Requirements

K3s is very lightweight, but has some minimum requirements as outlined below.

Whether you’re configuring K3s to run in a container or as a native Linux service, each node running K3s should meet the following minimum requirements. These requirements are baseline for K3s and its packaged components, and do not include resources consumed by the workload itself.

Prerequisites

Two nodes cannot have the same hostname.

If multiple nodes will have the same hostname, or if hostnames may be reused by an automated provisioning system, use the --with-node-id option to append a random suffix for each node, or devise a unique name to pass with --node-name or $K3S_NODE_NAME for each node you add to the cluster.

Architecture

K3s is available for the following architectures:

  • x86_64

  • armhf

  • arm64/aarch64

  • s390x

ARM64 Page Size

Prior to May 2023 releases (v1.24.14+k3s1, v1.25.10+k3s1, v1.26.5+k3s1, v1.27.2+k3s1), on aarch64/arm64 systems, the kernel must use 4k pages. RHEL9, Ubuntu, Raspberry PI OS, and SLES all meet this requirement.

Operating Systems

K3s is expected to work on most modern Linux systems.

Some OSs have additional setup requirements:

  • SUSE Linux Enterprise / openSUSE

  • Red Hat Enterprise Linux / CentOS / Fedora

  • Ubuntu / Debian

  • Raspberry Pi

It is recommended to turn off firewalld:

systemctl disable firewalld --now

If you wish to keep firewalld enabled, by default, the following rules are required:

firewall-cmd --permanent --add-port=6443/tcp #apiserver
firewall-cmd --permanent --zone=trusted --add-source=10.42.0.0/16 #pods
firewall-cmd --permanent --zone=trusted --add-source=10.43.0.0/16 #services
firewall-cmd --reload

Additional ports may need to be opened depending on your setup. See Inbound Rules for more information. If you change the default CIDR for pods or services, you will need to update the firewall rules accordingly.

It is recommended to turn off firewalld:

systemctl disable firewalld --now

If you wish to keep firewalld enabled, by default, the following rules are required:

firewall-cmd --permanent --add-port=6443/tcp #apiserver
firewall-cmd --permanent --zone=trusted --add-source=10.42.0.0/16 #pods
firewall-cmd --permanent --zone=trusted --add-source=10.43.0.0/16 #services
firewall-cmd --reload

Additional ports may need to be opened depending on your setup. See Inbound Rules for more information. If you change the default CIDR for pods or services, you will need to update the firewall rules accordingly.

If enabled, it is required to disable nm-cloud-setup and reboot the node:

systemctl disable nm-cloud-setup.service nm-cloud-setup.timer
reboot

Older Debian release may suffer from a known iptables bug. See Known Issues.

It is recommended to turn off ufw (uncomplicated firewall):

ufw disable

If you wish to keep ufw enabled, by default, the following rules are required:

ufw allow 6443/tcp #apiserver
ufw allow from 10.42.0.0/16 to any #pods
ufw allow from 10.43.0.0/16 to any #services

Additional ports may need to be opened depending on your setup. See Inbound Rules for more information. If you change the default CIDR for pods or services, you will need to update the firewall rules accordingly.

Raspberry Pi OS is Debian based, and may suffer from a known iptables bug. See Known Issues.

Cgroups

Standard Raspberry Pi OS installations do not start with cgroups enabled. K3S needs cgroups to start the systemd service. cgroups`can be enabled by appending `cgroup_memory=1 cgroup_enable=memory to /boot/firmware/cmdline.txt.

On Debian 11 and older Pi OS releases the cmdline.txt is located at /boot/cmdline.txt.

Example cmdline.txt:

console=serial0,115200 console=tty1 root=PARTUUID=58b06195-02 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait cgroup_memory=1 cgroup_enable=memory

Ubuntu Vxlan Module

With Ubuntu 21.10 to Ubuntu 23.10, vxlan support on Raspberry Pi was moved into a separate kernel module. This step in not required for Ubuntu 24.04 and later.

sudo apt install linux-modules-extra-raspi

For more information on which OSs were tested with Rancher managed K3s clusters, refer to the Rancher support and maintenance terms.

Hardware

Hardware requirements scale based on the size of your deployments. The minimum requirements are:

Node CPU RAM

Server

2 cores

2 GB

Agent

1 core

512 MB

Resource Profiling captures the results of tests and analysis to determine minimum resource requirements for the K3s agent, the K3s server with a workload, and the K3s server with one agent.

Disks

K3s performance depends on the performance of the database. To ensure optimal speed, we recommend using an SSD when possible.

If deploying K3s on a Raspberry Pi or other ARM devices, it is recommended that you use an external SSD. etcd is write intensive; SD cards and eMMC cannot handle the IO load.

Server Sizing Guide

When limited on CPU and RAM on the server (control-plane + etcd) node, there are limitations on the amount of agent nodes that can be joined under standard workload conditions.

Server CPU Server RAM Number of Agents

2

4 GB

0-350

4

8 GB

351-900

8

16 GB

901-1800

16+

32 GB

1800+
High Availability Sizing

When using a high-availability setup of 3 server nodes, the number of agents can scale roughly ~50% more than the above table. E.g., 3 server with 4 vCPU/8 GB can scale to ~1200 agents.

It is recommended to join agent nodes in batches of 50 or less to allow the CPU to free up space, as there is a spike on node join. Remember to modify the default cluster-cidr if desiring more than 255 nodes!

Resource Profiling contains more information how these recommendations were found.

Networking

The K3s server needs port 6443 to be accessible by all nodes.

The nodes need to be able to reach other nodes over UDP port 8472 when using the Flannel VXLAN backend, or over UDP port 51820 (and 51821 if IPv6 is used) when using the Flannel WireGuard backend. The node should not listen on any other port. K3s uses reverse tunneling such that the nodes make outbound connections to the server and all kubelet traffic runs through that tunnel. However, if you do not use Flannel and provide your own custom CNI, then the ports needed by Flannel are not needed by K3s.

If you wish to utilize the metrics server, all nodes must be accessible to each other on port 10250.

If you plan on achieving high availability with embedded etcd, server nodes must be accessible to each other on ports 2379 and 2380.

Important

The VXLAN port on nodes should not be exposed to the world as it opens up your cluster network to be accessed by anyone. Run your nodes behind a firewall/security group that disables access to port 8472.

Flannel relies on the Bridge CNI plugin to create a L2 network that switches traffic. Rogue pods with NET_RAW capabilities can abuse that L2 network to launch attacks such as ARP spoofing. Therefore, as documented in the Kubernetes docs, please set a restricted profile that disables NET_RAW on non-trustable pods.

Inbound Rules for K3s Nodes

Protocol Port Source Destination Description

TCP

2379-2380

Servers

Servers

Required only for HA with embedded etcd

TCP

6443

Agents

Servers

K3s supervisor and Kubernetes API Server

UDP

8472

All nodes

All nodes

Required only for Flannel VXLAN

TCP

10250

All nodes

All nodes

Kubelet metrics

UDP

51820

All nodes

All nodes

Required only for Flannel Wireguard with IPv4

UDP

51821

All nodes

All nodes

Required only for Flannel Wireguard with IPv6

TCP

5001

All nodes

All nodes

Required only for embedded distributed registry (Spegel)

TCP

6443

All nodes

All nodes

Required only for embedded distributed registry (Spegel)

Typically, all outbound traffic is allowed.

Additional changes to the firewall may be required depending on the OS used.

Large Clusters

Hardware requirements are based on the size of your K3s cluster. For production and large clusters, we recommend using a high-availability setup with an external database. The following options are recommended for the external database in production:

  • MySQL

  • PostgreSQL

  • etcd

CPU and Memory

The following are the minimum CPU and memory requirements for nodes in a high-availability K3s server:

Deployment Size Nodes vCPUs RAM

Small

Up to 10

2

4 GB

Medium

Up to 100

4

8 GB

Large

Up to 250

8

16 GB

X-Large

Up to 500

16

32 GB

XX-Large

500+

32

64 GB

Disks

The cluster performance depends on database performance. To ensure optimal speed, we recommend always using SSD disks to back your K3s cluster. On cloud providers, you will also want to use the minimum size that allows the maximum IOPS.

Network

You should consider increasing the subnet size for the cluster CIDR so that you don’t run out of IPs for the pods. You can do that by passing the --cluster-cidr option to K3s server upon starting.

Database

K3s supports different databases including MySQL, PostgreSQL, MariaDB, and etcd. See Cluster Datastore for more info.

The following is a sizing guide for the database resources you need to run large clusters:

Deployment Size Nodes vCPUs RAM

Small

Up to 10

1

2 GB

Medium

Up to 100

2

8 GB

Large

Up to 250

4

16 GB

X-Large

Up to 500

8

32 GB

XX-Large

500+

16

64 GB