Ntp: use values from network manager (via DHCP)

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Ok so I understand much better the status now (I’ve updated the wiki), depending on how you want to enable internet:

  • by default, NixOs uses a custom script to setup internet. This script, however, does not configure ntp from DHCP
  • networkd: networking.useNetworkd can be set to use systemd-networkd to configure the connections instead of the custom script (useNetworkd configures the systemd.network module to provide a decent default configuration). NTP from DHCP is automatically configured (the new NTP server is sent automatically to systemd-timesyncd) without any action, but in 2022 useNetworkd is considered experimental. But I tested it and it works great!
  • Network manager: Network Manager (networking.networkmanager.enable = true;), cannot directly configure NTP over DHCP… but there is a workaround: add a script that reconfigures systemd-timesyncd (with a new configuration file per connection) when a new connection is added and restart the service:
  ## To use, put this in your configuration, switch to it, and restart NM:
  ## $ sudo systemctl restart NetworkManager.service
  ## To check if it works, you can do `sudo systemctl status systemd-timesyncd.service`
  ## (it may take a bit of time to pick the right NTP as it may try the
  ## other NTP firsts)
  networking.networkmanager.dispatcherScripts = [
    {
      # https://wiki.archlinux.org/title/NetworkManager#Dynamically_set_NTP_servers_received_via_DHCP_with_systemd-timesyncd
      # You can debug with sudo journalctl -u NetworkManager-dispatcher -e
      # make sure to restart it:
      # sudo systemctl restart NetworkManager.service
      source = pkgs.writeText "10-update-timesyncd" ''
        [ -z "$CONNECTION_UUID" ] && exit 0
        INTERFACE="$1"
        ACTION="$2"
        case $ACTION in
        up | dhcp4-change | dhcp6-change)
            systemctl restart systemd-timesyncd.service
            if [ -n "$DHCP4_NTP_SERVERS" ]; then
              echo "Will add the ntp server $DHCP4_NTP_SERVERS"
            else
              echo "No DHCP4 NTP available."
              exit 0
            fi
            mkdir -p /etc/systemd/timesyncd.conf.d
            # <<-EOF must really use tabs to keep indentation correct… and tabs are often converted to space in wiki
            # so I don't want to risk strange issues with indentation
            echo "[Time]" > "/etc/systemd/timesyncd.conf.d/''${CONNECTION_UUID}.conf"
            echo "NTP=$DHCP4_NTP_SERVERS" >> "/etc/systemd/timesyncd.conf.d/''${CONNECTION_UUID}.conf"
            echo "config written"
            systemctl restart systemd-timesyncd.service
            echo "restarted"
            ;;
        down)
            rm -f "/etc/systemd/timesyncd.conf.d/''${CONNECTION_UUID}.conf"
            systemctl restart systemd-timesyncd.service
            ;;
        esac
        echo 'Done!'
      '';
    }
  ];

Note that it may take a bit of time (one minute maybe) for timesyncd to take the new configuration as it may try first the other NTP servers and timeout before trying the new NTP server.

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