If you've been used to managing Ubuntu 14.04 LTS and Ubuntu 16.04 LTS servers, Ubuntu 18.04 will be really confusing, because everything about networking just changed.

Let's say you want to make some changes to an interface, like DNS/nameservers or adding an interface. With previous versions, you'd just go to /etc/network/interfaces and make the change, and run service networking restart. Not so easy now.

If we take the DNS change as an example, you might grep /etc for the DNS server address, and then you'll end up with two filenames:

• /etc/cloud/cloud.cfg.d/50-curtin-networking.cfg
• /etc/netplan/50-cloud-init.yaml

It's not obvious, to put it mildly, where to make the change. The two files seem identical, so which one is the authoritative? In addition, the 50-cloud-init.yaml file says the following at the top:

Changes to it will not persist across an instance.

Oh, that must mean that the correct file is the one in /etc/cloud, right? Wrong!

The way it works is that when you set up the machine, "cloud" is in charge of writing the /etc/netplan/50-cloud-init.yaml on the first boot. That's why  it says it "will not persist": it will for your server, but it's a hint to image builders that when they're building a custom ISO that it's not the place to put such things.

Conclusion

The right place to make the change is /etc/netplan/50-cloud-init.yaml.

Then, to apply the change, you now have to use netplan apply.