dynamic DNS from clouDNS on GCP/Amazon linux VM instances with systemctl

cloudns_dynamic_dns_for_systemctl_boot_on_vms.txt

This is a work in progress. 

Using cloudns for dynamic DNS - get your AWS or EC2 server to use dynamic DNS at boot.
Get new dynamic DNS update via cloudns when systemctl starts on VM instances such as GCP and EC2.

I have a zone hosted in cloudns so this is a plus, I dont know if you can use this without a full zone of your own. 
These are the steps

1. Make a new A record for the VM that you want to use in a zone file on cloudns 

2. click the "Active Dynamic URL" button to popup the dynamic DNS info box. 
   You will see a wget one line command; plus links to perl and python scripts etc - for that specific hostname.
   You can use wget command in crontab - BUT - upload the perl or python script to /root/bin on the vm. 
   
      Perl works fine for Ubuntu - but AWS Linux has an OLD version of Tiny::HTTP so  USE PYTHON ON AWS Linux 
      WGET will not work via this systemctl service - I dont know why yet.

3. Download  ( and RENAME ) the perl or python script - 
      each script is custom for that single "A" hostname.
       Each script has a custom ID for that particular hostname.
    
4. Upload the script to the VM and place it in /root/bin/ or similar
5. Create the 2-3 line bach script to run the script provided by CloudNS
   i.e. /root/bin/update-cloudns-dynamic-ip.sh

     #!/bin/bash
    /usr/bin/python  /root/bin/al2-dynamic-url-python.py
    exit
    
    You can add whatever monitoring / sanity checks you want to this script etc. 

6. Create a new file to use as a new "simple service" for systemctl:
   Create this file as cloudns-dynamic-ip.service in the folder  /etc/systemd/system/
i.e.:

      [root@ip-172-23-23-23 ~]# vim /etc/systemd/system/cloudns-dynamic-ip.service
      [Unit]
      Description=UPDATE Dynamic DNS via cloudns

      [Service]
      Type=simple
      ExecStart=/bin/bash /root/bin/update-cloudns-dynamic-ip.sh

      [Install]
      WantedBy=default.target

      [root@ip-172-23-23-23 ~]# 

7. enable the new "service"
  [root@ip-172-23-23-23 ~]# systemctl enable cloudns-dynamic-ip
  Created symlink from /etc/systemd/system/default.target.wants/cloudns-dynamic-ip.service to /etc/systemd/system/cloudns-dynamic-ip.service.

8. Start the service ( this will execute the script )
  [root@ip-172-23-23-23 ~]# systemctl start cloudns-dynamic-ip
9. run this to see that it ran the script sucessfully:
  [root@ip-172-23-23-23 ~]# systemctl  status cloudns-dynamic-ip

You can also check with the DNS server to make sure it has the new value. 

Finally - using the wget command from CLOUDNS - make a  cron job for hourly updates in case the lease changes while server is up etc.

25 * * * * wget -q --read-timeout=0.0 --waitretry=5 --tries=30 --background https://ipv4.cloudns.net/api/dynamicURL/?q=base64ID