Networking

In-cluster networking

k0s supports any standard CNI network provider. For convenience, k0s does come bundled with two built-in providers, Kube-router and Calico.

Custom CNI configuration

You can opt-out of having k0s manage the network setup and choose instead to use any network plugin that adheres to the CNI specification. To do so, configure custom as the network provider in the k0s configuration file (k0s.yaml). You can deploy the CNI provider you want to use, either with Helm, plain Kubernetes manifests or any other way.

Notes

• When deploying k0s with the default settings, all pods on a node can communicate with all pods on all nodes. No configuration changes are needed to get started.
• Once you initialize the cluster with a network provider the only way to change providers is through a full cluster redeployment.

Kube-router

Kube-router is built into k0s, and so by default the distribution uses it for network provisioning. Kube-router uses the standard Linux networking stack and tools, and you can set up CNI networking without any overlays by using BGP as the main mechanism for in-cluster networking.

• Uses a bit less resources (~15%)
• Does NOT support Windows nodes

Calico

In addition to Kube-router, k0s also offers Calico as an alternative, integrated network provider. Calico is a layer 3 container networking solution that routes packets to pods. For example, it supports pod-specific network policies that help secure Kubernetes clusters in demanding use cases. Calico in k0s uses VXLAN by default. However, IP in IP is also supported via the bird mode.

• Uses a bit more resources
• Supports Windows nodes

Controller-Worker communication

One goal of k0s is to allow for the deployment of an isolated control plane, which may prevent the establishment of an IP route between controller nodes and the pod network. Thus, to enable this communication path (which is mandated by conformance tests), k0s deploys Konnectivity service to proxy traffic from the API server (control plane) into the worker nodes. This ensures that we can always fulfill all the Kubernetes API functionalities, but still operate the control plane in total isolation from the workers.

Note: To allow Konnectivity agents running on the worker nodes to establish the connection, configure your firewalls for outbound access, port 8132. Moreover, configure your firewalls for outbound access, port 6443, in order to access Kube-API from the worker nodes.

Required ports and protocols

Protocol	Port	Service	Direction	Notes
TCP	2380	etcd	controller ⟷ controller
TCP	6443	kube-apiserver	worker, CLI ⟶ controller	Authenticated Kubernetes API using mTLS, ServiceAccount tokens with RBAC
TCP	179	kube-router	worker ⟷ worker	BGP routing sessions between peers
UDP	4789	calico	worker ⟷ worker	Calico VXLAN overlay
TCP	10250	kubelet	controller, worker ⟶ host *	Authenticated kubelet API for the controller node kube-apiserver (and metrics-server add-ons) using mTLS
TCP	9443	k0s api	controller ⟷ controller	k0s controller join API, TLS with token auth
TCP	8132	konnectivity	worker ⟷ controller	Konnectivity is used as "reverse" tunnel between kube-apiserver and worker kubelets
TCP	112	keepalived	controller ⟷ controller	Only required for control plane load balancing VRRPInstances. Unless unicast is explicitly enabled, port 122 works on the ip address 224.0.0.18. 224.0.0.18 is a multicast IP address defined in RFC 3768.

You also need enable all traffic to and from the podCIDR and serviceCIDR subnets on nodes with a worker role.

iptables

iptables can work in two distinct modes, legacy and nftables. k0s auto-detects the mode and prefers nftables. To check which mode k0s is configured with check ls -lah /var/lib/k0s/bin/. The target of the symbolic link iptables reveals the mode which k0s selected. k0s has the same logic as other k8s components, but to ensure all component have picked up the same mode you can check via:

• kube-proxy: nsenter -t $(pidof kube-proxy) -m iptables -V
• kube-router: nsenter -t $(pidof kube-router) -m /sbin/iptables -V
• calico: nsenter -t $(pidof -s calico-node) -m iptables -V

There are known version incompatibility issues in iptables versions. k0s ships (in /var/lib/k0s/bin) a version of iptables that is tested to interoperate with all other Kubernetes components it ships with. However if you have other tooling (firewalls etc.) on your hosts that uses iptables and the host iptables version is different from the one that k0s (and other Kubernetes components) ship, it may cause networking issues. This is due to the fact that iptables, being user-space tooling, does not provide any strong version compatibility guarantees.

Firewalld & k0s

If you are using firewalld on your hosts you need to ensure it is configured to use the same FirewallBackend as k0s and other Kubernetes components use. Otherwise networking will be broken in various ways.

Here's an example configuration for a tested working networking setup:

[root@rhel-test ~]# firewall-cmd --list-all
public (active)
  target: default
  icmp-block-inversion: no
  interfaces: eth0
  sources: 10.244.0.0/16 10.96.0.0/12
  services: cockpit dhcpv6-client ssh
  ports: 80/tcp 6443/tcp 8132/tcp 10250/tcp 179/tcp 179/udp
  protocols:
  forward: no
  masquerade: yes
  forward-ports:
  source-ports:
  icmp-blocks:
  rich rules:

Basic single node firewalld setup:

# Allow traffic from podCIDR
$ firewall-cmd --add-source=10.244.0.0/16 --permanent
# Allow kubelet API traffic
$ firewall-cmd --add-port=10250/tcp --permanent
# Apply firewall changes
$ firewall-cmd --reload