How to resolve L2TP VPN connection issues on Nebula firewall?

Zyxel_Emily

Question:
Why can't my iPhone connect to the L2TP VPN server of Nebula firewall? What settings should I check on Nebula?

Answer:
If your iPhone shows the error "The L2TP VPN server is not responding" follow these steps to troubleshoot:

1. Verify if the WAN IP address assigned to your Nebula firewall is private or public:
If the WAN IP is private, ensure NAT Traversal is configured as "Auto".

2. Check if the WAN IP address is a private IP; this means your firewall is behind another router. Make sure the necessary ports (500 and 4500) are opened on that router.

3. Correct the VPN subnet configuration. Make sure VPN subnet does not conflict with the private IP address of WAN interface. Besides, do not configure /32 as subnet mask.
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