How to configure source server to accept SSH connections and allow user root during a migration process?

By default, SSH server is mainly configured to allow root user to log in. However, it may have been reconfigured to have root logins disabled as a security measure.

Note: 'root' user is disabled by default on AWS instances and Ubuntu.

1. Connect to the server via SSH.

2. Escalate privileges to 'root':

   # sudo su -
   # whoami
   root

3. Set password for 'root' user:

   # passwd

4. Make sure that the configuration file /etc/ssh/sshd_config has parameters PermitRootLogin and PasswordAuthentication are set to yes and not prepended by # symbol.
   Otherwise, open /etc/ssh/sshd_config in text editor and perform the necessary changes to look as below:

   # grep -i 'permitroot' /etc/ssh/sshd_config
   PermitRootLogin yes
   # grep -i 'passwordauthentication' /etc/ssh/sshd_config
   PasswordAuthentication yes

   Note: Before making this change, make sure that the password for user root is sufficiently complex (no less than 12 characters long, including numbers, upper/lowercase letters, and special symbols). Change the password if necessary.

5. Restart SSH server if any configuration changes have been made:

   # service sshd restart

If the server has a firewall, it should be configured to allow incoming SSH connections from Plesk server.

Review files /etc/hosts.allow and /etc/hosts.deny for any rules that may prevent access to SSH service from destination Plesk server.
For example, if access to sshd service is denied in hosts.deny:

sshd: ALL

ALL: ALL

Then, hosts.allow should have a record which would allow SSH access from destination server:

sshd: 192.0.2.2 192.0.2.3 : allow

Note: Replace IP addresses above with IP address of destination server. If destination server has several IP addresses, put all of them in one line, separated by spaces.