Monday, June 29, 2020

Unifi USG Pro 4 - L2TP Client VPN Issue

Hello!

I am at the end of my rope with this one. I know I'm missing something silly. Here's what's going on.

I have a unifi USG pro 4, it has a public address (no double NAT) running a client VPN server. Whenever I try to connect from Mac OS or Windows I get the same error message when looking at swanctl --log

03[ENC] invalid ID_V1 payload length, decryption failed?

03[ENC] could not decrypt payloads

03[IKE] message parsing failed

I've verified bi-directional communication between the client and USG, and checked the shared secret on both sides to make sure they match. This problem occurs regardless of whether the built in USG radius server is being used or a windows NPS server we have configured. In fact, it fails before it even gets to user authentication. I've checked client settings and they appear to match Ubiquiti's documentation. Here is ubiquiti's documentation on the setup.

Everything I have found on that error message listed above has said it's either a shared secret mismatch or firmware/software issue. My USG was running the latest, 4.51 and I rolled it back to 4.50 for kicks, same result.

I could use some creative ideas :) Thanks in advance

C



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