Red Hat

For some reason i cannot login using root or other accounts on my Linux system.

When logging in at the main console it says "Authentication failed" in a dialog box with an OK button.
The Linux system is Redhat 4.7.

I've already checked /etc/pam.d/login, /etc/security/access.conf and /etc/securetty files but everything is correct. I have already checked if /etc/nologin file exists.

I have accessed the files mentioned above using rescue mode.

Can anyone help me for this?

What does /var/log/secure say? What about /var/log/messages? They should give you a clue. Also, please do an "ls -l" on /bin/login and check if the permissions to that has been altered. It should have root:root ownership with 755 permission. Also, did you check if permissions on /etc/passwd and /etc/shadow are intact? Or even if the files exist?

Both files (/var/log/secure, /var/log/messages) have the information about the last time I was able to login successfully.

Also /bin/login has 755 permission, /etc/passwd has 644 permission and /etc/shadow has 600 permission.

This means PAM is not logging the login attempts (either successful or unsuccessful, it's supposed to log them) in /var/log/secure, which leads me to suspect that PAM is not doing what it's supposed to do.

Do you remember doing any changes that could affect PAM modules?

No I have not done or installed anything on the system that will affect PAM module.

My pam.d/login file contents are

#%PAM-1.0
auth required pam_securetty.so
auth required pam_stack.so service=system-auth
auth required pam_nologin.so
account required pam_stack.so service=system-auth
password required pam_stack.so service=system-auth
# pam_selinux.so close should be the first session rule
session required pam_selinux.so close
session required pam_stack.so service=system-auth
session required pam_loginuid.so
session optional pam_console.so
# pam_selinux.so open should be the last session rule
# session required pam_selinux.so open
session required /lib/security/pam_limits.so
session sufficient /lib/security/pam_lsass.so
session required pam_selinux.so open

Okay, could you execute "authconfig-tui" and make sure local user authentication is enabled? Also check /etc/nsswitch.conf after this. "files" should be the first option for passwd, shadow, and group in there.

I think somehow, PAM is not being used at all for authentication.

authconfig-tui command gave me an error saying command not found.

/etc/nsswitch.conf file first few lines are:

;passwd: files winbind
passwd files lsass
shadow files
group files lsass
;group: files winbind