Very simple procedure for testing a network change without losing access to the server.

Making a network change without losing access

If you are considering making a network change and don't want to risk a down machine that you would then have to console into... here's what to do. 

1. Copy your working network configuration 
cp /etc/network/interfaces /etc/network/interfaces.working.bak 

2. Make your changes to the network connection
cat /etc/network/interfaces.withchanges > /etc/network/interfaces

3. Create a script to undo the change 
vi /root/reset_net.sh 

In the file
#!/bin/bash

cat /etc/network/interfaces.working.bak > /etc/network/interfaces
systemctl restart networking.service

4. Change permissions 
chmod 750 /root/reset_net.sh 

5. Run crontab -e 
*/5 * * * * /root/reset_net.sh

6. Restart networking services (this will apply your changes from step 2. 
If your machine has no network after that, on the 5th minute, 
it will reapply the working configuration and restart networking services. 
systemctl restart networking.service

7. If you still have access to the system and everything is working okay, 
vi /root/reset_net.sh 
Comment out the line you added 
#*/5 * * * * /root/reset_net.sh

Done.