How to setup of Raspberry Pi 3 onboard WiFi for Ubuntu Server with 'netplan'?

The steps below were found to provide a persistent WiFi setup using netplan with Ubuntu Server 18.04 ubuntu-18.04.2-preinstalled-server-arm64+raspi3.img.xz image on a Raspberry Pi 3 B+.

Update system:

sudo apt update
sudo apt full-upgrade
sudo reboot

Determine interface names:

ip link show

# 1: lo: <LOOPBACK,UP,LOWER_UP> …
# 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> … state UP …
# 3: wlan0: <BROADCAST,MULTICAST> … state DOWN 

Determine your-cloud-init.yaml and open for editing.

cd /etc/netplan/
ls -l
# -rw-r--r-- 1 root root 666 May 15 22:00 50-cloud-init.yaml
### note your *cloud-init.yaml file name

### backup *cloud-init.yaml file
cp 50-cloud-init.yaml 50-cloud-init.yaml.bak
### restrict read access
sudo chmod 640 /etc/netplan/50-cloud-init.yaml
### edit *cloud-init.yaml
sudo nano 50-cloud-init.yaml

Add WiFi access information to your-cloud-init.yaml file.

# This file is generated from information provided by
# the datasource.  Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    version: 2
    ethernets:
        eth0:
            optional: true
            dhcp4: true
    # add wifi setup information here ...
    wifis:
        wlan0:
            optional: true
            access-points:
                "YOUR-SSID-NAME":
                    password: "YOUR-NETWORK-PASSWORD"
            dhcp4: true

Test, generate and apply the changed your-cloud-init.yaml config:

• Testing: sudo netplan --debug try (continue even if successful)
• Generate: sudo netplan --debug generate (provides more details in case of issues with the previous command)
• Apply: sudo netplan --debug apply (if no issues during the previous commands)

Confirmation Test:

sudo reboot

### wait, then without the wired ethernet connected ... 
ssh ubuntu@wifi-ip-address

The above sequence was distilled from the "Raspberry Pi 3B/B+ Wireless Bridge using Ubuntu Server 18.04 ARM Image and Netplan" gist link mentioned by Larnu. The gist goes well beyond just enabling WiFi since its turns the Pi into a Bridge.

---

Some additional useful WiFi setup steps.

Set hostname.

sudo hostnamectl set-hostname my-server-name

sudo nano /etc/hosts

127.0.0.1 localhost
# add host name
127.0.0.1 my-server-name

sudo nano /etc/cloud/cloud.cfg

# Set preserve_hostname to true for persistance after reboot
preserve_hostname: true

Verify from local Raspberry Pi commandline.

hostnamectl
#   Static hostname: my-server-name
#         Icon name: computer
#        Machine ID: …
#           Boot ID: …
#  Operating System: Ubuntu 18.04.2 LTS
#            Kernel: Linux 4.15.0-1036-raspi2
#      Architecture: arm64

Enable mDNS.

If desired, enable Multicast DNS by installing Avahi. Avahi supports the mDNS/DNS-SD/RFC 3927/Zeroconf/Bonjour specification.

sudo apt install avahi-daemon 

Remotely check mDNS resolution from another computer.

ping my-server-name.local
ssh [email protected]