kubeadm: kubeadm join for worker should not require identifying a bind address

BUG REPORT

Versions

kubeadm version (use kubeadm version): 1.12.1

Environment:

• Kubernetes version : 1.12.1
• Cloud provider or hardware configuration: bare metal
• OS (e.g. from /etc/os-release): CentOS 7

What happened?

When attempting to join a worker node that lacks a default route to an existing cluster by executing the kubeadm join ... command output from kubeadm token create <token> --print-join-command, the join fails because it can’t deduce a valid API server bind address. These lines are emitted on stderr from the kubeadm join ... command:

common.go:168] WARNING: could not obtain a bind address for the API Server: no default routes found in "/proc/net/route" or "/proc/net/ipv6_route"; using: 0.0.0.0
cannot use "0.0.0.0" as the bind address for the API Server

The VerifyAPIServerBindAddress(...) check was introduced to NewJoin(...) by 682b1b3.

What you expected to happen?

The kubeadm join ... command for a worker node should succeed because it doesn’t need to identify an API server bind address.

At first glance, my expectation is that the VerifyAPIServerBindAddress(...) check in NewJoin(...) should be conditional based on if internalcfg.ControlPlane == true.

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

On control plane node:

$ kubeadm token create <token> --print-join-command

On candidate worker node:

$ kubeadm join <apiserver> --token <token> --discovery-token-ca-cert-hash <hash> # output from above 'kubadm token create' command