Port / UDP
123NTP
NTP keeps clocks aligned with reference time sources, which is what makes logs correlatable and certificates valid.
If it is internet-facing: Port 123 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Internet-facing
Transport
Cleartext
Risk if public
Elevated
Exposure verdict
Port 123 normally answers from the internet, but the transport is not confidential by default. Enforce the encrypted variant before the traffic leaves your network.
Traffic crosses the network readable and modifiable. Anything sensitive on this port needs a tunnel, a VPN, or a different protocol.
Assigned by IANA through a review process. On Unix systems a process needs root or the CAP_NET_BIND_SERVICE capability to bind here, which is why containers so often remap these to a higher port.
Grouped with the other network services ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- The legacy monlist command turned NTP servers into enormous amplification reflectors; disable mode 6 and 7 queries.
- A host whose clock is pushed far enough out of true will reject valid certificates or accept expired ones.
Recommended firewall rule
Allow it, and harden the service
This port is expected to answer from anywhere, so the firewall is not where its security comes from.
ufw
sudo ufw allow 123/udpiptables
sudo iptables -A INPUT -p udp --dport 123 -j ACCEPTCheck what is really there
Listening locally
sudo ss -ulpn 'sport = :123'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iUDP:123macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 123 <host>A version probe confirms whether NTP is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 123The quick yes/no when you only need to know whether a path exists through the firewall.