Why the NTP service systemd-timesyncd is not able to synchronize to an offline NTP Server?

### Configuration of systemd-timesyncd ### On my linux distribution I'm using the NTP client systemd-timesyncd. Its configuration file (/etc/systemd/timesyncd.conf) is:

[Time]
NTP=192.168.127.11
FallbackNTP=192.168.127.11
#RootDistanceMaxSec=5
#PollIntervalMinSec=32
#PollIntervalMaxSec=2048

where 192.168.127.11 is the IP address of a Lubuntu workstation where is installed the package ntp. The Lubuntu workstation is **offline** (it is not connected to any other host other than the linux distribution where is in execution the systemd-timesyncd client). ### NTP Server configuration ### The Lubuntu workstation (ip address 192.168.127.201) executes the service ntpsec.service with the following configuration file /etc/ntpsec/ntp.conf:

# /etc/ntpsec/ntp.conf, configuration for ntpd; see ntp.conf(5) for help

driftfile /var/lib/ntpsec/ntp.drift
leapfile /usr/share/zoneinfo/leap-seconds.list

# This should be maxclock 7, but the pool entries count towards maxclock.
tos maxclock 11

# Comment this out if you have a refclock and want it to be able to discipline
# the clock by itself (e.g. if the system is not connected to the network).
#tos minclock 4 minsane 3

# Specify one or more NTP servers.

# Public NTP servers supporting Network Time Security:
# server time.cloudflare.com nts

# Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board
# on 2011-02-08 (LP: #104525). See https://www.pool.ntp.org/join.html  for
# more information.
#pool 0.ubuntu.pool.ntp.org iburst
#pool 1.ubuntu.pool.ntp.org iburst
#pool 2.ubuntu.pool.ntp.org iburst
#pool 3.ubuntu.pool.ntp.org iburst

# Use Ubuntu's ntp server as a fallback.
#server ntp.ubuntu.com
server 127.127.1.0
fudge 127.127.1.0 stratum 1

# By default, exchange time with everybody, but don't allow configuration.
restrict default kod nomodify nopeer noquery limited

# Local users may interrogate the ntp server more closely.
restrict 127.0.0.1
restrict ::1

### systemd-timesyncd is in Idle ### systemd-timesyncd client is in the "Idle" Status as showed by the following output message:

systemd-timesyncd.service - Network Time Synchronization
     Loaded: loaded (/lib/systemd/system/systemd-timesyncd.service; disabled; vendor preset: disabled)
     Active: active (running) since Thu 2025-07-10 10:28:13 CEST; 7min ago
       Docs: man:systemd-timesyncd.service(8)
   Main PID: 71449 (systemd-timesyn)
     Status: "Idle."
      Tasks: 2 (limit: 2199)
     Memory: 860.0K
     CGroup: /system.slice/systemd-timesyncd.service
             └─71449 /lib/systemd/systemd-timesyncd

Jul 10 10:28:12  systemd: Starting Network Time Synchronization...
Jul 10 10:28:13  systemd: Started Network Time Synchronization.

### Wireshark captured ### By Wireshark I can see that client and server exchange some datagrams. In the response datagram of the Server I can see the following information:

Frame 4379: 90 bytes on wire (720 bits), 90 bytes captured (720 bits) on interface 0
Ethernet II, Src:  (), Dst:  ()
Internet Protocol Version 4, Src: 192.168.127.11, Dst: 192.168.127.201
User Datagram Protocol, Src Port: 123, Dst Port: 45358
Network Time Protocol (NTP Version 4, server)
    Flags: 0xe4, Leap Indicator: unknown (clock unsynchronized), Version number: NTP Version 4, Mode: server
        11.. .... = Leap Indicator: unknown (clock unsynchronized) (3)
        ..10 0... = Version number: NTP Version 4 (4)
        .... .100 = Mode: server (4)
    Peer Clock Stratum: unspecified or invalid (0)
    Peer Polling Interval: invalid (0)
    Peer Clock Precision: 0,000000 sec
    Root Delay: 0 seconds
    Root Dispersion: 0,0037841796875 seconds
    Reference ID: (Initialization)
    Reference Timestamp: Jan  1, 1970 00:00:00.000000000 UTC
    Origin Timestamp: Jul 10, 2025 09:07:50.163961034 UTC
    Receive Timestamp: Jul 10, 2025 09:14:44.087945767 UTC
    Transmit Timestamp: Jul 10, 2025 09:14:44.088197457 UTC

In this captured datagram I would highlight the Flags byte=0xe4 and in particular the 2 bits:

11.. .... = Leap Indicator: unknown (clock unsynchronized)  # in the Flags byte

Furthermore I highlight the 2 bytes (equal to 0):

Peer Clock Stratum: unspecified or invalid (0)
Peer Polling Interval: invalid (0)

I have noted that with an other NTP server the byte Peer Clock Stratum is equal to 1 and the client is able to synchronize its date and time. ### Question ### Why in my context the service systemd-timesyncd is not able to synchronize to my offline NTP Server? ### links read but not useful ### I have checked many posts (for example [this link](https://unix.stackexchange.com/questions/323348/ntp-client-not-synchronizing-from-a-private-ntp-server) or [this other](https://unix.stackexchange.com/questions/714732/system-clock-not-synchronized-with-ntp-server-using-systemd-timesyncd)) but none of these have been useful.