====== Linux Timekeeping ======

<file>
ntpdate pool.ntp.org

ntpq -p

ntpstat

grep ntp /var/log/messages
</file>

===== Ubuntu =====

https://linuxconfig.org/how-to-change-timezone-on-ubuntu-18-04-bionic-beaver-linux

https://feeding.cloud.geek.nz/posts/time-synchronization-with-ntp-and-systemd/

:!: ''systemd-timesyncd'' is a client-only, lightweight, simple, replacement for ''ntpd''.  It cannot act as a time server.

<file>
timedatectl set-timezone America/Phoenix

timedatectl status
</file>

<file>
systemctl restart systemd-timesyncd.service
</file>

===== CentOS =====

Here we:

  - Set the correct timezone
  - Set the actual time
  - Set the NTP daemon to automatically start on system boot
  - Start the NTP daemon
  - Verify the NTP daemon is working properly

<file>
rm /etc/localtime

ln -s /usr/share/zoneinfo/America/Phoenix /etc/localtime

ntpdate pool.ntp.org

chkconfig ntpd on
service ntpd start

tail /var/log/messages
</file>

===== SME Server =====

Already functions as NTP server.

<file>
cat /var/log/ntpd/current |tai64nlocal
</file>

===== Ubuntu =====

<file>
sudo apt-get install ntp

sudo service ntp restart
</file>

===== Server Preference =====

Setting a local time server may noticeably speed up boot times.

Edit /etc/ntp.conf and add a line for your preferred server:

<file>
vi /etc/ntp.conf


server 192.168.2.1 prefer
</file>

If the initscripts run ntpdate, you can replace /etc/ntp/step-tickers:

<file>
sudo cp -a /etc/ntp/step-tickers /etc/ntp/step-tickers.orig

sudo vim /etc/ntp/step-tickers

192.168.2.1
</file>

===== NTP Servers =====

<file>
0.us.pool.ntp.org 
1.us.pool.ntp.org
2.us.pool.ntp.org
3.us.pool.ntp.org
</file>

===== Local Network Server =====

Edit the config file, uncomment and modify:

<file>
vim /etc/ntp.conf

# Hosts on local network are less restricted.
restrict 192.168.2.0 mask 255.255.255.0 nomodify notrap
broadcast 192.168.2.255
</file>

<file>
sudo service ntpd restart
</file>

===== Public Server =====

https://www.digitalocean.com/community/tutorials/how-to-configure-ntp-for-use-in-the-ntp-pool-project-on-ubuntu-16-04

===== Firewall =====

Open UDP port 123 on the firewall:

http://www.ghidinelli.com/2008/09/25/iptables-firewall-rules-for-ntpd-time-synchronization

<file>
/sbin/iptables -A INPUT -p udp -s 192.168.30.0/24 --dport 123 -j ACCEPT
/sbin/iptables -A OUTPUT -p udp --sport 123 -j ACCEPT

service iptables save
</file>
or:
<file>
system-config-securitylevel-tui

123:udp

netstat -uapn|grep 123

</file>