Port 123: NTP

Common Ports Reference: What runs on port 3389, 445 or 5985: common TCP and UDP ports for web, mail, Windows, AD, databases and VPNs, with security notes.

The ports that come up when opening a firewall, reading a scan or troubleshooting a connection: what runs on each, TCP or UDP, and what to watch out for. Search by number or name. Port numbers and service names were checked against IANA's port number registry; ports that are widely used but assigned by IANA to something else are marked unofficial.

Port number or name

123 is a system port (0-1023): assigned by IANA, and on Linux only root can listen on it. IANA's registry lists everything assigned to it: look up 123 at IANA.

1 of 96 ports

DNS, DHCP, time, SNMP, logging and VPNs.

Port	Protocol	Service	What it's for	IANA name
123	`UDP`	NTP	Time synchronisation. Kerberos fails when clocks drift more than five minutes, so domain members need it.	`ntp`

Is port 123 open on your network? Check it → · All common ports →

Network services 1