Active Directory Domain Services

This skill covers Active Directory Domain Services (AD DS) across all supported Windows Server versions (2016 through 2025). It has deep knowledge of:

• AD DS architecture (NTDS.dit, replication, sites and subnets, partitions)
• FSMO roles and their operational impact
• Multi-master replication topology and troubleshooting
• Group Policy processing, inheritance, and troubleshooting
• Kerberos and NTLM authentication
• Trust types and cross-forest authentication
• AD hardening and tiered administration model
• LAPS, gMSA, Protected Users, Authentication Policies/Silos
• AD recycle bin, fine-grained password policies, managed service accounts

This skill's coverage spans AD DS holistically. When a question is version-specific, read the matching file in references/versions/. When the version is unknown, apply general guidance and note where behavior differs across versions.

How to Approach Tasks

When you receive a request:

1. Classify the request type:

   • Troubleshooting -- Load references/diagnostics.md for repadmin, dcdiag, event ID reference
   • Architecture -- Load references/architecture.md for AD DS internals
   • Hardening / Security -- Load references/best-practices.md for tiered admin, GPO hardening
   • Administration -- Apply AD DS expertise directly
   • Migration / Upgrade -- Load the relevant references/versions/<v>.md files for source and target versions

2. Identify version -- Determine the forest/domain functional level and Windows Server version. If unclear, ask. Functional level determines available features.

3. Load context -- Read the relevant reference file for deep knowledge.

4. Analyze -- Apply AD DS-specific reasoning, not generic directory advice.

5. Recommend -- Provide actionable, specific guidance with PowerShell examples where applicable.

6. Verify -- Suggest validation steps (dcdiag, repadmin, event log checks).

Core Expertise

AD DS Architecture

AD DS is a multi-master replicated directory built on the Extensible Storage Engine (ESE). The database file is NTDS.dit, stored by default at C:\Windows\NTDS\.

Directory partitions:

Partition	Replication Scope	Contents
Schema	Forest-wide	Object class and attribute definitions
Configuration	Forest-wide	Sites, subnets, services, replication topology
Domain	Domain-wide	Users, groups, computers, OUs, GPOs
Application	Configurable	DNS zones (ForestDnsZones, DomainDnsZones), custom

Object model: Every AD object has a globally unique objectGUID, a security-aware objectSid (for security principals), and a distinguishedName (DN) reflecting its position in the hierarchy.

FSMO Roles

Five Flexible Single Master Operations roles that break the multi-master model for specific operations:

Role	Scope	Hosted On	Purpose	Impact if Unavailable
Schema Master	Forest	One DC	Schema modifications	Cannot extend schema (rare operation)
Domain Naming Master	Forest	One DC	Add/remove domains	Cannot add/remove domains (rare)
PDC Emulator	Domain	One DC per domain	Password changes, time sync, GPO editor, account lockout	Authentication failures, time drift, GPO edit issues
RID Master	Domain	One DC per domain	Allocates RID pools for SID creation	Cannot create new objects when RID pool exhausted
Infrastructure Master	Domain	One DC per domain	Cross-domain reference updates	Stale group membership display (multi-domain only)

Operational guidance:

• PDC Emulator is the most operationally critical role -- place it on your strongest DC
• Schema Master and Domain Naming Master can co-exist on the same DC
• Infrastructure Master should NOT be on a Global Catalog server (unless all DCs are GCs)
• Use netdom query fsmo or Get-ADForest/Get-ADDomain to identify role holders
• Seize roles only when the original holder is permanently offline

Replication

AD DS uses multi-master replication with a pull model. The Knowledge Consistency Checker (KCC) automatically generates a replication topology.

Intra-site replication:

• Change notification based (within 15 seconds of change)
• Compressed only if >50KB
• Uses RPC over IP

Inter-site replication:

• Schedule-based (default: every 180 minutes)
• Always compressed
• Can use RPC over IP or SMTP (schema/configuration only)
• Site links define cost, replication interval, and schedule

Replication metadata:

• Each attribute has a version number (USN), originating DC, and timestamp
• Conflict resolution: highest version wins; if tied, last writer wins (by timestamp)
• Lingering objects: objects deleted on one DC but not replicated before tombstone lifetime expires

Group Policy

Group Policy Objects (GPOs) apply configuration to users and computers:

Processing order (LSDOU):

1. Local -- Local Group Policy on the machine
2. Site -- GPOs linked to AD site
3. Domain -- GPOs linked to domain
4. OU -- GPOs linked to OUs (parent before child)

Last applied wins (closest OU overrides domain GPO for conflicting settings).

Modifiers:

• Enforced (formerly "No Override") -- Prevents child OUs from overriding
• Block Inheritance -- OU blocks all GPOs from above (except Enforced)
• Security filtering -- GPO applies only to specified users/groups/computers (default: Authenticated Users)
• WMI filtering -- GPO applies only if WMI query returns true

Group Policy troubleshooting:

# Generate RSoP report
gpresult /h C:\temp\gpresult.html /scope:computer
gpresult /h C:\temp\gpresult_user.html /scope:user

# Force Group Policy refresh
gpupdate /force

# Check GPO replication (SYSVOL vs AD)
Get-GPO -All | ForEach-Object {
    $gpo = $_
    $adVersion = $gpo.User.DSVersion, $gpo.Computer.DSVersion
    # Compare with SYSVOL version in gpt.ini
}

Kerberos Authentication in AD

AD DS is a Kerberos Key Distribution Center (KDC). Every DC runs the KDC service.

Default ticket lifetimes (configurable via GPO):

• TGT: 10 hours
• TGT renewal: 7 days
• Service ticket: 10 hours
• Clock skew tolerance: 5 minutes

Service Principal Names (SPNs):

# List SPNs for an account
setspn -L serviceaccount

# Find duplicate SPNs (causes Kerberos failures)
setspn -X

# Register SPN
setspn -S HTTP/webapp.example.com serviceaccount

Common Kerberos issues:

• Duplicate SPNs: KRB_AP_ERR_MODIFIED -- use setspn -X to find duplicates
• Clock skew: KRB_AP_ERR_SKEW -- sync time via NTP hierarchy (PDC Emulator is authoritative)
• Missing SPN: Falls back to NTLM (investigate with network trace)
• Delegation misconfigured: Double-hop authentication failures

Trust Types

Trust Type	Direction	Transitivity	Use Case
Parent-Child	Two-way	Transitive	Automatic between parent/child domains
Tree-Root	Two-way	Transitive	Automatic between tree roots in forest
Shortcut	One-way or Two-way	Transitive	Optimize authentication path between distant domains
External	One-way or Two-way	Non-transitive	Trust to a specific domain in another forest (NT4 compatible)
Forest	One-way or Two-way	Transitive	Trust between forest root domains
Realm	One-way or Two-way	Non-transitive or Transitive	Trust to non-Windows Kerberos realm

SID filtering: Enabled by default on forest trusts. Prevents SID history abuse across trust boundaries. Disable only with extreme caution.

AD Hardening Essentials

Tiered Administration Model:

Tier	Scope	Examples	Rule
Tier 0	Identity infrastructure	Domain controllers, AD DS, AD CS, Entra Connect	Tier 0 admins NEVER sign into Tier 1 or Tier 2 systems
Tier 1	Servers and applications	Member servers, SQL, Exchange, SCCM	Tier 1 admins NEVER sign into Tier 2 systems
Tier 2	Workstations and devices	User workstations, laptops, printers	Standard user and helpdesk tier

Key hardening controls:

• Protected Users group -- Prevents NTLM authentication, Kerberos delegation, and DES/RC4 encryption for members. Add all privileged accounts.
• Authentication Policies and Silos -- Restrict where privileged accounts can authenticate (2012 R2+). Enforce Tier 0 accounts can only authenticate to Tier 0 devices.
• LAPS (Local Administrator Password Solution) -- Randomizes and rotates local admin passwords. Windows LAPS (built into Windows 11/Server 2019+) replaces legacy Microsoft LAPS.
• Credential Guard -- Virtualizes LSA process to prevent credential theft (Mimikatz, Pass-the-Hash). Requires Secure Boot, UEFI.
• Privileged Access Workstations (PAWs) -- Dedicated admin workstations for Tier 0 administration. No internet, no email, no general-purpose use.
• gMSA (Group Managed Service Accounts) -- Automatic password rotation for service accounts. 240-character random password, rotated every 30 days.
• Fine-Grained Password Policies (FGPPs) -- Different password policies for different groups (e.g., stricter for admins). Requires 2008+ domain functional level.
• AdminSDHolder -- Protects privileged group membership. Runs every 60 minutes to enforce ACL consistency on protected objects.

Common Pitfalls

1. Domain Admins for everything -- Domain Admin is massively over-privileged. Delegate specific permissions to OUs instead.
2. Single site for distributed network -- Without proper sites/subnets, clients authenticate to random DCs across WAN links.
3. Ignoring tombstone lifetime -- Default is 180 days. DCs offline longer than tombstone lifetime reintroduce deleted objects as lingering objects.
4. SYSVOL replication issues -- DFSR replaced FRS in 2008+. FRS-to-DFSR migration is required before raising functional levels.
5. Not monitoring AdminSDHolder changes -- Attackers modify AdminSDHolder to persist admin access. Monitor Event ID 4780.
6. Unconstrained delegation -- Allows a service to impersonate any user to any service. Extremely dangerous. Audit with: Get-ADComputer -Filter {TrustedForDelegation -eq $true}.

Version-Specific Guidance

Version	Reference	What's version-specific
Windows Server 2016	references/versions/2016.md	Functional level 2016, Privileged Access Management, MIM integration, temporal group memberships
Windows Server 2019	references/versions/2019.md	Same functional level as 2016; hybrid identity improvements, security defaults, Windows Admin Center
Windows Server 2022	references/versions/2022.md	TLS 1.3 for LDAPS, Kerberos AES-256 improvements, security baseline updates
Windows Server 2025	references/versions/2025.md	Functional level 10 (first new FL since 2016), 32K database pages, NTLM deprecation, Kerberos with certificate trust

Reference Files

Load these when you need deep knowledge for a specific area:

• references/architecture.md -- AD DS internals: NTDS.dit, ESE database, replication protocols, partitions, FSMO mechanics, sites and subnets, Global Catalog, schema, trust authentication flow. Read for "how does X work" questions.
• references/diagnostics.md -- Troubleshooting playbooks: repadmin commands, dcdiag tests, critical event IDs, replication failure resolution, DNS issues, authentication failures, GPO troubleshooting. Read when diagnosing issues.
• references/best-practices.md -- Hardening and operational guidance: tiered administration, GPO security baselines, LAPS deployment, PAW architecture, monitoring and alerting, backup/recovery, DC placement. Read for design and operations questions.

Diagnostic Scripts

Ready-made RSAT ActiveDirectory audits (read-only) in scripts/. Defensive review of on-prem tier-0 exposure.

• scripts/01-privileged-group-audit.ps1 -- Recursive membership of Domain/Enterprise/Schema Admins and operators
• scripts/02-kerberoast-exposure.ps1 -- Kerberoastable SPN accounts and AS-REP-roastable accounts, privilege-weighted