How to override the ntp information sent by dhcp in debian?

I have a server in a network receiving all its network information from DHCP. The problem is that the ntp-server being sent is not set to UTC. I want to use the configuration for ntp found in /etc/ntp.conf but the DHCP information takes precedence. How can I force ntpd to use the configuration in /etc/ntp.conf as opposed to the one sent by DHCP? What is the Debian way to do this?

Solution 1:

I suggest you edit/create /etc/dhclient.conf and uncomment the line "request" with something like this:

request subnet-mask, broadcast-address, routers, domain-name, domain-name-servers, host-name;

Especially, if "ntp-servers" is present, remove it. Under some distribution ( aka Fedora ) it is sent by default, so you have to put this line explicitely so that the default is overriden.

Check man dhclient.conf for extended informations.

Solution 2:

Not directly answering the question but that may come in handy.

DHCP behavior

On Debian (at least) the DHCP client is overriding the existing ntp service configuration.

It is controlled by the script /etc/dhcp/dhclient-exit-hooks.d/ntp:

[contents from: /etc/dhcp/dhclient-exit-hooks.d/ntp]
NTP_CONF=/etc/ntp.conf
NTP_DHCP_CONF=/var/lib/ntp/ntp.conf.dhcp
...
[hundreds of lines of scripts]

The DHCP client receive ntp servers from the DHCP server, it grabs the current ntp service configuration from NTP_CONF path and alters it to include ntp servers received over dhcp, it generates a new ntp configuration file into NTP_DHCP_CONF path, and finally it forces the ntp service to use this new configuration file.

This results in always using the ntp servers advertised over dhcp. Bypassing system configuration.

This is hard to debug if not known because the service configuration in '/etc/ntp.conf' is left untouched and is correct on visual inspection. However it is actually ignored and overridden stealthy.

You can use 'ntpq -pn' to debug what ntp servers are actually being used by the deamon.

Mixing DHCP ntp options and system specific configuration

If there IS a ntp server in your network being advertised by DHCP and you want to ignore it:

You will need to reconfigure the dhcp client in /etc/dhclient.conf to skip the ntp-servers dhcp option. Refer to the answer from @Oliver.

Then you can customize the system configuration in /etc/ntp.conf

If there WAS a ntp server advertised by DHCP at some point but not anymore:

The system ntp configuration might still be overridden by the dhcp client, forcing obsolete ntp servers advertised a long time ago. This dhcp overriding can survive for days, across ntp service restart and reboot.

To put an end to it, you will need to remove /var/lib/ntp/ntp.conf.dhcp and restart the ntp service on all your servers.

Final Word

This is all undocumented, unexpected and hard to debug behavior. That caused me quite the headache today which is why I am documenting this in here.

Solution 3:

You can also just remove this file: /etc/dhcp/dhclient-exit-hooks.d/ntp, instead of editing your DHCP-requests. This file creates ntp.conf.dhcp.

In case ntp.conf.dhcp got created at a previous boot, be sure to remove it as well.