Can anyone tell me if it's possible to logon automatically a predefined user without need to input username and password (in DSL)?

Logon from where to where?

Within a network it is often done using ssh "trusts".

Essentially you create an ssh key for the user on the system you want to login from then add its public key to the authorized_keys files on the host yu want to login to.

To generate on from system:
-Login to that user on that system
-Run "ssh-keygen -t dsa" to generate dsa key (type "man ssh-keygen" for more dtails). It generates two files in $HOME/.ssh which are the private key (id_dsa) and the public key (id_dsa.pub).

To create trust on the to system:
-Login as the user (supplying password first time)
-cd $HOME/.ssh (mkdir $HOME/.ssh if the directory doesn't already exist)
-"vi authorized_keys"
add the contents of id_dsa.pub from the "from" system to this file.

To login with the trust:
-Login to as the user on the from system
-Type ssh tosystem
If the trust is working it shouldn't prompt for a password.

Note: ssh is "secure" shell so expects to be "secure". You need to insure the home directory of both users is writable only by the user itself and that $HOME/.ssh is only writable/readable by the user itself. Otherwise it won't work.

Local login.
Thank you