Raspberry PI pass NTP config with DHCP.

Configure NTP on a Raspberry PI with DHCP option 42 NTP Servers. DHCP allows to send a whole list of options along with the IP address for the Raspberry PI.

NTP client

Raspberry PI OS is debian based and uses systemd-timesyncd as NTP client. The default configuration contains a list of debian pool ntp servers but these require internet access. This can be seen with the command

timedatectl show-timesync --all

When you want to run a Raspberry PI in a vlan with no internet access you will need to setup a local NTP server that the Raspberry PI can use. This could be a local router or a different Raspberry PI (with internet access).

Raspberry PI OS uses dhcpcd as DHCP client. If instead systemd-networkd was used, the NTP config would be passed automatically. Another alternative would be to use Gnome NetworkManager.

Configure dhcpcd

The first step is to configure dhcpcd to use the option ntp_servers. Uncomment the following line in /etc/dhcpcd.conf

option ntp_servers

Add dhcpcd hook

The next step is to alter the systemd-timesyncd configuration when the DHCP option is received. dhcpcd allows hook scripts to be configured that can do this. dhcpcd-run-hooks are configured in /lib/dhcpcd/dhcpcd-hooks. Add the following hook /lib/dhcpcd/dhcpcd-hooks/60-timesyncd.conf

add_ntp_conf()
{
    exec > /etc/systemd/timesyncd.conf.d/$interface.conf
    echo "[Time]"
    echo "NTP=$new_ntp_servers"
    systemctl restart systemd-timesyncd
}

remove_ntp_conf() 
{
    rm -f /etc/systemd/timesyncd.conf.d/$interface.conf
    systemctl restart systemd-timesyncd
}

# For ease of use, map DHCP6 names onto our DHCP4 names
case "$reason" in
BOUND6|RENEW6|REBIND6|REBOOT6|INFORM6)
	new_ntp_servers="$new_dhcp6_sntp_servers"
;;
esac

if $if_up; then
	add_ntp_conf
elif $if_down; then
	remove_ntp_conf
fi

Now reboot the Raspberry PI and the NTP configuration will be added to /etc/systemd/timesyncd.conf.d/eth0.conf. You can also check the output of timedatectl show-timesync --all to validate that timesyncd is configured correctly.