How to configure a Linux NTP server

Network Time Protocol (NTP) provides algorithms and defines messages for the synchronization of time customers for an accurate time reference. This article describes how to configure a Linux NTP time server to synchronize time with an Internet-based public NTP server.

NTP server systems fall into two categories: primary reference servers and secondary reference. Primary reference servers an external reference is used when sufficient time, such as GPS or radio clocks. Secondary reference servers synchronize with slightly reduced accuracy primary reference NTP servers and offer. Primary reference servers are designated stratum 1 servers, while secondary servers have a stratum greater than 1.

The Distribution NTP

NTP source code is freely available from the site of Network Time Protocol Internet. The current version is available for download 4.2.4. NTP is available for Linux operating systems with ports available for Windows NT. After downloading the source code, must be configured, built and installed on the host machine. Many Linux operating systems, such as RedHat, RPM provide NTP packets.

Configuring NTP

The 'ntp.conf' is the main source of configuration information from an NTP server installation. Among other things, contains a list of reference clocks that the installation is synchronization. A list of references NTP server is specified with the 'server' configuration command like this:

the server time-a.nist.gov # NIST, Gaithersburg, Maryland NTP server

the server time-c.timefreq.bldrdoc.gov # NIST, Boulder, Colorado NTP server

Controlling the NTP server daemon

Once configured, the NTP daemon to start, stop and restart with the commands: 'ntpd Start "," ntpd stop' and 'ntpd restart'. The NTP server daemon can be accessed through the "ntpq-p 'command. Ntpq command queries the NTP server for synchronization status and provides a list of servers with synchronization information for each server.

NTP Access Control

NTP server access can be restricted through the "restrict" directive in the ntp.conf file. You can restrict access to the NTP server:

restrict default ignore

To allow only the machinery of its own network to synchronize with the server use:

restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

Multiple restriction directives can be specified in the ntp.conf file to restrict access to a particular set of equipment.

Authentication options

Authentication matching allows access to be specified by the NTP server and client partners. NTP keys are stored in the file ntp.keys in the following format: number M-number key (the M means for MD5 encryption), for example:

1 Secret M

5 M RaBBiT

7 M timely

10 M MyKey

In NTP configuration ntp.conf, specify which of the keys listed above are trusted, ie are safe to use. Every key is specified in the key file but not trusted will not be used for authentication, for example:

trustedkey 1 7 10

The NTP server is configured for authentication.

Client Configuration for Authentication

The client must be configured with similar information to the server, however, you can use a subset of the specified keys on the server. A subset of different keys can be used on different clients, including:

Client A)

1 Secret M

7 M timely

trustedkey 1 7

Client B)

1 Secreto M

5 M RaBBiT

7 M timely

10 M MyKey

trustedkey 7 10

Essentially, authentication is used by the client to authenticate the time server is who they claim to be and that no rogue server intervenes. The key is encrypted and sent to the client by the server that is unencrypted and checked that the client keys to ensure a match.

About the Author

Dave Evans develops Linux NTP Server synchronisation systems to ensure accurate time on PC’s and computer networks. Click here to find out more about Linux NTP Time Server systems.

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