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We are facing a problem with connecting android devices to our VPN server. iOS devices do not have any problems connecting.

Android devices trying to connect via StrongSwan official app from Google Play. And iOS from system settings. VPN server based on StrongSwan+Charon

We tried to make a dump from VPN server and found the difference between successful and unsuccessful connections

  • In success: server receives SPI key, and responds with Responder SPI key and connection successful
  • In failure: server receives SPI key, but responds with empty Responder SPI key and connection failure with "INVALID_KE_PAYLOAD" error

Screenshot of success server response (connecting from iOS device): success

Screenshot of failure server response (connecting from Android device): error

Also dump files .pcap we also have for failure and success connections, if it will help to suggest a solution. We do not know how to fix this, maybe you can help us with this problem, or say what we need to read?

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  • Easier to debug if you add output from the log instead of dump from Wireshark. Sometimes it is a certificate issue, sometimes it is an encryption algorithm issue and sometimes it is just a user authentication issue. I can come up with a working profile for Android strongswan client that works with Let's Encrypt, buy it does not work with Android native client because of an issue with Let's Encrypt certificates in general.
    – Lasse Michael Mølgaard
    Commented Jan 13, 2023 at 3:17
  • INVALID_KE_PAYLOAD is not a fatal error. It just tells the initiator to use a different key exchange method. Read/post the logs (server and client) to see what the actual error is if the connection is not established successfully.
    – ecdsa
    Commented Jan 13, 2023 at 9:33

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