Port 123 — NTP

UDP well-known

What runs on port 123

Network Time Protocol — clock synchronisation.

Security considerations

Another amplification vector. Disable the monlist command and do not run an open NTP server.

Checking whether something is listening

# Linux / macOS — what is bound to the port
sudo lsof -i :123
sudo ss -lntp | grep :123

# Windows
netstat -ano | findstr :123
Get-NetTCPConnection -LocalPort 123

# is it reachable from outside?
nc -zv example.com 123
curl -v telnet://example.com:123

Freeing the port

# find the process, then stop it
sudo lsof -ti :123 | xargs kill        # Linux / macOS
netstat -ano | findstr :123            # note the PID, then:
taskkill /PID <pid> /F                    # Windows

Should this port be open to the internet?

Usually not. Expose it only if a specific external client needs it, and restrict by source address where you can.

Quick reference

Frequently asked questions

Other common ports

• Port 20 — FTP data
• Port 21 — FTP control
• Port 22 — SSH / SFTP
• Port 23 — Telnet
• Port 25 — SMTP
• Port 53 — DNS
• Port 67 — DHCP server
• Port 68 — DHCP client
• Port 69 — TFTP
• Port 80 — HTTP
• Port 110 — POP3
• Port 143 — IMAP
• Port 161 — SNMP
• Port 389 — LDAP
• Port 443 — HTTPS
• Port 445 — SMB
• Port 465 — SMTPS
• Port 514 — Syslog

All port numbers