ntp

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NTP (Network Time Protocol) is used to synchronize the system time with other devices over the network. This usually happens in a client-server model.

Installation

USE flags

Optional USE flags for net-misc/ntp:
USE flag (what is that?) Default Recommended Description
caps No Use Linux capabilities library to control privilege
debug No No Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see http://www.gentoo.org/proj/en/qa/backtraces.xml
ipv6 Yes Add support for IP version 6
openntpd No Allow ntp to be installed alongside openntpd
parse-clocks No Add support for PARSE clocks
samba No Provide support for Samba's signing daemon (needed for Active Directory domain controllers)
selinux No  !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur
snmp No Add support for the Simple Network Management Protocol if available
ssl Yes Add support for Secure Socket Layer connections
vim-syntax No Pulls in related vim syntax scripts
zeroconf No Support for DNS Service Discovery (DNS-SD)

Emerge

Install the suite of NTP programs:

root #emerge --ask net-misc/ntp

Alternatively OpenNTPD can be use instead of net-misc/ntp.

Configuration

Ntp-Client

To adjust ntp-client's command and upstream servers, edit the ntp-client configuration file:

FILE /etc/conf.d/ntp-client
NTPCLIENT_CMD="ntpdate"
NTPCLIENT_OPTS="-s -b -u \
	0.gentoo.pool.ntp.org 1.gentoo.pool.ntp.org \
	2.gentoo.pool.ntp.org 3.gentoo.pool.ntp.org"

Server

In /etc/ntp.conf the servers that will be used to synchronize the local time for ntpd can be specified. The default configuration is populated with:

FILE /etc/ntp.conf
server 0.gentoo.pool.ntp.org
server 1.gentoo.pool.ntp.org
server 2.gentoo.pool.ntp.org
server 3.gentoo.pool.ntp.org
Note
Time zones and location of the server do not matter for NTP, it synchronizes via UTC time.

Per default the Gentoo servers are listed and enabled. A list of available servers can be found here: ntp.org. A home or company server here can used, given that ntpd is running and the machine is allowed to.

On systems, where network connection is not always available at boot (laptops, etc.) it might be helpful to add the following lines to server configuration:

FILE /etc/ntp.conf
server 127.127.1.0
fudge  127.127.1.0 stratum 10

This sets localhost as a server with low priority, so that the daemon starts properly even without network connection and switches to using network servers when connection is established.

Permissions

Permission are used to control who is allowed to synchronize or change permissions.

Access to NTP service allowed only from localhost. noquery can be added to help prevent the server from being abused to conduct DDOS attacks:

FILE /etc/ntp.conf
# To deny other machines from changing the
# configuration but allow localhost:
restrict default nomodify nopeer noquery
restrict 127.0.0.1

Access to NTP service allowed only from the 192.168.0.0/24 network:

FILE /etc/ntp.conf
# To allow machines within the local network to synchronize
# their clocks with this server, but ensure they are
# not allowed to configure the server or used as peers
# to synchronize against, uncomment this line.
#
restrict 192.168.0.0 mask 255.255.255.0 nomodify nopeer notrap

Denying access to NTP's monlist functionality, used for querying traffic stats but also exploited in a denial-of-service attack.

FILE /etc/ntp.conf
disable monitor

Usage

Basic tools and common usage.

Client

ntp-client

To start the ntp-client:

root #rc-service ntp-client start

To view the status of the client:

root #rc-service ntp-client status

To have the client start at boot:

root #rc-update add ntp-client default

ntpdate

This used to be the client, but its functionality is now moved into ntpd and ntp-client itself. It is purely to set the local time when started and then exits (not a service):

root #ntpdate pool.ntp.org

Server

The server is both a client, and server. If the setup can not access net early in init, use server only instead.

ntpd service

If ntpd is run as a service, the time will automatically synchronize as long as the difference between the local time and the time on the server is less than 1000s (~17 mins). So it is pretty common to adjust the time initially to whatever the server time is as a trusted source:

root #ntpd -g -c /etc/ntp.conf
Note
If ntpd is already running, it will not start a second time.

Add ntpd to the default runlevel to have the time synchronized automatically. There is no need to run a client when the service is running. In this case verify that ntp-client or ntpdate are not in any runlevels.

root #rc-update

When it is confirmed the configuration is clear from ntp-client or ntpdate, add the ntpd service:

root #rc-service ntpd start
root #rc-update add ntpd default

To monitor status of the server:

root #rc-service ntpd status

See also

External resources