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Local NTP (Chrony) over Hotspot / LAN

A step-by-step guide to use a laptop/PC as a time server and Raspberry Pis as clients, with or without Internet.

Overview

You can keep your Raspberry Pis in sync by running Chrony on a computer (the “server”) and pointing the Pis (the “clients”) at it. This works even with no Internet (server serves its own clock), and across hotspots (NetworkManager shared AP or phone hotspot) or normal Wi-Fi LANs.

This guide covers:

  • Server setup (Ubuntu 24/25+ or similar)
  • Raspberry Pi client setup (Raspberry Pi OS Bookworm+)
  • Hotspot/LAN subnet rules
  • Verification & troubleshooting

Network assumptions

  • Server hostname (example): roadking → reachable as roadking.local via mDNS.
  • Hotspot/LAN subnets (pick what you actually use):
    • NetworkManager Wi-Fi Hotspot: typically 10.42.0.0/24 (server IP often 10.42.0.1)
    • Android phone hotspot: often 192.168.43.0/24
    • Any other router: check with ip -4 addr

Find your active subnet on the server:

ip -4 addr show             # identify the Wi-Fi interface (e.g., wlp… or wlan0)
ip -4 addr show <iface>     # look for: inet X.Y.Z.W/24 → your subnet is X.Y.Z.0/24

1) Server setup (Ubuntu 24/25+)

Note: in this example the server is called 'roadking'.

1.1 Set hostname & enable mDNS (so roadking.local resolves). This may already be enabled on the computer, so if it already resolves (ping), then can skip this and go to 1.2.

sudo hostnamectl set-hostname roadking
sudo apt update
sudo apt install -y avahi-daemon libnss-mdns
sudo systemctl enable --now avahi-daemon

Test from another machine on the same Wi-Fi:

ping -c1 roadking.local

1.2 Install Chrony

sudo apt install -y chrony

1.3 Configure Chrony

Back up and edit the main config:

sudo cp /etc/chrony/chrony.conf /etc/chrony/chrony.conf.bak.$(date +%s)
sudo nano /etc/chrony/chrony.conf

If you are on Ubuntu 25+, Chrony may include extra snippets via:

sourcedir /etc/chrony/chrony.d

Comment that out if you want a minimal, explicit config:

#sourcedir /etc/chrony/chrony.d

A) If the server HAS Internet and you want it to sync upstream:

  • Keep a couple of pool lines or add your preferred NTP servers.
  • Add allow for your hotspot/LAN subnet so Pis can query you.
  • (Optional) cmdallow if you want to run chronyc clients from other hosts without sudo.

Example:

# Upstream time (keep 1–2 pools)
pool 0.pool.ntp.org iburst
pool 1.pool.ntp.org iburst

# Allow clients on my subnet to use me as NTP server
allow 10.42.0.0/24
# or: allow 192.168.43.0/24

# (Optional) allow chronyc control commands from subnet (e.g., 'chronyc clients')
cmdallow 10.42.0.0/24

B) If the server has NO Internet and should serve its own clock:

  • Remove/comment any pool/server lines.
  • Add local stratum 10 so Chrony serves its own clock.
  • Add allow for your subnet.
  • (Optional) cmdallow.

Example (standalone):

# Act as a standalone local time source (no upstream)
local stratum 10

# Allow clients on hotspot/LAN
allow 10.42.0.0/24
# or: allow 192.168.43.0/24

# Optional control channel permissions
cmdallow 10.42.0.0/24

Save & exit.

1.4 Restart & enable Chrony

sudo systemctl restart chrony
sudo systemctl enable chrony

1.5 (Optional) Open firewall for NTP/UDP 123

If UFW is enabled:

sudo ufw allow proto udp to any port 123
sudo ufw reload

1.6 Verify on the server

chronyc sources -v      # shows upstream (case A) or local clock (case B)
sudo chronyc clients    # shows clients once Pis start querying
journalctl -u chrony --no-pager | grep -i denied   # check for refused requests

2) Raspberry Pi client setup (Bookworm+)

2.1 Ensure mDNS (so roadking.local resolves quickly)

sudo apt update
sudo apt install -y avahi-utils libnss-mdns
getent ahosts roadking.local   # should print an IP

Tip: If mDNS lookups feel slow or your hotspot renumbers, you can pin a static entry in /etc/hosts on each Pi:

echo "10.42.0.1 roadking.local roadking" | sudo tee -a /etc/hosts

(Replace 10.42.0.1 with Roadking’s current hotspot IP.)

2.2 Install Chrony

sudo apt install -y chrony

2.3 Point at the server

sudo cp /etc/chrony/chrony.conf /etc/chrony/chrony.conf.bak.$(date +%s)
sudo nano /etc/chrony/chrony.conf

Comment out default pools/servers and add your server:

# Comment out any defaults:
# pool ...
# server ...
# sourcedir ...


# Use the local server (by name)
server roadking.local iburst minpoll 3 maxpoll 8

# (Optional) add a fallback server if Roadking is sometimes unavailable:
# server thria.local iburst minpoll 3 maxpoll 8

Save & exit.

2.4 Restart & enable Chrony

sudo systemctl restart chrony
sudo systemctl enable chrony

2.5 Verify on the Pi

Immediately:

chronyc sources -v
chronyc tracking

After ~30–60s you should see ^* roadking.local in sources and a sensible offset/skew in tracking.

Quick one-shot query (also tests UDP 123 reachability):

sudo apt install -y ntpdate
ntpdate -q roadking.local

3) Hotspot notes

  • NetworkManager Wi-Fi Hotspot (Linux) defaults to 10.42.0.0/24, server IP 10.42.0.1.
  • Phone hotspots often use 192.168.43.0/24 (Android) or other 192.168.x.0/24.
  • Always match the server’s active subnet in allow (and cmdallow if you use it).
  • If you change hotspot types, update allow and restart Chrony on the server.

4) Troubleshooting

Symptom: chronyc clients on server says 501 Not authorised

  • That message refers to Chrony’s control channel, not NTP service.
  • Run as root: sudo chronyc clients
  • Or add cmdallow <your-subnet>/24 to server’s chrony.conf and restart.

Symptom: Pi shows:

chronyc sources
^? roadking.local ...
chronyc tracking → Stratum: 0 / Not synchronised
  • Give it 30–60 seconds after the first query.
  • Verify name resolution: getent ahosts roadking.local
  • Try direct IP: server 10.42.0.1 iburst ... (replace with server IP).
  • On server, confirm allow <subnet>/24 matches the Pi’s actual IP subnet.
  • Check firewall: allow UDP/123 on server.

Symptom: “Refusing NTP request” or “denied” in server logs

  • journalctl -u chrony | grep -i denied
  • Fix allow subnet (e.g., allow 10.42.0.0/24) and restart Chrony.

Symptom: Name resolution slow (mDNS delay)

  • Ensure libnss-mdns is installed and /etc/nsswitch.conf has: 
    hosts: files mdns4_minimal [NOTFOUND=return] dns myhostname
  • Optionally add static /etc/hosts lines for instant lookups.

Symptom: Server has no Internet but must still serve time

  • Use the standalone config (remove pools; add local stratum 10).

5) Quick automation snippets

Server (standalone, hotspot 10.42.0.0/24):

sudo apt update
sudo apt install -y avahi-daemon libnss-mdns chrony
sudo cp /etc/chrony/chrony.conf /etc/chrony/chrony.conf.bak.$(date +%s)
sudo bash -c 'cat >/etc/chrony/chrony.conf' <<'CONF'
#sourcedir /etc/chrony/chrony.d
local stratum 10
allow 10.42.0.0/24
#cmdallow 10.42.0.0/24
CONF
sudo systemctl restart chrony
sudo systemctl enable chrony

Client (Pi → roadking.local):

sudo apt update
sudo apt install -y avahi-utils libnss-mdns chrony
sudo cp /etc/chrony/chrony.conf /etc/chrony/chrony.conf.bak.$(date +%s)
sudo bash -c 'cat >/etc/chrony/chrony.conf' <<'CONF'
server roadking.local iburst minpoll 3 maxpoll 8
CONF
sudo systemctl restart chrony
sudo systemctl enable chrony
chronyc sources -v
chronyc tracking

6) Summary

  • Install Chrony on the server and Pis.
  • On the server: use allow <subnet>/24 (and local stratum 10 if offline). Comment out sourcedir on Ubuntu 25+ if you want a minimal config.
  • On the Pis: set server roadking.local iburst (or the server IP).
  • Enable mDNS or add /etc/hosts entries for instant resolution.
  • Verify with chronyc sources/tracking (Pi) and chronyc clients (server).