USG FLEX H Series - NAT Traversal Support for IPSec Remote Access VPN

Zyxel_Claudia

With the latest uOS firmware update, Zyxel's H Series firewalls now support NAT Traversal (NAT-T) for IPSec Remote Access VPNs. This feature is essential for devices deployed behind a NAT or firewall, allowing them to maintain a stable VPN connection when the device itself is assigned a private IP.

Why NAT Traversal is Important for IPSec VPNs

Previously, remote access VPNs using IPSec on Zyxel devices did not support NAT Traversal. This limitation meant that if a firewall was behind a NAT router or an ISP-managed gateway, it would only receive a private IP. In such cases, remote users would struggle to establish a successful VPN connection, as the VPN server address would appear as a private IP in the downloaded VPN configuration, causing connection failures.

How to Enable NAT Traversal for IPSec Remote Access VPN

1. Verify Network Setup:
   • Ensure your firewall is behind a NAT device or ISP gateway that assigns a private IP.
   • Obtain a public IP address or set up a Dynamic DNS (DDNS) service.
2. Configure NAT Traversal:
   • Log in to the uOS web interface of your firewall.
   • Navigate to the VPN > IPSec VPN > Remote Access VPN settings.
   • Enter the public IP address in the NAT Traversal field.
   • If using DDNS, you can enter the domain name in this field to dynamically resolve the VPN server address.
3. Download VPN Configuration:
   • After configuring NAT Traversal, download the VPN configuration file for remote users.
   • Open the configuration file and verify that the Server Address shows the public IP or DDNS, ensuring remote clients will be able to connect correctly.

The new NAT Traversal support in uOS firmware offers a straightforward solution for establishing reliable IPSec VPN connections, especially in NAT-constrained environments. Allows remote users to connect seamlessly from behind NAT networks or when the VPN server is behind a NAT router. This enhancement not only simplifies remote connectivity but also opens up new deployment scenarios for secure access across diverse network setups.