Port 123: NTP
Network Time Protocol: keeps clocks in sync. Old NTP servers answering monlist queries were used for huge amplification floods. Only run a public NTP server if you mean to.
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Port Lookup
Port 123 at a glance
Port123
ProtocolUDP
ServiceNTP
Open to the Internet?Only to addresses that need it
See What Is Listeningss -lntup | grep ':123 '
Test From Outsidenmap -sU -p 123 your-server
Block with UFWufw deny 123/udp
Block with nftablesnft add rule inet filter input udp dport 123 drop
NTP uses UDP port 123. Network Time Protocol: keeps clocks in sync.
Old NTP servers answering monlist queries were used for huge amplification floods. Only run a public NTP server if you mean to.
Allow port 123 only from the addresses that need it: your office, a VPN or a bastion host. The firewall rule generator builds those rules for nftables, iptables and UFW.
Other Ports
20 FTP Data21 FTP22 SSH23 Telnet25 SMTP53 DNS67 DHCP Server80 HTTP110 POP3123 NTP143 IMAP161 SNMP389 LDAP443 HTTPS445 SMB465 SMTPS514 Syslog587 Mail Submission853 DNS over TLS993 IMAPS995 POP3S1194 OpenVPN1433 Microsoft SQL Server1900 SSDP / UPnP2049 NFS2083 cPanel (SSL)2087 WHM (SSL)3306 MySQL / MariaDB3389 RDP3478 STUN / TURN5060 SIP5432 PostgreSQL5900 VNC6379 Redis8006 Proxmox VE8080 HTTP Alternate8443 HTTPS Alternate11211 Memcached25565 Minecraft27017 MongoDB51820 WireGuard
Questions
What is port 123 used for?
NTP. Network Time Protocol: keeps clocks in sync.
Is it safe to open port 123?
Only to addresses that need it. Old NTP servers answering monlist queries were used for huge amplification floods. Only run a public NTP server if you mean to.
How do I check if port 123 is open?
On the server, ss -lntup | grep ':123 ' shows what is listening. From another machine, nmap -sU -p 123 your-server shows whether the firewall lets it through.
How do I block port 123?
With UFW: ufw deny 123/udp. With nftables, add a drop rule for dport 123 in your input chain.
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