By default, all SONiC devices support both the serial console based login and the SSH based login. The default credential (if not modified at image build time) for login is admin/YourPaSsWoRd. In case of SSH login, users can login to the management interface (eth0) IP address after configuring the same using serial console. Refer the Configuring Management Interface and Loopback Interface section for configuring the IP address for management interface.

Example:

At Console: Debian GNU/Linux 9 sonic ttyS1 

sonic login: admin 
Password: YourPaSsWoRd 

SSH from any remote server to sonic can be done by connecting to SONiC IP 

user@debug:~$ ssh admin@sonic_ip_address(or SONIC DNS Name) 
admin@sonic's password:

By default, login takes the user to the default prompt from which all the show commands can be executed.

On successful login, SONiC Welcome Message of the Day will be displayed.