I got message as 'Authentication failed' when I tried to login as a user.And I did the same as root ,got the same result. How can I overcome this PROBLEM?. Do I need to reinstall my REDHAT linux.As long as you have physical ACCESS to a Linux machine, you can gain root access. What version of RedHat are you using. I'm sure your using the GRUB bootloader. What you'll need to do is boot to single user mode.

1.If you have a GRUB password configured, type P and enter the password.

2.Select Red Hat Linux with the version of the kernel that you wish to boot and type e for edit. You will be presented with a list of items in the configuration file for the title you just selected.

3.Select the line that starts with kernel and type e to edit the line.

4.Go to the end of the line and type the NUMBER 1 as a separate word (press the SPACEBAR and then type the number 1). Press Enter to exit edit mode.

5.Back at the GRUB screen, type b to boot into single user mode.

From there it will boot into single user mode, once you get a prompt. You'll be able to change your password by typing in the passwd command. If you have any more questions, you can contact me at ResourcePC

Dave