Active Directory Overview (Windows Server) for Interview Preparation

What is Active Directory ?

Active Directory (AD) is Microsoft's directory service used to Manage below Network objects and Resources

• Manage users
• Manage Computers
• Manage Groups
• Control access to resources
• apply security policies   

We can call AD as :

A centralized brain of windows network

Without AD:

• Every computer has its own users
• Passwords aren't Shared
• Policies must be configured one-by-one.

With AD:

• one login works everywhere.
• Permission are controlled centrally.
• Security is enforced automatically.

Active Directory Web Services Port number : 9389

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Why Active Directory Exists ?

Imagine a company having 1000 Employees:

Without AD

• 1000 local accounts per Server.
• Manual access control.
• No central security.
• Difficult to Manage.

With AD:

• One user account per employee
• login from any company PC
• Central password rules
• Easy onboarding or Offboarding

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Core Components of Active Directory ?

• Domain
• Domain Controller (DC)
• Objects
• Organizational units (OUs)

Domain : A domain is a logical group of users, computers and other network resources

Example: Company,local

Everything inside domain follows the same:

• Security riles
• Authentication system
• Policies
  

Domain Controller (DC) : A Domain controller is a server that stores the database, Authenticates users,

and enforces security policies.

Key Points:

• We usually have multiple DCs
• They replicate data between each other
• if one fails, other DC still work

Objects: Everything in AD is an object

• Users
• Computers
• Groups
• Printers
• Service accounts

Each object has Attributes:

• Username
• Password hash
• Email
• Group membership

Organizational Units (OUs): OUs are folders used to organize objects

OUs help to:

• Apply Group Polices
• Delegate admin control
• Keep AD Clean

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Authentication : Steps to show how login works

    Kerberos (Authentication Protocol)

    When a user logs in: 

1. User enters username/Password
2. DC verifies credentials
3. DC issues a Ticket if credentials are correct
4. User uses ticket to access resources

• Secure
• Encrypted
• No password sent repeatedly

    Alternative Option:

• NTLM (older, less secure) : Windows New Technology LAN Manager

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Group Policy (GPO) :

Group policy allows admins to:

• Enforce password rules
• Disable USB ports
• Set desktop wallpapers
• Install software automatically
• Lock down security settings and many more. 

GPOs are linked to:

• Sites
• Domains
• OUs

GPO Processing order is LSDOU

1. Local
2. Site
3. Domain 
4. OU

LSDOU. It's group policy inheritance model, where the policies are applied to Local machines, Sites, Domains and Organizational Units.

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Active Directory Structure

Forest

A forest is the top-level container.

• Contains one or more domains
• Shares a common schema
• Trusts exist automatically

 Tree

A tree is a group of domains with:

• Shared namespace
• Automatic trust

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Trusts

Trusts define how domains access each other.

Types:

• One-way
• Two-way
• Transitive
• External

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AD Replication

• DCs copy changes between each other
• Uses multi-master replication
• Based on sites and subnets

Key concepts:

• Intra-site replication (fast)
• Inter-site replication (compressed, scheduled)

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FSMO Roles (Critical AD Roles)

There are 5 FSMO roles:

Forest-wide:

1. Schema Master
2. Domain Naming Master

Domain-wide:

3. RID Master
4. PDC Emulator
5. Infrastructure Master

Why they matter:

• Certain AD tasks require these roles
• Losing them can break AD operations

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Active Directory Security

Common Attacks:

• Pass-the-Hash
• Kerberoasting
• Golden Ticket
• DCSync

Best Practices:

• Tiered admin model
• Least privilege
• Secure service accounts
• Protect Domain Admins
• Monitor logs regularly 

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AD Modern Use & Azure Integration

Today, AD often works with:

• Azure AD / Entra ID
• Hybrid identity
• SSO (Single Sign-On)
• Cloud authentication

Tools Every AD Admin Uses

• Active Directory Users and Computers (ADUC)
• Group Policy Management Console (GPMC)
• PowerShell (essential)
• Event Viewer
• ADSI Edit (danger zone)

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Common AD Mistakes

• Flat OU structure
• Too many Domain Admins
• Ignoring replication
• Weak password policies
• No documentation

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