Error Nebula VPN L2TP

GreenBureautique

Hello,

I have a problem with my L2TP VPN on my USG FLEX 200. Previously the VPN worked well but suddenly an error message appeared on Windows 10 client workstations "the l2tp connection attempt failed because the security has encountered a "nebulous" error. Anyone have a solution?

Thanks

mMontana

After a Windows Update patch?

GreenBureautique

We tested with several computers but always the same error message.

Zyxel_Cooldia

Hi @GreenBureautique,
Could you send me your org, and site name in PM. 
We would like to check if it is device issue, or windows client issue.

GreenBureautique

Hi, OK
Thanks 

GreenBureautique

I sent you a private message. Have you been able to look at the problem ?

Zyxel_Cooldia

Hi @GreenBureautique,
Your firewall both wan1 and wan2 are behind NAT. As we test, there is no any connection log when we test L2TP dial up from Internet to FLEX500. 
Please help to check if uplink router have port mapping for IKE, ESP, and NATT.

GreenBureautique

Hi, thanks for your reply, no there is no port mapping.

Zyxel_Cooldia

Hi @GreenBureautique,
Since our gateway is behind NAT(firewall wan is private IP. ), uplink router must have port mapping for IKE, ESP, and NATT. Without port mapping, Internet hosts cannot reach our firewall interface for l2tp access.  the scenario is similar to below link.
https://community.zyxel.com/en/discussion/675/how-do-i-configure-the-zywall-for-a-l2tp-server-behind-nat