How can I remote ssh login to the Ubuntu 16.04 LTS via root?

By default you are unable to ssh login to the Ubuntu 16.04 LTS via root, but you can enable it step by step as following:

1. Login to the system via regular user account.

2. Set password to root account (Due to root password is empty by default)

sudo passwd root

3. To modify the configuration file of sshd_config (Marked PermitRootLogin prohibit-password, and added PermitRootLogin yes )

sudo vi /etc/ssh/sshd_config

# Authentication:

LoginGraceTime 120

#PermitRootLogin prohibit-password 

PermitRootLogin yes 

StrictModes yes

4. Restart sshd

 sudo systemctl restart ssh

Done.

Unable to logon to the Ubuntu Core with 2nd SSH public key?

After I added a new SSH public key into my Ubuntu One. However, I am unable to logon to the Ubuntu Core system with this new key, therefore I logon to the Ubuntu Core by original public key.

Afterward, I noticed that my system didn't sync up a new public key from Ubuntu One automatically.

Until now I have no idea if this is a defect or limitation, please advice, if any.

Workaround:

Added 2nd public key via manually into the ~/.ssh/authorized_keys

How to deploy Ubuntu Core from KVM to VirtualBox?

Steps:
1. Download Ubuntu Core of KVM image from URL below:
https://developer.ubuntu.com/core/get-started/kvm

2. To decompress xz.

3. To convert the format of img to the vdi via VBoxManage command below:

VBoxManage.exe convertdd ubuntu-core-16-amd64.img ubuntu-core-16-amd64.vdi

4. Import vdi file above to the VirtualBox without issue.