Enable the Telnet or SSH Service, on the Synology control panel.
Then use the ssh client of your choice, in this case I use my internal terminal to connect to my server.
Following is the ssh connect command
ssh — OpenSSH SSH client (remote login program)
ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
[-D [bind_address:]port] [-e escape_char] [-F configfile] [-I pkcs11]
[-i identity_file] [-L [bind_address:]port:host:hostport] [-l login_name]
[-m mac_spec] [-O ctl_cmd] [-o option] [-p port]
[-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port]
[-w local_tun[:remote_tun]] [user@]hostname [command]
I only use the -l switch for specify the username to login
Specifies the user to log in as on the remote machine. This also may be
specified on a per-host basis in the configuration file.
Here is the full command example
ssh server_ip_address -l login_name
And it will prompt for password, input the corresponding login_name’s password and you have the access to the server through ssh protocol.