Master Active Directory: A Complete Beginner's to Intermediate Guide

What is Active Directory

Active Directory (AD) is a directory service developed by Microsoft that provides a centralized database for managing and organizing network resources in a Windows domain. It handles essential functions like authentication and authorization, allowing administrators to control who can access what resources on the network.

Why does it matter?

Active Directory (AD) matters because it is the central hub for managing and securing an organization’s entire IT infrastructure. By providing a single, consistent framework for identity and access, it simplifies administration, strengthens security, and supports the efficient operation of a network. For most organizations operating in a Windows environment, AD is a foundational and mission-critical service.

Core Components of Active Directory

To effectively work with Active Directory (AD), it’s essential to understand its foundational building blocks. These core components define how AD is structured, how users and devices are managed, and how permissions are applied across an organization.

Domain

A domain is the foundation of an Active Directory environment. It’s a logical group of network objects, such as users, computers, printers, and security groups that share the same database and security policies. When a user logs in to a domain, their credentials are verified by a domain controller, and access is granted based on the permissions assigned to them.

Tree

A tree is a collection of one or more domains that are logically connected in a hierarchical structure. These domains share a common namespace and are linked together through trust relationships.

Forest

A forest is the topmost level in Active Directory architecture and serves as the overall security and administrative boundary. It consists of one or more trees that may or may not share a common namespace but are linked through trust relationships. Domains within the same forest trust each other automatically, which allows for resource sharing, centralized authentication, and unified administration.

Organizational Units (OUs)

Organizational Units are used to organize objects within a domain into manageable sections. They are like folders that help structure the domain logically based on departments, teams, or functions. For example, you might create OUs named HR, IT, or Finance and place the relevant user accounts and computers in each. OUs make it easier for administrators to apply group policies and delegate specific management tasks. For instance, an IT manager could be given control over the IT OU without having full access to the rest of the domain. Unlike domains or forests, OUs are purely for administrative convenience and do not have any security boundaries of their own.

Objects

In Active Directory, everything is considered an object. An object is any individual item that is stored in the directory, such as a user, a computer, a printer, or even a group. Each object has a set of attributes that describe it, like a user object having a name, email address, and login credentials.

Domain Controller (DC)

A Domain Controller is a Windows server that runs the Active Directory Domain Services (AD DS) role. It is responsible for handling authentication and authorization requests in the domain. When a user logs in, the domain controller checks their username and password against the AD database and determines what resources they can access.

It also enforces group policies, manages trust relationships, and replicates AD data to other domain controllers to keep everything synchronized. In a secure environment, having multiple domain controllers is recommended for redundancy and load balancing. If one fails, the others continue to serve authentication requests without downtime.

Global Catalog

The Global Catalog is a specialized database that stores a partial replica of all objects in the Active Directory forest. It is used to perform searches across all domains and to locate objects quickly, regardless of which domain they belong to.

Understanding the Active Directory Authentication System

Authentication is the process of verifying a user’s identity, and in an Active Directory environment, it’s at the heart of everything. Every time a user logs in, accesses a file share, or tries to open an internal application, AD is working behind the scenes to determine: “Is this person really who they say they are?” and “Are they allowed to do this?”

Active Directory primarily uses two authentication protocols:

Kerberos

Kerberos is the default and preferred authentication protocol in Active Directory environments starting with Windows 2000 and later. It’s fast, secure, and uses encrypted tickets instead of passwords to verify identity.

1. User Login: When a user logs into their machine and enters their credentials, the system sends a request to the Key Distribution Center (KDC), which runs on the Domain Controller.
2. Ticket Granting Ticket (TGT): If the credentials are correct, the KDC sends back a Ticket Granting Ticket, which is an encrypted proof that the user is authenticated. The TGT is stored in the user’s session, so they don’t have to keep re-entering their password.
3. Service Request: When the user wants to access a resource (like a file share or a web app), their system sends the TGT to the KDC and asks for a Service Ticket for that specific resource.
4. Access Granted: The KDC issues a Service Ticket, which is presented to the target service. If valid, the user is granted access, all without sending their password again.

This process is seamless and secure because the password is never transmitted after login, and everything is encrypted.

NTLM Authentication

NTLM is an older authentication protocol that’s still supported for compatibility reasons. It’s less secure than Kerberos and should be avoided whenever possible in modern environments.

In NTLM, authentication happens through a challenge-response mechanism:

1. The user sends a request to access a resource.
2. The server sends a challenge (random string).
3. The client responds with a hash based on the challenge and the user’s password hash.
4. The server checks the hash with the domain controller to validate it.

Since NTLM doesn’t use tickets or encryption the way Kerberos does, it’s vulnerable to pass-the-hash, relay, and brute force attacks, making it a common target in red team operations.

Managing Objects, Applying Group Policies, and Maintaining Security in Active Directory

Once the Active Directory environment is set up with its core components like domains, OUs, and objects. The real day-to-day work for administrators begins. Managing these objects efficiently is key to keeping the network organized, secure, and compliant with organizational policies.

Administrators typically use tools like Active Directory Users and Computers (ADUC), Group Policy Management Console (GPMC), and PowerShell to manage the directory. These tools allow them to create, modify, and delete objects like users, groups, and computers, as well as assign roles or access permissions based on business needs.

To enforce consistent rules across users and devices, administrators rely heavily on Group Policy. Group Policy is a feature that allows them to push out specific settings or restrictions, such as password policies, desktop configurations, software installations, and security rules across the domain or to specific OUs. For example, you can enforce that all users in the Finance OU have a strong password policy and restricted access to certain applications.

Security is maintained through a mix of access control, group membership, and authentication protocols. Permissions are usually assigned to groups rather than individual users, which makes it easier to manage large environments. For instance, placing a user in the “HR_Managers” group might automatically give them access to HR folders and internal tools. Behind the scenes, authentication is handled by Kerberos, the default protocol used by Active Directory to verify identities securely.

Administrators also use audit policies, event logs, and Active Directory Certificate Services (AD CS) to monitor activity and secure communications within the network. Regular monitoring and periodic reviews of user permissions, group memberships, and policy compliance are essential practices for preventing insider threats or privilege misuse.

Conclusion

Active Directory remains the backbone of identity and access management for many organizations. With the right knowledge and hands-on practice, you’ll not only understand how AD supports a secure environment, you’ll also be better prepared to protect, audit, or even test it effectively.

Thanks for reading, and feel free to reach out if you have questions.