Error when creating a cluster with _ in it's name

[finalspy]

What happened:

 ✗ Starting control-plane 🕹️ 
ERROR: failed to create cluster: failed to init node with kubeadm: command "docker exec --privileged name_with_underscore-control-plane kubeadm init --skip-phases=preflight --config=/kind/kubeadm.conf --skip-token-print --v=6" failed with error: exit status 1
Command Output: I0413 10:45:15.583742     171 initconfiguration.go:200] loading configuration from "/kind/kubeadm.conf"
[config] WARNING: Ignored YAML document with GroupVersionKind kubeadm.k8s.io/v1beta2, Kind=JoinConfiguration
hostport name_with_underscore-control-plane:6443: host 'name_with_underscore-control-plane' must be a valid IP address or a valid RFC-1123 DNS subdomain
k8s.io/kubernetes/cmd/kubeadm/app/util.ParseHostPort
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/util/endpoint.go:111
k8s.io/kubernetes/cmd/kubeadm/app/util/config.SetClusterDynamicDefaults
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/util/config/initconfiguration.go:157
k8s.io/kubernetes/cmd/kubeadm/app/util/config.SetInitDynamicDefaults
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/util/config/initconfiguration.go:56
k8s.io/kubernetes/cmd/kubeadm/app/util/config.documentMapToInitConfiguration
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/util/config/initconfiguration.go:305
k8s.io/kubernetes/cmd/kubeadm/app/util/config.BytesToInitConfiguration
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/util/config/initconfiguration.go:235
k8s.io/kubernetes/cmd/kubeadm/app/util/config.LoadInitConfigurationFromFile
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/util/config/initconfiguration.go:207
k8s.io/kubernetes/cmd/kubeadm/app/util/config.LoadOrDefaultInitConfiguration
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/util/config/initconfiguration.go:219
k8s.io/kubernetes/cmd/kubeadm/app/cmd.newInitData
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/cmd/init.go:333
k8s.io/kubernetes/cmd/kubeadm/app/cmd.NewCmdInit.func3
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/cmd/init.go:193
k8s.io/kubernetes/cmd/kubeadm/app/cmd/phases/workflow.(*Runner).InitData
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/cmd/phases/workflow/runner.go:183
k8s.io/kubernetes/cmd/kubeadm/app/cmd.NewCmdInit.func1
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/cmd/init.go:141
k8s.io/kubernetes/vendor/github.com/spf13/cobra.(*Command).execute
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/vendor/github.com/spf13/cobra/command.go:842
k8s.io/kubernetes/vendor/github.com/spf13/cobra.(*Command).ExecuteC
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/vendor/github.com/spf13/cobra/command.go:950
k8s.io/kubernetes/vendor/github.com/spf13/cobra.(*Command).Execute
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/vendor/github.com/spf13/cobra/command.go:887
k8s.io/kubernetes/cmd/kubeadm/app.Run
	/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/kubeadm.go:50
main.main
	_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/kubeadm.go:25
runtime.main
	/usr/local/go/src/runtime/proc.go:204
runtime.goexit
	/usr/local/go/src/runtime/asm_amd64.s:1374

What you expected to happen:

Creation successful

How to reproduce it (as minimally and precisely as possible):

kind create cluster name_with_underscore

Anything else we need to know?:

This "phenomenom" was also spotted in the following issues afaik :

• RROR: failed to create cluster: failed to init node with kubeadm #1437
• Error: Failed to create cluster: failed to init node with kubeadm on Windows #2111

May be the regular expression ^[a-z0-9_.-]+$ L32 of kind/pkg/internal/apis/config/validate.go :

kind/pkg/internal/apis/config/validate.go

Line 32 in 5e58a0d

var validNameRE = regexp.MustCompile(`^[a-z0-9_.-]+$`)

should be replaces by
^[a-z0-9.-]+$ (note that I don't know if this is used to validate something else... so maybe this correction isn't accurate).

Environment:

• kind v0.9.0 go1.15.2 linux/amd64
• Client Version: version.Info{Major:"1", Minor:"21", GitVersion:"v1.21.0", GitCommit:"cb303e613a121a29364f75cc67d3d580833a7479", GitTreeState:"clean", BuildDate:"2021-04-08T16:31:21Z", GoVersion:"go1.16.1", Compiler:"gc", Platform:"linux/amd64"}
• Docker Server Version: 19.03.6, Kernel Version: 5.3.0-24-generic, Operating System: Ubuntu 19.10
• Ubuntu 19.10 (Eoan Ermine)

[neolit123]

k8s.io/kubernetes/cmd/kubeadm/app/util.ParseHostPort
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/cmd/kubeadm/app/util/endpoint.go:111
k8s.io/kubernetes/cmd/kubeadm/app/util/config.SetClusterDynamicDefaults
must be a valid IP address or a valid RFC-1123 DNS subdomain

as you can see the error comes from kubeadm when setting the controlPlaneEndpoint field in the ClusterConfiguration object.
this field must be an IP or a DNS name.

kubeadm uses a k8s utility function that misleadingly claims that it follows RFC-1123.
yet that RFC does allow underscores. this is more of a limitation for hostnames.

in any case, hostnames with underscores or capital letters should not reach the kubelet - i.e. the kind node containers ideally should not have _

so i think your regexp fix is valid.