Prior to Ubuntu 18.04, DNS nameservers would traditionally be configured using the /etc/resolv.conf configuration file. For a long time, this file allowed quick and easy configuration of DNS nameservers, as can be seen in an example resolv.conf file below.
Starting in Ubuntu 18.04, network settings, including nameservers, are now controlled through the NetPlan interface. Even prior to 18.04, you were still allowed to make changes to the original resolv.conf, but this file was actually was controlled by NetworkManager and its configuration files, typically located in the /etc/network/interfaces directory.
So, what prompted this change to using the new NetPlan interface? Simplicity and flexibility. Using a human-readable YAML file makes configuring complex networking configurations far easier to do. In addition, debugging is quicker and errors are easier to track.
Changing NameServers in NetPlan
With this change in interfaces, how do we actually update the nameservers for an Ubuntu 18.04 system? Instead of using /etc/resolv.conf, navigate to /etc/netplan, and you may find a series *.yaml files there. If there are none yet defined, you should create one in the recommended naming format of, ##-name.yaml, such as 01-netconfig.yaml. NetPlan will load these files in the following directory order of precedence and then by the preceding number order.
/run/netplan/. yaml /etc/netplan/. yaml /lib/netplan/*. yaml
Find the Existing IP Configuration
Before changing the NetPlan network configuration, it is prudent to learn what the existing interface configuration is. NetPlan itself doesn’t monitor the active status, instead we use the ip command to display this.
The output should look similar to the below screenshot, but keep in mind that each network interface may be different depending on the system.
Within this configuration, you will find your active interfaces. In this case, we are looking for the eth0 configuration for purposes of this tutorial.
Updating the Configuration
Either choose an existing YAML file to edit or create a new one and add the following configuration, modified as necessary for your configuration.
This configuration assumes that you are using a static IP and eth0 is your primary interface as learned from the above ip a command.
The nameservers that are in use here are the public Cloudflare nameservers, but use any that works best for you. Included are the IPv6 resolvers as well.
If you want to use DHCP, you can replace the static IP configuration with dhcp4: yes under the eth0 section.
Additionally, if you use DHCP, you will need to set an override for nameservers.
You might notice that the addresses are listed in a list underneath addresses but it’s possible to also define these addresses in an array configuration as well.
Renderers
In some configurations, you might see a renderer configuration listed. There are two available types: NetworkManager and networkd. By default, networkd is used. The difference is that NetworkManager is the GUI manager used by Ubuntu, whereas networkd is used by Systemd. It’s unnecessary to define this, as the default configuration should work in most cases.
Testing the NameServer Change
After the configuration file has been created, run the following command to temporarily try the new settings and revert if there is a misconfiguration.
If there is a failure, the configuration will automatically revert after 120 seconds. You can open up a second shell to test out any configurations while this is applying. If the settings work as expected, you can hit enter to keep those settings or revert if they do not. This allows you to modify the configurations until it works.
Applying the Change
Finally, after making sure the changes are accurate, the changes need to be applied. To do so, run the netplan apply command like so.
If the interface is just switching nameservers, a reboot is generally unnecessary. If larger configuration changes are made, then a reboot of the system may be necessary to effect the changes.
Bringing Interfaces Up and Down
Previously, ifconfig was used to bring an interface up and down. As noted previously though, the ip command is taking the place of ifconfig. In the example below, eth0 is brought down and then back up.
If eth0 is your only interface, be careful not to bring the interface down without having a way to connect to the system remotely.
Debugging NetPlan
What happens when a configuration goes wrong? There are a few ways to explore the NetPlan configuration and figure out where a failure might have occurred. One way to determine if the correct configuration was actively applied is to verify the configuration files generated in /run/systemd/network. For example, you can navigate to that directory and find a matching *.network configuration file, then verify that the configuration matches what you expect.
An additional command that works well to verify that the generated config is correct is to use the following:
Keep in mind that the following screenshot may be different depending on your configuration.
Conclusion
NetPlan is a big change in the existing Ubuntu networking, but it is intended to simplify and create more flexible configurations. Using a combination of existing commands and the new NetPlan one’s we can quickly create and apply complex networking configurations.
With the introduction of a YAML-based configuration and updated abilities through the ip command, it is easier than ever to create complex configurations to handle even the most unique situations.
