When dealing with password management on Debian systems, you might encounter the frustrating error:

passwd: User not known to the underlying authentication module
passwd: password unchanged

This occurs in several scenarios:

• When non-root users try to change their own password
• When root attempts to change another user's password
• When Kerberos authentication is involved but misconfigured

Debian uses PAM (Pluggable Authentication Modules) for authentication. The error suggests a mismatch between:

/etc/passwd
/etc/shadow
/etc/pam.d/common-password

First verify the user exists in shadow:

sudo grep claudiu /etc/shadow

Check PAM configuration:

cat /etc/pam.d/common-password

Solution 1: Rebuild Authentication Databases

For systems without pwconv/pwunconv:

sudo apt-get install libpam-runtime
sudo pam-auth-update --force

Solution 2: Manual Shadow Entry Creation

If the user lacks a shadow entry:

sudo cp /etc/shadow /etc/shadow.backup
sudo usermod -a -G shadow claudiu
sudo chmod g+r /etc/shadow
sudo vipw -s

Solution 3: Kerberos Configuration

For systems using Kerberos:

sudo apt-get install libpam-krb5
sudo dpkg-reconfigure krb5-config

If issues persist, examine auth logs:

sudo tail -f /var/log/auth.log

Test PAM stack directly:

sudo pamtest passwd claudiu

Remember to restore original permissions after fixing:

sudo chmod g-r /etc/shadow
sudo gpasswd -d claudiu shadow

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When working with Debian systems, you might encounter a puzzling situation where password changes fail with the error:

passwd: User not known to the underlying authentication module
passwd: password unchanged

This typically happens when the system is configured to use PAM (Pluggable Authentication Modules) and there's a mismatch between the user accounts in /etc/passwd and the authentication service being used.

First, let's check the authentication configuration. Run:

cat /etc/nsswitch.conf | grep passwd

If you see output like passwd: files ldap or passwd: compat, it indicates your system is using multiple authentication sources.

Here are the most frequent cases and how to resolve them:

Case 1: Missing Shadow Password Suite

The pwconv and pwunconv utilities are part of the shadow package. Install it with:

sudo apt-get install shadow

After installation, you can synchronize password databases:

sudo pwconv
sudo grpconv

Case 2: Kerberos Authentication Misconfiguration

If your system uses Kerberos but the user isn't properly registered, you'll need to:

sudo kadmin.local -q "addprinc username"

Or modify the PAM configuration in /etc/pam.d/common-password to include both local and Kerberos authentication.

Case 3: PAM Module Issues

Check your PAM configuration for the passwd service:

cat /etc/pam.d/passwd

Ensure it includes lines like:

password        requisite       pam_unix.so obscure sha512
password        [success=1 default=ignore]      pam_ldap.so use_authtok try_first_pass

If the above methods don't work, try these:

# Force password change using chpasswd
echo "username:newpassword" | sudo chpasswd

# Or edit directly (not recommended for production)
sudo vipw

To avoid similar problems:

• Keep your authentication methods consistent across all services
• Regularly check /etc/nsswitch.conf and PAM configurations
• When adding users, ensure they exist in all required authentication systems