Hybrid Identity Required Ports and Protocols

Summary

When you are planning to implementing a hybrid identity solution, remember that this following ports and protocols are required.
In this post, I will cover the prerequisite of the Microsoft Entra Connect(Formerly known as Azure ad connect) and On-premises AD.

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1. Overview

This is the overal design but note that 'On-premises Active Directory and Azure AD Connect Server'

2. Microsoft Entra Connect and On-premises AD

This section describes the ports and protocols that are required for communication between the Microsoft Entra Connect server and on-premises AD.

3. If you don't allow those ports and protocols..

You will encounter following issues.
Example:

389 Ports error.

Test with this powershell on the Microsoft Entra Connect server

Test-Netconnection 'Forest name' -Port 389

If the AD has not opened port 53, you need to use IP address.

Test-Netconnection 'IP address' -Port 389

Reference

https://learn.microsoft.com/en-us/entra/identity/hybrid/connect/reference-connect-ports