This is a cloud-config example from a working PXE boot to disk installation for CoreOS

pxe-cloud-config_internal-network.yml

#cloud-config

#Credit: many portions of this config came from the great work detailed here by:
#Behner, brianclements, Metamogul and especially crawford for the cloud-config inside a cloud-config idea:
#https://github.com/coreos/coreos-cloudinit/issues/205

ssh_authorized_keys:
  - ssh-rsa <core user key goes here>

#This writes a cloud-config to the PXE booted system that 
#we will use to install the machine to disk
write_files:
    - path: /run/cloud-config.yml
      permissions: '0644'
      content: |
        #cloud-config

        ssh_authorized_keys:
          - ssh-rsa <core user key goes here>
        #Add addition users you need to log into the box. 
        #We have one set here for ease of testing
        #users:
          #- name: testuser
            #passwd: <create a hash of the password and put it here>
            #groups:
              #- sudo
              #- systemd-journal
        write_files:
            - path: /etc/environment
              permissions: 0644
              owner: root
              content: |
                COREOS_PUBLIC_IPV4=$_public_ipv4
                COREOS_PRIVATE_IPV4=$_public_ipv4
        coreos:
            update:
                reboot-strategy: etcd-lock
            etcd:
                # generate a new token for each unique cluster from https://discovery.etcd.io/new?size=2
                #Ensure its a size 2 token so you can test a cluster using only two machines
                discovery: https://discovery.etcd.io/<cluster ID goes here>
                #You'll notice we use the variables that cloud providers use, except
                #we renamed them and populate them on our own
                addr: $_public_ipv4:4001
                peer-addr: $_public_ipv4:7001
                peer-election-timeout: 500
                peer-heartbeat-interval: 100
            fleet:
                #A test to see how metadata is added for use in building out a profile of a box
                metadata: ip=$_public_ipv4
            units:
                - name: etcd.service
                  command: start
                - name: fleet.service
                  command: start
                - name: docker-tcp.socket
                  command: start
                  content: |
                    [Unit]
                    Description=Docker Socket for the API

                    [Socket]
                    ListenStream=2375
                    Service=docker.service
                    BindIPv6Only=both
                    
                    [Install]
                    WantedBy=sockets.target
                - name: enable-docker-tcp.service
                  command: start
                  content: |
                    [Unit]
                    Description=Enable the Docker Socket for the API

                    [Service]
                    Type=oneshot
                    ExecStart=/usr/bin/systemctl enable docker-tcp.socket
    #This script will loop until the network card comes up and gives us an IP
    #to latch onto and save in /etc/environment for variable substitution
    - path: /run/setup-environment.sh
      permissions: '0755'
      content: |
        #!/bin/bash

        ENV="/etc/environment"

        # Test for read write access to $1
        touch $ENV
        if [ $? -ne 0 ]; then
            echo exiting, unable to modify: $ENV
            exit 1
        fi

        # Setup environment target
        sed -i -e '/^COREOS_PUBLIC_IPV4=/d' \
            -e '/^COREOS_PRIVATE_IPV4=/d' \
            "${ENV}"

        # We loop until the the IP addresses are set
        function get_ip () {
            IF=$1
            IP=
            while [ 1 ]; do
                IP=$(ifconfig $IF | awk '/inet / {print $2}')
                if [ "$IP" != "" ]; then
                    break
                fi
                sleep .1
            done
            echo $IP
        }

        # Echo results of IP queries to environment file as soon as network interfaces
        # get assigned IPs
        # Note: in a PXE setup where you're not guaranteed to get a network interface
        #name of eth0, we need another way to get the name of the interface as seen below
        #We use this interface name to get the IP of that interface
        INTERFACE=$(ip route get 8.8.8.8 | awk '{ print $5; exit }')
        echo COREOS_PUBLIC_IPV4=$(get_ip $INTERFACE) >> $ENV
        echo COREOS_PRIVATE_IPV4=$(get_ip $INTERFACE) >> $ENV
    #This will substitute the variables in our second cloud-config to the
    #values we pulled earlier
    - path: /run/prep-cloud-config.sh
      permissions: '0755'
      content: |
        #!/bin/bash
        
        #Grab the IP address from the /etc/environment file
        IP=$(awk 'BEGIN { FS = "=" }; /COREOS_PUBLIC_IPV4/ {print $NF};' /etc/environment)
        sed "s/\$_public_ipv4/$IP/g" /run/cloud-config.yml > /tmp/cconfigtmp
        sed -i "s/\$_public_ipv4/$IP/g" /tmp/cconfigtmp
        mv /tmp/cconfigtmp /run/cloud-config.yml
    - path: /etc/resolv.conf
      permissions: 0644
      owner: root
      content: |
        nameserver <set your nameserver here>
        nameserver <set another nameserver here>
        nameserver 8.8.8.8
        nameserver 8.8.4.4
        domain <your domain name here>
#This is a duplicate of the user section in the second cloud-config above
#So we can log into the templorary PXE boot version and the installed version with the same creds
#users:
  #- name: testuser
    #passwd: <password hash goes here>
    #groups:
      #- sudo
      #- systemd-journal
coreos:
  units:
    - name: setup-environment.service
      command: start
      runtime: true
      content: |
        [Unit]
        Description=Setup environment with private (and public) IP addresses

        [Service]
        Type=oneshot
        RemainAfterExit=yes
        ExecStart=/run/setup-environment.sh
    - name: prep-cloud-config.service
      command: start
      runtime: true
      content: |
        [Unit]
        Description=Re-write variables in cloud-config for proper expansion.
        Requires=setup-environment.service
        After=setup-environment.service

        [Service]
        Type=oneshot
        RemainAfterExit=yes
        ExecStart=/run/prep-cloud-config.sh