If we want to access external server via SSH, it is possible that if we are behind a proxy we will not have access to port 22 used by SSH. As root, edit the file sshd_config:

Edit the line which states 'Port 22' and changed it by 'Port 443'. This port is used by 'https' so it will not be closed in the proxy. Switch over to the new port by restarting SSH.