The Tool Desk
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What you are building
K3s is a lightweight Kubernetes distribution suited to small VPS clusters, labs, edge systems and game-service infrastructure. The installation below uses one Ubuntu VPS as the K3s server (the control plane) and, if needed, other Ubuntu VPSs as agents (workers). The server runs the Kubernetes API and can also run workloads. K3s documentation describes a single-node server installation as a fully functional Kubernetes cluster, so you do not need multiple machines for a usable test cluster.
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This guide assumes Ubuntu LTS on every machine, root access or a user that can run sudo, and network reachability between the VPSs. Commands are shown exactly as shell commands; run server commands on the server VPS and agent commands on each worker VPS.
Hardware and operating-system requirements
Official minimums
The K3s requirements documentation lists these configuration minimums, excluding CPU and memory consumed by your workloads:
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| Node role | Minimum CPU | Minimum RAM | What the figure means |
|---|---|---|---|
| Server | 2 cores | 2 GB | Minimum for the K3s server itself; pods, logging and system services need additional capacity. |
| Agent | 1 core | 512 MB | Minimum for the agent; application pods require memory and CPU on top of this. |
These are not comfortable production sizes. A 2026 Ubuntu 24.04 walkthrough recommends about 4 GB of RAM for a practical single-node test installation. Treat that as a third-party recommendation, not an official K3s minimum. If you plan to run a database, game server, ingress controller, monitoring or several replicas, budget those workloads separately and leave headroom for the operating system.
Ubuntu and hostnames
- Use an Ubuntu LTS release supported by your VPS provider. The procedure is intended for Ubuntu 24.04 LTS as well as other current Ubuntu LTS releases.
- Give every machine a unique hostname. K3s uses the hostname to identify a node. Duplicate names can silently corrupt cluster membership.
- If unique hostnames are not possible, set a unique
K3S_NODE_NAMEvalue on each machine before installation. - Keep the system clocks and package state current using your normal Ubuntu administration process, and confirm that you can obtain the server’s private or public IP address from each agent.
Required network ports
Apply firewall rules at the VPS provider and on the operating system only when those firewalls are enabled. Ubuntu’s UFW is often reported as disabled by default, but verify your own image rather than assuming it.
| Port | Protocol | Direction and scope | Purpose |
|---|---|---|---|
| 6443 | TCP | Reachable by all nodes; agents connect to the server | Kubernetes API server |
| 8472 | UDP | Between all nodes | Flannel VXLAN, the default networking backend |
| 10250 | TCP | As required for metrics collection | Kubelet metrics, when metrics-server is used |
| 51820 | UDP | Between nodes when selected | Flannel WireGuard backend |
| 51821 | UDP | Between nodes when selected | Flannel WireGuard backend |
| 2379–2380 | TCP | Server nodes only | Embedded etcd traffic for a highly available server cluster |
| 5001 | TCP | Only when enabled | Embedded registry mirror |
Ports 51820/UDP, 51821/UDP, 2379–2380/TCP and 5001/TCP are conditional. Do not expose them simply because they appear in a generic port list. Port 6443 must be reachable from every node. K3s uses reverse tunnelling, so agents initiate outbound connections to the server; you normally do not need to open an inbound agent port for the server to contact the worker.
K3s quick start: prepare each Ubuntu VPS
- Confirm the role. Choose one VPS as the initial server. Decide which machines, if any, will be agents.
- Set a unique hostname. Check the current value with
hostnamectl. Change it using Ubuntu’s standard hostname tools if two machines share a name, then reconnect so the change is applied consistently. - Record addresses. From every agent, make sure the server address you intend to use is reachable. A private address is preferable when all VPSs share a private network; otherwise use the server’s reachable public address and restrict port 6443 at the provider firewall to trusted sources.
- Check the firewall. Inspect UFW and any cloud firewall. Allow TCP 6443 from agents and UDP 8472 between all nodes. Add 10250 only if you will use metrics-server, and add the conditional ports only for the features that require them.
- Check capacity. Verify at least 2 CPU cores and 2 GB RAM on the server and 1 core and 512 MB RAM on each agent. For a single-node test, around 4 GB RAM is a more practical starting point according to a third-party Ubuntu 24.04 guide.
Install the K3s server
Log in to the selected server VPS as root or use a sudo-capable account. The standard installer is:
curl -sfL https://get.k3s.io | sh -The installer places the K3s binary at /usr/local/bin/k3s, creates a systemd unit named k3s, and starts the service. Setting no K3S_URL makes this machine a server rather than an agent.
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After the command returns, check the service before doing anything else:
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sudo kubectl get nodesOn a single-node installation, the server should appear in the output. A single server can schedule workloads, so you can stop here if your goal is a one-VPS laboratory or small deployment. If you add agents, leave this terminal available for the token and verification steps.
Install server, agent, cluster: joining workers
Read the node-token
K3s writes the join credential on the server at:
sudo cat /var/lib/rancher/k3s/server/node-tokenCopy the complete token exactly, including punctuation, and treat it like a password. Do not publish it in a repository, chat room or shell history that other users can read. If your terminal wraps the line, copy the underlying value rather than the visual line break.
Run the agent installer
On each worker VPS, substitute the server’s reachable address and the token you just copied:
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curl -sfL https://get.k3s.io | K3S_URL=https://<server-ip>:6443 K3S_TOKEN=<token> sh -For example, the URL must include the scheme and port: https://203.0.113.10:6443. The angle-bracket text is a placeholder; replace it with real values and do not type the brackets. Providing K3S_URL is what tells the installer to configure an agent instead of installing another server.
The agent installer creates its systemd service and starts the worker. Repeat the command on every additional worker, using the same server URL and token. Each worker still needs its own unique hostname. If you had to use custom names, provide a distinct K3S_NODE_NAME for each agent in the installer environment.
Verify that every node is Ready
Return to the server and run:
sudo kubectl get nodesThe output should list the server and each joined agent, with STATUS shown as Ready. This is the requested end state: the API is responding, node registration succeeded and the control plane can see the workers.
Use the following checks when a node is missing or remains NotReady:
- On the affected machine, inspect the service with
sudo systemctl status k3s-agent --no-pagerfor an agent orsudo systemctl status k3s --no-pagerfor a server. - Confirm that the agent can reach the server on TCP 6443 and that the server address in
K3S_URLis correct. - Confirm UDP 8472 is allowed between every node. The default Flannel VXLAN backend needs this path for pod networking.
- Recheck the hostname on every machine. Duplicate hostnames can create confusing membership failures rather than a clear installer error.
- Re-copy the token from
/var/lib/rancher/k3s/server/node-token. A missing character or an extra line break prevents authentication. - Check memory and CPU pressure. The documented minimums exclude workloads; a node can register and still become unstable when it is overcommitted.
Firewall and provider rules in practice
There are usually two independent filtering layers: the VPS provider’s security group and Ubuntu’s local firewall. A rule in only one layer is insufficient. Permit TCP 6443 from the agents to the server, and permit UDP 8472 in both directions between nodes. If metrics-server is installed, ensure the path it uses to reach kubelets includes TCP 10250. For a highly available setup with embedded etcd, restrict TCP 2379 and 2380 to server nodes; workers do not need those ports.
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Do not open the Kubernetes API to the entire internet unless you have a specific reason and compensating controls. Prefer a private network or a narrow source range. The join token grants cluster access, so store it with the same care as an administrator credential.
Single-node versus multi-node layouts
Single-node test cluster
One server VPS is the shortest path to a working cluster. It contains the control plane and can run pods itself. This layout is appropriate for learning, CI experiments and small services where a server failure is acceptable. Size it above the bare minimum if you will install ingress, monitoring, a registry or stateful applications; the practical 4 GB RAM recommendation is a useful test baseline, not a guarantee for every workload.
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Adding agents separates application capacity from the control-plane host. Agents connect outbound to the server, and the server’s kubectl get nodes output becomes the authoritative view of membership. This layout does not by itself make the control plane highly available: one server remains a single point of failure.
Multiple servers for embedded etcd
K3s can use embedded etcd for a highly available server cluster. In that design, allow TCP 2379–2380 between server nodes only. Plan the server count, storage and failure domains deliberately; do not turn on HA ports or add servers casually to a cluster that was created as a single-node installation. Consult the current K3s documentation for the supported HA bootstrap and version-specific flags before building this topology, because installer channels and options can change.
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The one-line installer follows K3s’s current channel behavior unless you explicitly select a version. Exact channel names and installer flags change over time, and they were not established here from current release notes. Before pinning a production version, check the live K3s documentation and release notes, then test the chosen version on a disposable VPS. Keep server and agent versions within the compatibility guidance for that release, and schedule upgrades rather than mixing arbitrary binaries.
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Common mistakes and their fixes
“The installer made another server”
If you intended an agent but omitted K3S_URL, the installer has no instruction to join an existing server. Re-run the agent installation with both K3S_URL and K3S_TOKEN set, after removing or rebuilding the unintended node according to your operational policy.
“Connection refused” or timeout on port 6443
Check that the server service is running, that the address resolves to the correct VPS, and that provider and host firewalls allow TCP 6443 from the agent. A private address that is not routed between VPS networks produces the same symptom as a closed firewall.
Nodes register but pods cannot communicate
First verify UDP 8472 between all nodes. That port carries the default Flannel VXLAN traffic. If you selected a WireGuard Flannel backend instead, allow the corresponding UDP 51820 and 51821 ports as required by that configuration.
A node appears twice or replaces another node
Inspect hostnames. Two machines with the same hostname can silently collide in cluster membership. Rename the machines or set unique K3S_NODE_NAME values, then rejoin the affected node cleanly.
Metrics are unavailable
Metrics-server requires access to kubelet metrics. Confirm that TCP 10250 is permitted where your metrics deployment needs it, and distinguish a metrics configuration problem from basic node readiness.
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- Record which VPS is the server and which are agents, including their unique hostnames and addresses.
- Restrict API access and protect the node token.
- Monitor disk, RAM and CPU; the K3s minimums do not include application workloads.
- Document the enabled Flannel backend and corresponding firewall ports.
- Back up application data and any cluster state that matters to you. A single-node cluster has no control-plane redundancy.
- Read the current K3s documentation before enabling embedded etcd, an embedded registry mirror, a non-default networking backend or a pinned release channel.
What success looks like
The setup is complete when the server’s k3s systemd service is running, every intended worker has joined with the server token, and sudo kubectl get nodes lists the expected machines as Ready. From there, deploy workloads using your normal Kubernetes manifests and continue treating capacity, firewall scope, credentials and version selection as ongoing operational responsibilities rather than one-time installation details.
Quick Recap
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