Ntpd is a daemon used to keep system time of server synchronized with Internet standard time servers. Ntpd is the acronymous of Network Time Protocol (NTP) daemon. Your server could be a client (asks time) or a server (gives time). Even if you don’t have ntpd set, your server has an internal clock which keep your server time updated (but of course not synchronized with Internet standard time servers).
You can find ntp configuration in the file:
In this file are indicated the ip address of the ntp servers you need for sync
server ntpserver1 server ntpserver2
Hardware Clock and System Clock
Any server has two different clock: hardware and system.
Hardware Clock is set on BIOS and it is independent from the system you have installed in your server. Server Battery keeps Hardware clock update even if you switch off your server.
System Clock is kept by Kernel Linux, It is a software Clock.
date and hwclock are two command necessary to show or set system clock (date) or hardware clock (hwclock).
[root@server ~]# date gio 21 mag 2015, 15.19.37, UTC [root@server ~]# hwclock gio 21 mag 2015 15:20:21 UTC -0.562774 seconds
If your server is not synchronized or you have a mismatch between hardware clock and system clock, you can manually synchronize your server using the following:
service ntpd stop ntpdate ntp_server_name_or_ip hwclock --systohc service ntpd start
hwclock –systohc sync hardware and system clock. You can set it as UTC or Localtime as you prefer
hwclock --systohc --utc hwclock --systohc --localtime
In case you need to the clock difference between your server and another_server you need clockdiff command
clockdiff another_server host=another_server rtt=750(187)ms/0ms delta=79685ms/79685ms Thu May 21 15:35:11 2015
Open iptables NTP port
NTP uses port 123 udp. So it is important that you open this port in your IPTABLES if you are configuring an ntp server.
-A INPUT -p udp --dport 123 -j ACCEPT -A OUTPUT -p udp --sport 123 -j ACCEPT