304.1 Linux Authentication Clients (weight: 5)

Tech Tutorial: 304.1 Linux Authentication Clients #

Introduction #

In this tutorial, we will delve into the management and authentication of user accounts in Linux, focusing on configuration and use of Name Service Switch (NSS), Pluggable Authentication Modules (PAM), System Security Services Daemon (SSSD), and Kerberos. Understanding these components is crucial for securing and managing authentication in Linux environments, especially in networked or multi-user scenarios.

Key Knowledge Areas: #

  • Configuration and usage of NSS
  • Understanding and configuring PAM
  • Setting up and managing SSSD
  • Configuring Kerberos for authentication
  • Enforcing password policies

Utilities: #

  • nsswitch.conf
  • pam.conf, pam.d/
  • sssd.conf
  • krb5.conf
  • Various PAM and NSS related tools

Step-by-Step Guide #

1. Configuring Name Service Switch (NSS) #

The Name Service Switch (NSS) configuration file, /etc/nsswitch.conf, controls how the Linux system resolves names to information like user identities or hostnames.

Example: /etc/nsswitch.conf #

passwd:         files sss
group:          files sss
shadow:         files sss
hosts:          files dns
networks:       files
services:       db files
protocols:      db files
rpc:            db files
ethers:         db files
netgroup:       nis

In this example, the system uses local files and SSSD (sss) for password and group information, DNS for host resolution, and a combination of local database files for other services.

2. Understanding and Configuring PAM #

Pluggable Authentication Modules (PAM) provide a flexible mechanism for authenticating users. PAM configurations are typically located in /etc/pam.d/.

Example: /etc/pam.d/common-auth #

auth    required    pam_unix.so nullok
auth    optional    pam_sss.so use_first_pass

In this configuration, pam_unix.so handles standard Unix authentication, and pam_sss.so integrates with SSSD for additional authentication mechanisms.

3. Setting Up and Managing SSSD #

System Security Services Daemon (SSSD) provides access to remote identity and authentication providers. Configuration is handled in /etc/sssd/sssd.conf.

Example: /etc/sssd/sssd.conf #

[sssd]
services = nss, pam
config_file_version = 2
domains = EXAMPLE

[domain/EXAMPLE]
id_provider = ldap
auth_provider = krb5
ldap_uri = ldap://ldap.example.com
ldap_search_base = dc=example,dc=com
krb5_server = kerberos.example.com
krb5_realm = EXAMPLE.COM

This configuration enables SSSD with LDAP as the identity provider and Kerberos for authentication.

4. Configuring Kerberos for Authentication #

Kerberos requires configuration of the /etc/krb5.conf file to specify the realms and servers for authentication.

Example: /etc/krb5.conf #

[libdefaults]
    default_realm = EXAMPLE.COM
    ticket_lifetime = 24h
    renew_lifetime = 7d

[realms]
    EXAMPLE.COM = {
        kdc = kerberos.example.com
        admin_server = kerberos.example.com
    }

[domain_realm]
    .example.com = EXAMPLE.COM
    example.com = EXAMPLE.COM

5. Enforcing a Password Policy #

Password policies can be enforced using PAM’s pam_cracklib.so or pam_pwquality.so modules.

Example: /etc/pam.d/common-password #

password    requisite    pam_pwquality.so retry=3 minlen=8 dcredit=-1 ucredit=-1
password    required     pam_unix.so use_authtok nullok shadow sha512

This configuration enforces a minimum password length and requires both digits and uppercase letters.

Conclusion #

Understanding and configuring NSS, PAM, SSSD, and Kerberos are crucial for secure and efficient user authentication management in Linux. By following the examples and configurations outlined in this tutorial, you can strengthen your system’s security and ensure compliant authentication practices. Whether you’re preparing for an exam or securing a production environment, these tools are indispensable in the Linux ecosystem.