Active Directory authentication non domain joined Linux Virtual Machines

Currently Linux distribution can work as member of Active Directory domains, which gives them access to the AD authentication system. To take advantage of AD authentication in some cases, we can avoid the AD join. To let users sign in on Azure Linux VM with Active Directory account you have different choices. One possibility is to Join in Active Directory the VM. Another possibility is to base the authentication flow through LDAP to your Active Directory without Join the VM on AD. This article shows you how to authenticate with AD credential on your Linux system (CentosOS) based on LDAP.

Prerequisites

To complete the authentication flow we assume, you already have:

  • An Active Directory Domain Services already configured.
  • A Linux VM (for the test we use CentosOS based machine).
  • A network infrastructure that allows communication between Active Directory and the Linux VM.
  • A dedicated User Account for read AD objects.
  • The Linux VM need to have these packages installed:
    • sssd
    • sssd-tools
    • sssd-ldap
    • openldap-clients
  • An LDAPS Certificate correctly configured on the Linux VM.
  • A CA Certificate correctly imported into Certificate Store of the Linux VM (the path varies depending on the Linux distro).

Active Directory User Configuration

To read Users in your Active Directory Domain Services create a ReadOnlyUser in AD. For create a new user follow the steps below:

  1. Connect to your Domain Controller.
  2. Click Start, point to Administrative Tools, and then click Active Directory Users and Computers to start the Active Directory Users and Computers console.
  3. Click the domain name that you created, and then expand the contents.
  4. Right-click Users, point to New, and then click User.
  5. Type the first name, last name, and user logon name of the new user, and then click Next. In lab environment we used a user called ReadOnlyUser.
  6. Type a new password, confirm the password, and then click to select one of the following check boxes if needed:
    • Users must change password at next logon (recommended for most user)
    • User cannot change password
    • Password never expires
    • Account is disabled (If you disable the account the authentication will fail)
  7. Click Next.

Review the information that you provided, and if everything is correct, click Finish.

Note

The lab environment is based on:

  • Windows Server 2016 Domain and Forest Functional Level.
  • Linux client Centos 8.5.

Linux Virtual Machines Configuration

Note

You must run these command with sudo permission

On your Linux VM, install the following packages: sssd sssd-tools sssd-ldap openldap-client:

yum install -y sssd sssd-tools sssd-ldap openldap-clients

After the installation check if LDAP search works. In order to check it try an LDAP search following the example below:

ldapsearch -H ldaps://contoso.com -x \
        -D CN=ReadOnlyUser,CN=Users,DC=contoso,DC=com -w Read0nlyuserpassword \
        -b CN=Users,DC=contoso,DC=com

If the LDAP query works fine, you will obtain an output with some information like follow:

extended LDIF

LDAPv3
base <CN=Users,DC=contoso,DC=com> with scope subtree
filter: (objectclass=*)
requesting: ALL

Users, contoso.com
dn: CN=Users,DC=contoso,DC=com
objectClass: top
objectClass: container
cn: Users
description: Default container for upgraded user accounts
distinguishedName: CN=Users,DC=contoso,DC=com
instanceType: 4
whenCreated: 20220913115340.0Z
whenChanged: 20220913115340.0Z
uSNCreated: 5660
uSNChanged: 5660
showInAdvancedViewOnly: FALSE
name: Users
objectGUID:: i9MABLytKUurB2uTe/dOzg==
systemFlags: -1946157056
objectCategory: CN=Container,CN=Schema,CN=Configuration,DC=contoso,DC=com
isCriticalSystemObject: TRUE
dSCorePropagationData: 20220930113600.0Z
dSCorePropagationData: 20220930113600.0Z
dSCorePropagationData: 20220930113600.0Z
dSCorePropagationData: 20220930113600.0Z
dSCorePropagationData: 16010101000000.0Z

Note

If your get and error run the following command:

ldapsearch -H ldaps://contoso.com -x
-D CN=ReadOnlyUser,CN=Users,DC=contoso,DC=com -w Read0nlyuserpassword
-b CN=Users,DC=contoso,DC=com -d 3

Troubleshoot according to the output.

Create sssd.conf file

Create /etc/sssd/sssd.conf with a content like the following. Remember to update the ldap_uri, ldap_search_base and ldap_default_bind_dn.

Command for file creation:

vi /etc/sssd/sssd.conf

Example sssd.conf:

[sssd]
config_file_version = 2
domains = default
services = nss, pam
full_name_format = %1$s

[nss]

[pam]

[domain/default]
id_provider = ldap
cache_credentials = True
ldap_uri = ldaps://contoso.com
ldap_search_base = CN=Users,DC=contoso,DC=com
ldap_schema = AD
ldap_default_bind_dn = CN=ReadOnlyUser,CN=Users,DC=contoso,DC=com
ldap_default_authtok_type = obfuscated_password
ldap_default_authtok = generated_password

# Obtain the CA root certificate for your LDAPS connection.
ldap_tls_cacert = /etc/pki/tls/cacerts.pem

# This setting disables cert verification.
#ldap_tls_reqcert = allow

# Only if the LDAP directory doesn't provide uidNumber and gidNumber attributes
ldap_id_mapping = True

# Consider setting enumerate=False for very large directories
enumerate = True

# Only needed if LDAP doesn't provide homeDirectory and loginShell attributes
fallback_homedir = /home/%u
default_shell = /bin/bash
access_provider = permit
sudo_provider = ldap
auth_provider = ldap
autofs_provider = ldap
resolver_provider = ldap

Save the file with ESC + wq! command.

Note

If you don't have a valid TLS certificate under /etc/pki/tls/ called cacerts.pem the bind doesn't work

Change permission for sssd.conf and create the obfuscated password

Set the permission to sssd.conf to 600 with the following command:

chmod 600 /etc/sssd/sssd.conf

After that create an obfuscated password for the Bind DN account. You must insert the Domain password for ReadOnlyUser:

sss_obfuscate --domain default

The password will be placed automatically in the configuration file.

Configure the sssd service

Start the sssd service:

service sssd start

Now configure the service with the authconfig tool:

authconfig --enablesssd --enablesssdauth --enablemkhomedir --updateall

At this point restart the service:

systemctl restart sssd

Test the configuration

The final step is to check that the flow works properly. To check this, try logging in with one of your AD users in Active Directory. We tried with a user called ADUser. If the configuration is correct, you will get the following result:

[centosuser@centos8 ~]su - ADUser@contoso.com
Last login: Wed Oct 12 15:13:39 UTC 2022 on pts/0
[ADUser@Centos8 ~]$ exit

Now you are ready to use AD authentication on your Linux VM.