123 NTPNetwork Time Protocol for clock synchronization.
Essential everywhere; allow UDP/123 outbound to time sources. Restrict inbound to avoid amplification.
| Transport | Status | Meaning |
|---|---|---|
| TCP | Assigned | Assigned by IANA, but not standardized, specified, or widely used on this port. |
| UDP | Yes | Assigned by IANA and standardized, specified, or widely used on this port. |
| SCTP | — | Not applicable for this transport. |
| DCCP | — | Not applicable for this transport. |
Well-known (0–1023). System port for a core service. On Unix-like systems, binding it requires superuser privileges.
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Copy-ready inbound rules for the active transports. Replace every <source> placeholder with the specific host, subnet, or CIDR that should reach this port.
sudo ufw allow proto udp from <source> to any port 123Allow UDP 123 (NTP). Replace <source> with the network/host that should reach it.
sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="<source>" port port="123" protocol="udp" accept'Allow UDP 123 (NTP) from <source>. Run 'firewall-cmd --reload' afterwards.
nft add rule inet filter input ip saddr <source> udp dport 123 acceptAllow UDP 123 (NTP). Assumes table 'inet filter' with an 'input' chain.
sudo iptables -A INPUT -p udp -s <source> --dport 123 -j ACCEPTAllow UDP 123 (NTP). Persist with iptables-save/netfilter-persistent.
az network nsg rule create -g <resource-group> --nsg-name <nsg> -n Allow-ntp-123 --priority 1000 --access Allow --direction Inbound --protocol UDP --source-address-prefixes <source> --destination-port-ranges 123Inbound rule for 123 (NTP). Set <source> to a CIDR; lower priority number = higher precedence.
aws ec2 authorize-security-group-ingress --group-id <sg-id> --ip-permissions IpProtocol=udp,FromPort=123,ToPort=123,IpRanges="[{CidrIp=<source-cidr>}]"Inbound UDP 123 (NTP). Replace <source-cidr> (e.g. 10.0.0.0/16).
gcloud compute firewall-rules create allow-ntp-123 --direction=INGRESS --action=ALLOW --rules=udp:123 --source-ranges=<source-cidr> --network=<network>Ingress rule for 123 (NTP). Scope with --source-ranges and optionally --target-tags.
netsh advfirewall firewall add rule name="Allow NTP 123/UDP" dir=in action=allow protocol=UDP localport=123 remoteip=<source>Inbound UDP 123 (NTP). Set remoteip to a host/subnet; 'any' is discouraged.
New-NetFirewallRule -DisplayName "Allow NTP 123/UDP" -Direction Inbound -Action Allow -Protocol UDP -LocalPort 123 -RemoteAddress <source>Inbound UDP 123 (NTP). Replace <source> with a host or subnet.
Rules are generated only for transports marked active for this port. They default to an inbound allow with an explicit source placeholder — never any. Review direction, scope, and rule precedence before applying in production.