Podman ?

[carlocorradini]

It's possible to use Podman as container runtime?

[brandond]

As far as I know podman doesn't provide a CRI socket for the Kubelet to connect to, since it's just a daemonless replacement for the CLI portion of docker. I believe you would need to use something else like cri-o.

[carlocorradini]

Thanks!

[mtalexan]

Some background for future searchers of this same topic:

CRI is the interface needed by k3s/k8s.
docker has 2 parts to it, both under the same name: a CLI client and a daemon.
containerd is a daemon only, though there's a related simplistic CLI tool, ctr, that can do some very simple things with it.
podman has a primarly daemonless rootless CLI client, but can optionally also run a daemon and interact with that daemon using the normally daemonless rootless CLI client (with the --remote option). The optional daemon is intended to be used as a replacement for the docker daemon and attempts to be a fully compatible daemon for even the docker client.
CRI-O (crio) is a tool developed by and sharing design/code with podman that is primarily a daemon, though it has crictl for some interaction.

The docker daemon supports 2 different endpoint interface sets from the same socket: CRI and the CLI client interface.
The containerd daemon only supports CRI.
The optional podman daemon supports only the docker CLI client interface with some minor additions that only the podman CLI knows how to use if you use --remote.
crio only supports CRI (from what I can tell).

CRI-O and podman use the same image cache storage technology, and it's possible to use podman --root=... to point at the CRI-O image storage location to manage the cache using the podman CLI. Supposedly you can also use the podman CLI to talk to the CRI-O daemon as well, though I've yet to find documentation on how to do this (maybe --remote?).

---

K8s and K3s technically only need a daemon supporting the CRI interface. Historically this was dockerd or containerd, but dockerd support has been deprecated/removed from K8s. At some point the default backend for K8s was changed to CRI-O.
K3s also deprecated dockerd when K8s did, but as of now doesn't officially support CRI-O as a backend. One of the benefits of k3s is that it packages what you need with it, so it has containerd packaged with it still and used as the default.

It is technically possible, though not directly supported, to use CRI-O as the K3s backend if you setup CRI-O yourself and then point the K3s server at it with the --container-runtime-endpoint option, but it's not a Rancher supported configuration.