Port 123 NTP
Port 123 is the IANA-assigned ntp port for Network Time Protocol (TCP / UDP).
What is port 123 used for?
Port 123 is assigned to ntp and uses TCP / UDP. The IANA registry describes the service as “Network Time Protocol”. It sits in the well-known range, which operating systems reserve for core network services, so software expects it at this number by default.
Ports are the transport-layer address a service listens on. A client that wants to reach ntp connects to port 123. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 123 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.
Browse all Time synchronization ports
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [Dave_Mills]. Reference: [RFC 5905].
Check and block port 123
Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.
Check on Linux
sudo ss -tlpn 'sport = :123'
sudo ss -ulpn 'sport = :123'Check on Windows
Get-NetTCPConnection -LocalPort 123 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 123 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 123/tcp
sudo ufw deny 123/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 123 -j DROP
sudo iptables -A INPUT -p udp --dport 123 -j DROPThese commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.
Is port 123 safe to expose?
An openly reachable NTP server can be abused for reflection and amplification; use authenticated NTP and rate limiting. Expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Where port 123 is usually used
Most traffic on port 123 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Servers synchronizing clocks with an internal or public time source.
- Troubleshooting clock drift that breaks TLS, logs or authentication.
- Restricting time services to trusted hosts so they cannot be used for amplification.
Check clock sources
Query the NTP daemon's own status; chronyc reports the selected source and current offset.
chronyc sources -vCommon problems with port 123
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 123. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.
Port already in use
Find the process holding the port with sudo ss -tlpn 'sport = :123' or sudo lsof -i :123. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.
Firewall rule did not survive a reboot
UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.
The port is blocked but still reachable
A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.
Port 123 at a glance
| Port number | 123 |
|---|---|
| Service name | ntp |
| Transport protocol | TCP / UDP |
| Category | Time synchronization |
| Range | Well-known port (0–1023) |
| Risk level | Medium |
| Registered assignee | [Dave_Mills] |
| Reference | [RFC 5905] |
| Registry description | Network Time Protocol |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 123 used for?
Port 123 (NTP) uses TCP / UDP. Network Time Protocol.
How do I check if port 123 is open?
On Linux run `sudo ss -tlpn 'sport = :123'` and `sudo ss -ulpn 'sport = :123'`. On Windows run `Get-NetTCPConnection -LocalPort 123 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 123 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 123; empty output usually means nothing is bound to it.
How do I block port 123?
With UFW run `sudo ufw deny 123/tcp` and `sudo ufw deny 123/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 123 -j DROP` and `sudo iptables -A INPUT -p udp --dport 123 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 123 safe to expose?
An openly reachable NTP server can be abused for reflection and amplification; use authenticated NTP and rate limiting. Nevertheless, expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Related ports
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy