Enabling Dual-stack on Existing IPv4-only Cluster

[optix2000]

I have a very old k3s cluster that started on 1.21 and is now on 1.30 that I would like to upgrade to using dual-stack.

Docs state that: Dual-stack networking must be configured when the cluster is first created. It cannot be enabled on an existing cluster once it has been started as IPv4-only.

However, these docs are 2 years old (at time of writing), and there doesn't seem to be any indication why this wouldn't be possible, given that regular k8s lets you do an in place "upgrade" to dual-stack.

Is there any technical reason why this is not possible?

Ignoring the docs and just blindly setting cluster-cidr with IPv6 addresses doesn't work:

--cluster-cidr=10.42.0.0/16,fd00:aaaa::/64
--service-cidr=10.43.0.0/16,fd00:aaaa:0:1::/112

K3's knows that nodes have an IPv6 address (as the annotations show IPv6) k3s.io/internal-ip: 38.46.x.x,2602:x:x:x::1:0
But k3s doesn't use the IPv6 addresses, even though the underlying nodes have IPv6 addresses and have IPv6 connectivity.

Addresses:
  InternalIP:  38.46.x.x
  Hostname:    foo.bar.baz
...
PodCIDR:                      10.42.1.0/24
PodCIDRs:                     10.42.1.0/24

tldr: What stops us from enable Dual-Stack after cluster creation?

[brandond]

Nodes are only assigned Pod CIDRs when they first join the cluster. There is no logic in the Kubernetes IPAM controller to assign a new IPv6 CIDR if the cluster is later reconfigured to enable IPv6 support.

You can try deleting and rejoining the nodes, so that they get a new CIDR assigned, but this is not something we test or support.

[optix2000]

Thanks for the response!

Does this mean that a CNI that doesn't respect podCIDR could also work around this restriction if all the pods get recreated?