In an Active Directory, an accurate time synchronisation is absolutely necessary. E. g. Kerberos relies on correct timestamps to prevent replay attacks and AD needs it for resolving replication conflicts. The maximum time tolerance in an Active Directory is 5 minutes per default. If e. g. your domain members (clients, Member Servers, DCs) clock differs more than that to your servers clock, accessing the server is denied. As the default time source in an Active Directory forest, is the Domain Controller with the PDC emulator FSMO role. See http://technet.microsoft.com/en-us/library/cc773013%28v=ws.10%29.aspx#w2k3tr_times_how_izcr for information about time synchronisation in an AD DS hierarchy.
ntpd from http://ntp.org allows allows time syncronisation with external sources and can also be configured to be a time source for others. Please note, that ntpd does not support authenticated time to Windows 2000 clients! This is due to these clients not behaving as the ntpd server expects. As these clients are now very old and unsupported, you may need to find another way to keep these clocks in sync.
See the server information used in documentation page for used paths, hostnames, etc.
Configuring time synchronisation on a DC
- ntpd >= 4.2.6 from ntp.org, with enabled signed ntp support ("--enable-ntp-signd"), installed via package or self compiled
- Make sure, that the socket permissions are set correct. It must be readable by the account your ntpd uses and should not be accessable by other
# chown root:ntp /usr/local/samba/var/lib/ntp_signd/ # chmod 750 /usr/local/samba/var/lib/ntp_signd/ # ls -ld /usr/local/samba/var/lib/ntp_signd/ drwxr-x--- 2 root ntp 4096 1. May 09:30 /usr/local/samba/var/lib/ntp_signd/
Typically ntpd's configuration file is /etc/ntpd.conf in most distributions. In the following, you see a working minimum ntpd.conf, that retrieves its time from an external NTP server and provides time via signed NTP for others.
# Local clock (Note: This is not the localhost address!) server 127.127.1.0 fudge 127.127.1.0 stratum 10 # The source, where we are receiving the time from server 0.pool.ntp.org iburst prefer driftfile /var/lib/ntp/ntp.drift logfile /var/log/ntp ntpsigndsocket /usr/local/samba/var/lib/ntp_signd/ # Access control # Default restriction: Only allow querying time (incl. ms-sntp) from this machine restrict default kod nomodify notrap nopeer mssntp # Allow everything from localhost restrict 127.0.0.1 # Allow that our time source can only provide time and do nothing else restrict 0.pool.ntp.org mask 255.255.255.255 nomodify notrap nopeer noquery
For further information about ntpd access control, see http://support.ntp.org/bin/view/Support/AccessRestrictions. Examples about ntpd SELinux labeling and policy you will find here.
Configuring time synchronisation on a Samba Domain Members
- ntpd from ntp.org, installed via package or self compiled
Typically ntpd's configuration file is /etc/ntpd.conf in most distributions. In the following, you see a working minimum ntpd.conf, that retrieves its time from an Samba AD Domain Controller and doesn't provide NTP for others.
# Local clock (this is not the localhost address!) server 127.127.1.0 fudge 127.127.1.0 stratum 10 # The source, where we are receiving the time from server DC1.samdom.example.com iburst prefer driftfile /var/lib/ntp/ntp.drift logfile /var/log/ntp # Access control # Default restriction restrict default ignore # Allow everything from localhost restrict 127.0.0.1 # Allow that our time source can only provide time and do nothing else restrict DC1.samdom.example.com mask 255.255.255.255 nomodify notrap nopeer noquery
For further information about ntpd access control, see http://support.ntp.org/bin/view/Support/AccessRestrictions
Configuring time synchronisation on Windows clients
Per default, Windows clients in an Active Directory, automatically synchronize their time with the DC, owning the PDC emulator role. If you don't want to use a different source or to configure multiple time server, etc. you don't have to take any action.
Setting user defined time source(s) and options
If you require your Windows clients to synchronize time with a different server than your DC owning the PDC role, you can configure this via Group Policies. Using the following way, you can define multiple time servers and adjust time sycronisation related options:
- Create a new Group Policy Object in the Group Policy Management Console (part of the Remote Server Administration Tools) and edit it.
- In the Group Policy Management Editor, go to "Computer Configuration" / "Administrative Templates" / "System" / "Windows Time Service" / "Time Providers".
- Edit the "Configure Windows NTP Client" policy:
- This example changes the NTP server setting to a DC that provides time, but is not owner of the PDC role. For further explanations on the possible options, see the description in the policy and, visit http://technet.microsoft.com/de-de/library/cc779145%28v=ws.10%29.aspx.
- Save the GPO and link it to the desired OU.
Configuring time synchronisation on Linux clients