Time Synchronization

Active Directory uses the Kerberos protocol for authentication purposes. However, this protocol is sensitive to clock timings between systems collaborating in a Kerberos domain. Consequently, if the time difference between the Ubuntu system and the Active Directory controller is more than five minutes, authentication against the Active Directory controller will ultimately fail [3].

Primarily, Unix and Linux systems use the Network Time Protocol (NTP) for time synchronization (e.g., Ubuntu uses the default NTP server at ntp.ubuntu.com ). No timing differences can exist between the Ubuntu machine and the Active Directory controller, so you can't use an external timing source.

To configure the Ubuntu machine to use the Active Directory controller as its NTP server, install and configure a service like NTP or chrony. If everything has been set up correctly, enter

$ ping example.com
$ ping WIN-2RIFAMT88GR.example.com

to make sure the Ubuntu machine can now ping the domain name and the Active Directory FQDN.

Discovering a Domain

The realmd system D-Bus service manages discovery and registration in realms, such as Active Directory domains. To install the package and check the members already enrolled or registered with the Active Directory domain, enter

$ sudo apt install realmd
$ realm list

An empty result shows that no machines are enrolled so far. Now discover your domain (example.com here) by running the command

$ realm discover example.com

After successfully discovering the domain, you'll see output similar to that shown in Listing 1. Interestingly, the result also lists the requisite and most appropriate packages for joining the domain. The command

vagrant@node1: $ realm discover example.com
example.com
  type: kerberos
  realm-name: EXAMPLE.COM
  domain-name: example.com
  configured: no
  server-software: active-directory
  client-software: sssd
  required-package: sssd-tools
  required-package: sssd
  required-package: libnss-sss
  required-package: libpam-sss
  required-package: adcli
  required-package: samba-common-bin

$ sudo apt install sssd sssd-tools libnss-sss libpam-sss adcli samba-common-bin

installs those packages.

Joining a Domain

The configured: no option shown in Listing 1 indicates that the system isn't ready to join the Active Directory domain. To do so, you can simply run the realm join command (Figure 2):

$ sudo realm join example.com -v

Figure 2: Joining the domain with the realm join command.

A second realm discover command shows the change (Figure 3).

Figure 3: The configured domain after issuing the realm join command.

The default approach of realm is to log in from the domain administrator's account. However, you can use the -U switch to pass a different username [4]. The -v option provides verbose output.

Additionally, the realm tool also creates a configuration for the SSSD and adds modules for PAM and NSS. Furthermore, it has taken care of starting the required services. You can also check the /etc/sssd/sssd.conf<file> file (Listing 2):

vagrant@node1: $ sudo cat /etc/sssd/sssd.conf
[sssd]
domains = example.com
config_file_version = 2
services = nss, pam
[domain/example.com]
default_shell = /bin/bash
krb5_store_password_if_offline = True
cache_credentials = True
krb5_realm = EXAMPLE.COM
realmd_tags = manages-system joined-with-adcli
id_provider = ad
fallback_homedir = /home/%u@%d
ad_domain = example.com
use_fully_qualified_names = True
ldap_id_mapping = True
access_provider = ad

Moving on, you need to create a home directory for every AD user that will log on to the Ubuntu machine. To set up pam_mkhomedir, run the command (Figure 4)

$ sudo pam-auth-update --enable mkhomedir

Figure 4: Creating the user's home directory and fetching its details.

Additionally, you can fetch the details of an AD user with the command

$ getent passwd Administrator@example.com

At last, you can log in on the Ubuntu machine with the credentials of the Active Directive user and verify that you get the user home directory on the Ubuntu machine with the commands

$ sudo login
$ ls /home/

The login command prompts you to supply the username (appended with the domain name) and password (Figure 5).

Figure 5: Logging in with the Active Directory user's credentials.