Example CoreOS cloud-config file (referenced from http://www.beginswithdata.com/2016/12/30/centos7-kvm-coreos/)

user_data

#cloud-config

ssh_authorized_keys:
 - ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAgEAt4qwYNjFYPnPJObaNC+VoMYBFcEuUzCCBw6YzcU5f27bm0AwKvZKbVbwCo7IJ7gpNP0tIKvDK9z/qRQmcjWlW9p/cqpCa2a1fb0+8O2BhFXaD0BxuZWeuX8SF+mfJ2NzhgpTnR= kvm
 - ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQCh1LOHY/929X7qVXqpFQZRJt4uz3rkLVaCHdVD57Y+CEXjEQpoDBBdD6AEkPCvJ+vzFm9LC6KKUFVS7VXxZRUvXvKSWTKUrdsYV9vhNRLFGXYwzyUJov05w+/fxlels/L5Zc== coreos

hostname: @@VMNAME@@

users:
  - name: "core"
    passwd: "$6$rounds=4096$z2H2MkliS3J$c4hDSsKR3dkdkdjKKEK88dQdUhbS0gxKgAjNmUVRrgYOxMbvygrtrXr8QvFTeqfxAe1/M62O/CdyaNxaFgWu/"

coreos:
  update:
    reboot-strategy: off
  units:
    - name: systemd-networkd.service
      command: stop
    - name: static.network
      runtime: true
      content: |
        [Match]
        Name=eth0

        [Network]
        Address=@@IP@@/24
        Gateway=10.64.28.1
        DNS=10.64.28.200
    - name: down-interfaces.service
      command: start
      content: |
        [Service]
        Type=oneshot
        ExecStart=/usr/bin/ip link set eth0 down
        ExecStart=/usr/bin/ip addr flush dev eth0
    - name: systemd-networkd.service
      command: restart

https://coreos.com/validate/

Validation Results:

Config Invalid!
Document: incorrect type for "" (want struct)
Line: 6found character that cannot start any token

hostname: "@@VMNAME@@"
all ok now