Resolving 'Invalid DN Syntax' Error in Active Directory Authentication for VPN Users

Zyxel_Kevin

Question:

How to resolve the "Invalid DN Syntax" error when configuring Active Directory authentication for VPN users on a Zyxel Firewall?

Answer:

The "Invalid DN Syntax" error typically occurs during Active Directory (AD) connection testing when the Bind DN setting does not correctly specify the location of the VPN user within the AD structure.

• Root Cause: The default Active Directory configuration on Zyxel firewalls often assumes that the administrative user (Bind DN) used for authentication queries is located directly under the cn=users container (e.g., CN=VPNADMIN,CN=Users,DC=yourdomain,DC=local). However, if your VPN user (e.g., VPNADMIN) is located in a different Organizational Unit (OU) or path within your Active Directory, the firewall's default query will fail, resulting in an "Invalid DN Syntax" error.
• Resolution:
  • Identify the correct Distinguished Name (DN) for your VPN user:
  • Open the Advance settings, Find the "Bind DN Base"
  • This is the full path to your VPN user in Active Directory. For example, if your user is VPNADMIN within the TEST OU under zyxel.local domain, the Bind DN Base would be CN=VPNADMIN,OU=TEST,DC=zyxel,DC=local.