How can I tell Rancher which CNI to use for setting up a new K3S cluster? https://ranchermanager.docs.rancher.com/faq/container-network-interface-providers doesn't tell; it is all about RKE/RKE2.

If your CNI is not in the dropdown list you can modify the cluster yaml, specify:

cni:
  - none

And provide your own CNI manifest as an addon

Does Kubernetes work with CNI: none? I thought some kind of CNI is mandatory.

It is. But you need to supply your own CNI manifest if you don't want to deploy one of the CNI's from the dropdown

AFAICT there is no dropdown to select a CNI for K3s in Rancher v2.7.1. And the cluster.yaml doen't mention a CNI, either, but of course I will try your suggestion. Thank you very much.

Ah yes, you're right. You'd need to pass the equivalent of

--flannel-backend=none

. I've not tested this but in the cluster yaml in the

machineGlobalConfig

block add

flannel-backend: none

and supply the CNI manifest via addons or fleet etc

K3s only ships with a single CNI, unlike RKE2 which allows a selection. If you want to deploy your own you should do as David said.

It turned out that the built-in Flannel in K3s is fine for my needs. I just got a problem with RKE2 (calico by default). Posted yesterday on the rke2 channel, but no reply yet.