5).

Once ntpd_enable="YES" has been added to /etc/rc.conf, ntpd can be started now without rebooting the system by typing:

# service ntpd start

28.11.2. Using NTP with a PPP Connection

ntpd does not need a permanent connection to the Internet to function properly. However, if a PPP connection is configured to dial out on demand, NTP traffic should be prevented from triggering a dial out or keeping the connection alive. This can be configured with filter directives in /etc/ppp/ppp.conf. For example:

 set filter dial 0 deny udp src eq 123
 # Prevent NTP traffic from initiating dial out
 set filter dial 1 permit 0 0
 set filter alive 0 deny udp src eq 123
 # Prevent incoming NTP traffic from keeping the connection open
 set filter alive 1 deny udp dst eq 123
 # Prevent outgoing NTP traffic from keeping the connection open
 set filter alive 2 permit 0/0 0/0

For more details, refer to the PACKET FILTERING section in ppp(8) and the examples in /usr/share/examples/ppp/.

Note:

Some Internet access providers block low-numbered ports, preventing NTP from functioning since replies never reach the machine.

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.