Altering entered PAM username

["Julián de Navascués" <julian navascues gmail com>]

Hi all, 

I wonder if it's possible to change the username inside a PAM auth module. I would like to do something like login with a user "guest" and map it to a real unix user account, for example "real_user" (like a switch user command, "su real_user")...

I have tried something like this:

int pam_sm_authenticate(pam_handle_t *pamh,int flags,int argc,const char **argv)
{
   
int retval = pam_set_item(pamh, PAM_USER, "real_user");
return PAM_SUCCESS;

}

Using this auth module with a ssh server I would expect to login as "real_user" and see a prompt like real_user machine$ and /home/real_user as the working directory... but it does not and logs that "guest" is not a valid unix user on the machine.

I would like to know what else is needed to map users...

Thank for your help