How to confirm an user logged in is a remote user?
 

How do I confirm if a user logged in, is remote or local? In the case the user is remote, how to be sure what authentication/method is it using, like LDAP, NIS or other?

just use the "w" command. The authentication mechanism is not relevant though once connected. You can look in /var/log/secure for authentication logs and marry them up if need be.

actually theres a user "abc123". Its not listed in /etc/passwd. so i wanted to know it gets authenticated to login to this machine.

Thanks'

Try "getent passwd" to see all possible users.

getnet passwd showed a lot of users, but the problem is fixed now.
I tried to diable nis in etc/nsswitch.conf, the user could not login.
I uncomemnted the nis option for passwd. the user can login./
Hence we can conclude that the user needed NIS to login.