Port 6443 (TCP) — Kubernetes API: what it is, how to check

Port 6443 Kubernetes API · TCP

The Kubernetes API server. kubectl, kubelets, controllers and CI pipelines all talk to it, with TLS and certificate, token or OIDC authentication.

Port	6443
Transport	TCP
Service	Kubernetes API
Category	Infrastructure
Range	registered (1024–49151)
Encrypted	Yes, always (TLS or equivalent)
Exposure	Internet-facing only with authentication and care

What runs on port 6443

The Kubernetes API server. kubectl, kubelets, controllers and CI pipelines all talk to it, with TLS and certificate, token or OIDC authentication. It is often exposed to the internet on managed clusters (restricted by authorised networks) and is the one port that, if authentication is misconfigured, hands over the whole cluster. Older distributions and some tutorials use 8443 or 443 instead.

Software that uses it: kube-apiserver · k3s, RKE2, kubeadm clusters · EKS, GKE, AKS endpoints

Should port 6443 be open?

Internet-facing only with authentication and care. It can be exposed when the service behind it is authenticated, rate-limited and kept up to date; most deployments are better off behind a VPN, an allow-list or a reverse proxy.

kubectl cluster-info and curl -k https://host:6443/version are the basic checks.
Disable anonymous auth (--anonymous-auth=false) and restrict the source addresses if the endpoint is public.

Check port 6443

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :6443'
sudo lsof -i :6443

# from another machine: is it reachable?
nc -zv host 6443
nmap -p 6443 host
curl -v telnet://host:6443      # raw TCP connect test, Ctrl-C to leave

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 6443

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 6443 proto tcp
sudo ufw deny 6443/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 6443 accept
nft add rule inet filter input tcp dport 6443 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 6443 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 6443 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=6443 protocol=tcp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 6443" with no source is just "open".

Related ports

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions

Is port 6443 TCP or UDP?

TCP. Kubernetes API is a connection-oriented protocol; there is no UDP service on 6443 by default (a scanner reporting UDP 6443 open is usually seeing a filtered port).

Is port 6443 encrypted?

Yes, always (TLS or equivalent). The port number does not make it safe by itself: certificate validation, authentication and patching still decide.

How do I check whether port 6443 is open?

On the server, ss -ltnp (or -lunp for UDP) shows whether something listens on 6443 and which process. From another machine, nc -zv host 6443 or nmap -p 6443 host tests reachability. "Open" in a scan means something answered; a firewall usually makes the port filtered (no answer) rather than closed (RST).

Can Kubernetes API run on a different port?

Yes; the number is a convention, not a requirement. Changing it reduces log noise from scanners but does not add security, and every client, firewall rule and monitoring check then needs the new number. Keep the default unless you have a concrete reason.