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# 27.11. Clock Synchronization with NTP

Contributed by Tom Hukins.

## 27.11.1. Overview

Over time, a computer's clock is prone to drift. The Network Time Protocol (NTP) is one way to ensure your clock stays accurate.

Many Internet services rely on, or greatly benefit from, computers' clocks being accurate. For example, a web server may receive requests to send a file if it has been modified since a certain time. In a local area network environment, it is essential that computers sharing files from the same file server have synchronized clocks so that file timestamps stay consistent. Services such as cron(8) also rely on an accurate system clock to run commands at the specified times.

FreeBSD ships with the ntpd(8) NTP server which can be used to query other NTP servers to set the clock on your machine or provide time services to others.

## 27.11.2. Choosing Appropriate NTP Servers

In order to synchronize your clock, you will need to find one or more NTP servers to use. Your network administrator or ISP may have set up an NTP server for this purpose——check their documentation to see if this is the case. There is an online list of publicly accessible NTP servers which you can use to find an NTP server near to you. Make sure you are aware of the policy for any servers you choose, and ask for permission if required.

Choosing several unconnected NTP servers is a good idea in case one of the servers you are using becomes unreachable or its clock is unreliable. ntpd(8) uses the responses it receives from other servers intelligently——it will favor unreliable servers less than reliable ones.

## 27.11.3. Configuring Your Machine

### 27.11.3.1. Basic Configuration

If you only wish to synchronize your clock when the machine boots up, you can use ntpdate(8). This may be appropriate for some desktop machines which are frequently rebooted and only require infrequent synchronization, but most machines should run ntpd(8).

Using ntpdate(8) at boot time is also a good idea for machines that run ntpd(8). The ntpd(8) program changes the clock gradually, whereas ntpdate(8) sets the clock, no matter how great the difference between a machine's current clock setting and the correct time.

To enable ntpdate(8) at boot time, add `ntpdate_enable="YES"` to `/etc/rc.conf`. You will also need to specify all servers you wish to synchronize with and any flags to be passed to ntpdate(8) in `ntpdate_flags`.

### 27.11.3.2. General Configuration

NTP is configured by the `/etc/ntp.conf` file in the format described in ntp.conf(5). Here is a simple example:

    server ntplocal.example.com prefer
    server timeserver.example.org
    server ntp2a.example.net

    driftfile /var/db/ntp.drift

The `server` option specifies which servers are to be used, with one server listed on each line. If a server is specified with the `prefer` argument, as with `ntplocal.example.com`, 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 without any consideration to other responses. The `prefer` argument is normally used for NTP servers that are known to be highly accurate, such as those with special time monitoring hardware.

The `driftfile` option specifies which file is used to store the system clock's frequency offset. The ntpd(8) program 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.

The `driftfile` option specifies which file is used to store information about previous responses from the NTP servers you are using. This file contains internal information for NTP. It should not be modified by any other process.

### 27.11.3.3. Controlling Access to Your Server

By default, your NTP server will be accessible to all hosts on the Internet. The `restrict` option in `/etc/ntp.conf` allows you to control which machines can access your server.

If you want to deny all machines from accessing your NTP server, add the following line to `/etc/ntp.conf`:

    restrict default ignore

If you only want to allow machines within your own network to synchronize their clocks with your server, but ensure they are not allowed to configure the server or used as peers to synchronize against, add

    restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

instead, where `192.168.1.0` is an IP address on your network and `255.255.255.0` is your network's netmask.

`/etc/ntp.conf` can contain multiple `restrict` options. For more details, see the `Access Control Support` subsection of ntp.conf(5).

## 27.11.4. Running the NTP Server

To ensure the NTP server is started at boot time, add the line `ntpd_enable="YES"` to `/etc/rc.conf`. If you wish to pass additional flags to ntpd(8), edit the `ntpd_flags` parameter in `/etc/rc.conf`.

To start the server without rebooting your machine, run `ntpd` being sure to specify any additional parameters from `ntpd_flags` in `/etc/rc.conf`. For example:

    # ntpd -p /var/run/ntpd.pid

> 注: Under FreeBSD 4.X, you have to replace every instance of `ntpd` with `xntpd` in the options above.

## 27.11.5. Using ntpd with a Temporary Internet Connection

The ntpd(8) program does not need a permanent connection to the Internet to function properly. However, if you have a temporary connection that is configured to dial out on demand, it is a good idea to prevent NTP traffic from triggering a dial out or keeping the connection alive. If you are using user PPP, you can use `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 see the `PACKET FILTERING` section in ppp(8) and the examples in `/usr/share/examples/ppp/`.

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

## 27.11.6. Further Information

Documentation for the NTP server can be found in `/usr/share/doc/ntp/` in HTML format.

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