Port / TCP
6443Kubernetes API server
The Kubernetes control plane API, the single endpoint every kubectl command and controller talks to.
If it is internet-facing: Finding 6443 open to the internet is a finding in its own right — it warrants an explanation, not just a rule.
Exposure
Behind a gateway
Transport
Encrypted by default
Risk if public
High
Exposure verdict
Port 6443 should sit behind a reverse proxy, VPN, or source allow-list rather than being forwarded at the perimeter. Publishing it directly puts an administrative or internal interface on the open internet.
The session is protected from the first byte, so there is no plaintext phase for an on-path attacker to strip or read.
Registered with IANA on request, but bindable by any unprivileged user. Registration records intent — it does not reserve the port, so collisions between products are common.
Grouped with the other containers & platform ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- TLS is mandatory here, so the real questions are RBAC bindings and whether anonymous auth is disabled.
- A managed cluster's API can usually be restricted to an address allow-list — do that rather than leaving it globally reachable.
Recommended firewall rule
Allow only from 10.0.0.0/8, drop the rest
Swap in the CIDR your administrators or application tier actually come from — the point is that the default answer is no.
ufw
sudo ufw allow from 10.0.0.0/8 to any port 6443 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 6443 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 6443 -j DROPIf you do need it open to everyone
sudo ufw allow 6443/tcpWrite down why. An unexplained allow rule outlives the person who added it, and the next reviewer has no way to tell a deliberate exception from an accident.
Check what is really there
Listening locally
sudo ss -tlpn 'sport = :6443'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 -iTCP:6443 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 6443 <host>A version probe confirms whether Kubernetes API server is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 6443The quick yes/no when you only need to know whether a path exists through the firewall.