Every server should have accurate time information, this is especially important for mail server. To make sure our servers time is always up to date we are going to install ntp package which enables our server to use NTP protocol for syncing your servers clock.
apt-get install ntp
If your data center or VPS provider offers an NTP service use their
NTP server. Open up
/etc/ntp.conf with nano and
server 0.debian.pool.ntp.org iburst dynamic server 1.debian.pool.ntp.org iburst dynamic server 2.debian.pool.ntp.org iburst dynamic server 3.debian.pool.ntp.org iburst dynamic
You should have at least two servers in the ntp.conf.
After saving the changes you have to restart the NTP service by running
invoke-rc.d ntp restart
Note: invoke-rc.d is Debians shortcut to /etc/init.d/. You can run
/etc/init.d/ntp restart as well.
You can examine if the NTP server is working as expected if you run the ntpq -p command:
example:~# ntpq -p remote refid st t when poll reach delay offset jitter ============================================================================== zg1.ntp.CARNet. 18.104.22.168 2 u 58 64 3 7.157 106.619 4.158 zg2.ntp.CARNet. 22.214.171.124 2 u 57 64 3 7.677 25.934 4.792 morcic.RI.CARNe 126.96.36.199 2 u 58 64 3 11.796 26.527 7.751
After a while the reach column should have values greather that 0. Which means that the NTP servers we are pooling for time information are reachable.
If your clock was way off from the current time NTP will not sync your servers time. In that case stop the NTP daemon. Set the time and date manually (or you can install and use the ntpdate utility) and run the NTP daemon again.