Port 10250 (TCP) — kubelet: what it is, how to check

Port 10250 kubelet · TCP

The kubelet API on every Kubernetes node: pod logs, exec into containers, metrics.

Port	10250
Transport	TCP
Service	kubelet
Category	Infrastructure
Range	registered (1024–49151)
Encrypted	Yes, always (TLS or equivalent)
Exposure	Private network or VPN only

What runs on port 10250

The kubelet API on every Kubernetes node: pod logs, exec into containers, metrics. The API server calls it; nobody else should. With anonymous auth left on (older defaults), anyone reaching 10250 can exec into any pod on the node, which is cluster compromise. 10255 was the read-only port, now removed.

Software that uses it: kubelet · metrics-server (as a client) · k3s, kubeadm, managed-cluster nodes

Should port 10250 be open?

Private network or VPN only. Nothing good comes from this port being reachable from the internet. Keep it on an internal network, reach it over a VPN or an SSH tunnel, and firewall it from everything that does not need it.

curl -sk https://node:10250/pods: a 401 is good; a list of pods is a critical finding.
Set --anonymous-auth=false and --authorization-mode=Webhook on the kubelet.

Check port 10250

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :10250'
sudo lsof -i :10250

# from another machine: is it reachable?
nc -zv host 10250
nmap -p 10250 host
curl -v telnet://host:10250      # 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 10250

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 10250 proto tcp
sudo ufw deny 10250/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 10250 accept
nft add rule inet filter input tcp dport 10250 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 10250 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 10250 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=10250 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 10250" 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 10250 TCP or UDP?

TCP. kubelet is a connection-oriented protocol; there is no UDP service on 10250 by default (a scanner reporting UDP 10250 open is usually seeing a filtered port).

Is port 10250 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 10250 is open?

On the server, ss -ltnp (or -lunp for UDP) shows whether something listens on 10250 and which process. From another machine, nc -zv host 10250 or nmap -p 10250 host tests reachability. "Open" in a scan means something answered; a firewall usually makes the port filtered (no answer) rather than closed (RST).

Can kubelet 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.