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ntp
net-misc/ntp is an implementation of the Network Time Protocol.
Installation
USE flags
USE flags for net-misc/ntp Network Time Protocol suite/programs
caps
|
Use Linux capabilities library to control privilege |
debug
|
Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces |
ipv6
|
Add support for IP version 6 |
openntpd
|
Allow ntp to be installed alongside openntpd |
parse-clocks
|
Add support for PARSE clocks |
readline
|
Enable support for libreadline, a GNU line-editing library that almost everyone wants |
samba
|
Provide support for Samba's signing daemon (needed for Active Directory domain controllers) |
selinux
|
!!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur |
snmp
|
Add support for the Simple Network Management Protocol if available |
ssl
|
Add support for SSL/TLS connections (Secure Socket Layer / Transport Layer Security) |
threads
|
Add threads support for various packages. Usually pthreads |
vim-syntax
|
Pulls in related vim syntax scripts |
zeroconf
|
Support for DNS Service Discovery (DNS-SD) |
Emerge
Install the suite of NTP programs:
root #emerge --ask net-misc/ntpAlternatively OpenNTPD can be used instead of net-misc/ntp.
Configuration
Ntp-Client
To adjust ntp-client's command and upstream servers, edit the ntp-client configuration file. The default configuration is populated with:
/etc/conf.d/ntp-clientNTPCLIENT_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:
/etc/ntp.confserver 0.gentoo.pool.ntp.org server 1.gentoo.pool.ntp.org server 2.gentoo.pool.ntp.org server 3.gentoo.pool.ntp.org
Time zones and location of the server do not matter for NTP; it synchronizes via UTC.
By default the Gentoo servers are listed and enabled. A list of available servers can be found on ntp.org. A home or company server here can be used, if ntpd is running and the machine is allowed access.
On systems where a network connection is not always available at boot (laptops, etc.), it might help to add the following lines to server configuration:
/etc/ntp.confserver 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 a network connection and switches to using network servers when a 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:
/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:
/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.
/etc/ntp.confdisable monitor
Usage
Basic tools and common usage.
Client
ntp-client
To start the ntp-client:
root #rc-service ntp-client startTo view the status of the client:
root #rc-service ntp-client statusTo have the client start at boot:
root #rc-update add ntp-client defaultntpdate
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.orgServer
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.confIf 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-updateWhen 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 defaultTo monitor status of the server:
root #rc-service ntpd status