Ubuntu changed networking. Embrace the YAML.

If I'm being completely honest, I still dislike the switch from eth0, eth1, eth2 to names like, enp3s0, enp4s0, enp5s0 . I've learned to accept it and mutter to myself while I type in unfamiliar interface names. Then I installed the new LTS version of Ubuntu and typed vi /etc/network/interfaces . Yikes. After a technological lifetime of entering my server's IP information in a simple text file, that's no longer how things are done. Sigh. The good news is that while figuring out Netplan for both desktop and server environments, I fixed a nagging DNS issue I've had for years (more on that later).

The Basics of Netplan

The old way of configuring Debian-based network interfaces was based on the ifupdown package. The new default is called Netplan, and although it's not terribly difficult to use, it's drastically different. Netplan is sort of the interface used to configure the back-end dæmons that actually configure the interfaces. Right now, the back ends supported are NetworkManager and networkd .

If you tell Netplan to use NetworkManager, all interface configuration control is handed off to the GUI interface on the desktop. The NetworkManager program itself hasn't changed; it's the same GUI-based interface configuration system you've likely used for years.

If you tell Netplan to use networkd , systemd itself handles the interface configurations. Configuration is still done with Netplan files, but once "applied", Netplan creates the back-end configurations systemd requires. The Netplan files are vastly different from the old /etc/network/interfaces file, but it uses YAML syntax, and it's pretty easy to figure out.

The Desktop and DNS

If you install a GUI version of Ubuntu, Netplan is configured with NetworkManager as the back end by default. Your system should get IP information via DHCP or static entries you add via GUI. This is usually not an issue, but I've had a terrible time with my split-DNS setup and systemd-resolved . I'm sure there is a magical combination of configuration files that will make things work, but I've spent a lot of time, and it always behaves a little oddly. With my internal DNS server resolving domain names differently from external DNS servers (that is, split-DNS), I get random lookup failures. Sometimes ping will resolve, but dig will not. Sometimes the internal A record will resolve, but a CNAME will not. Sometimes I get resolution from an external DNS server (from the internet), even though I never configure anything other than the internal DNS!

I decided to disable systemd-resolved . That has the potential to break DNS lookups in a VPN, but I haven't had an issue with that. With resolved handling DNS information, the /etc/resolv.conf file points to 127.0.0.53 as the nameserver. Disabling systemd-resolved will stop the automatic creation of the file. Thankfully, NetworkManager itself can handle the creation and modification of /etc/resolv.conf. Once I make that change, I no longer have an issue with split-DNS resolution. It's a three-step process:

Do sudo systemctl disable systemd-resolved.service . Then sudo rm /etc/resolv.conf (get rid of the symlink). Edit the /etc/NetworkManager/NetworkManager.conf file, and in the [main] section, add a line that reads DNS=default .

Once those steps are complete, NetworkManager itself will create the /etc/resolv.conf file, and the DNS server supplied via DHCP or static entry will be used instead of a 127.0.0.53 entry. I'm not sure why the resolved dæmon incorrectly resolves internal addresses for me, but the above method has been foolproof, even when switching between networks with my laptop.

Netplan CLI Configuration

If Ubuntu is installed in server mode, it is almost certainly configured to use networkd as the back end. To check, have a look at the /etc/netplan/config.yaml file. The renderer should be set to networkd in order to use the systemd-networkd back end. The file should look something like this:

network: version: 2 renderer: networkd ethernets: enp2s0: dhcp4: true

Important note: remember that with YAML files, whitespace matters, so the indentation is important. It's also very important to remember that after making any changes, you need to run sudo netplan apply so the back-end configuration files are populated.

The default renderer is networkd , so it's possible you won't have that line in your configuration file. It's also possible your configuration file will be named something different in the /etc/netplan folder. All .conf files are read, so it doesn't matter what it's called as long as it ends with .conf. Static configurations are fairly simple to set up:

network: version: 2 renderer: networkd ethernets: enp2s0: dhcp4: no addresses: - 192.168.1.10/24 - 10.10.10.10/16 gateway4: 192.168.1.1 nameservers: addresses: [192.168.1.1, 8.8.8.8]

Notice I've assigned multiple IP addresses to the interface. Netplan does not support virtual interfaces like enp3s0:0 , rather multiple IP addresses can be assigned to a single interface.

Unfortunately, networkd doesn't create an /etc/resolv.conf file if you disable the resolved dæmon. If you have problems with split-DNS on a headless computer, the best solution I've come up with is to disable systemd-resolved and then manually create an /etc/resolv.conf file. Since headless computers don't usually move around as much as laptops, it's likely the /etc/resolv.conf file won't need to be changed. Still, I wish networkd had an option to manage the resolv.conf file the same way NetworkManager does.

Advanced Network Configurations

The configuration formats are different, but it's still possible to do more advanced network configurations with Netplan:

Bonding:

network: version: 2 renderer: networkd bonds: bond0: dhcp4: yes interfaces: - enp2s0 - enp3s0 parameters: mode: active-backup primary: enp2s0

The various bonding modes ( balance-rr , active-backup , balance-xor , broadcast , 802.3ad , balance-tlb and balance-alb ) are supported.

Bridging:

network: version: 2 renderer: networkd bridges: br0: dhcp4: yes interfaces: - enp4s0 - enp3s0

Bridging is even simpler to set up. This configuration creates a bridge device using the two interfaces listed. The device ( br0 ) gets address information via DHCP.

CLI Networking Commands

If you're a crusty old sysadmin like me, you likely type ifconfig to see IP information without even thinking. Unfortunately, those tools are not usually installed by default. This isn't actually the fault of Ubuntu and Netplan; the old ifconfig toolset has been deprecated. If you want to use the old ifconfig tool, you can install the package:

sudo apt install net-tools

But, if you want to do it the "correct" way, the new "ip" tool is the proper way to do it. Here are some equivalents of things I commonly do with ifconfig :

Show network interface information.

Old way:

ifconfig

New way:

ip address show

(Or you can just do ip a , which is actually less typing than ifconfig .)

Bring interface up.

Old way:

ifconfig enp3s0 up

New way:

ip link set enp3s0 up

Assign IP address.

Old way:

ifconfig enp3s0 192.168.1.22

New way:

ip address add 192.168.1.22 dev enp3s0

Assign complete IP information.

Old way:

ifconfig enp3s0 192.168.1.22 net mask 255.255.255.0 broadcast ↪192.168.1.255

New way:

ip address add 192.168.1.22/24 broadcast 192.168.1.255 ↪dev enp3s0

Add alias interface.

Old way:

ifconfig enp3s0:0 192.168.100.100/24

New way:

ip address add 192.168.100.100/24 dev enp3s0 label enp3s0:0

Show the routing table.

Old way:

route

New way:

ip route show

Add route.

Old way:

route add -net 192.168.55.0/24 dev enp4s0

New way:

ip route add 192.168.55.0/24 dev enp4s0

Old Dogs and New Tricks

I hated Netplan when I first installed Ubuntu 18.04. In fact, on the particular server I was installing, I actually started over and installed 16.04 because it was "comfortable". After a while, curiosity got the better of me, and I investigated the changes. I'm still more comfortable with the old /etc/network/interfaces file, but I have to admit, Netplan makes a little more sense. There is a single "front end" for configuring networks, and it uses different back ends for the heavy lifting. Right now, the only back ends are the GUI NetworkManager and the systemd-networkd dæmon. With the modular system, however, that could change someday without the need to learn a new way of configuring interfaces. A simple change to the renderer line would send the configuration information to a new back end.