SE VPN server is not responding to initial isakmp packets

[renrafal]

Hi,

I'm experiencing an issue to run IPSEC L2TP from android.

Server is "listening" on UDP 500 and 4500 aftes enabling L2TP.
udp UNCONN 0 0 192.168.100.5:500 0.0.0.0:* users:(("vpnserver",pid=460,fd=62))
udp UNCONN 0 0 192.168.100.5:4500 0.0.0.0:* users:(("vpnserver",pid=460,fd=63))

In TCPDump on SE server i can see requests for isakmp
11:54:51.510621 IP 37.225.20.254.5842 > 192.168.100.5.500: isakmp: parent_sa ikev2_init
11:54:52.597739 IP 37.225.20.254.5842 > 192.168.100.5.500: isakmp: parent_sa ikev2_init
But VPN server is not responding...

When L2TP is disabled below is right reaction of operating system
11:57:20.118299 IP 37.225.20.254.9005 > 192.168.100.5.500: isakmp: parent_sa ikev2_init
11:57:20.118392 IP 192.168.100.5 > 37.225.20.254: ICMP 192.168.100.5 udp port 500 unreachable, length 556

OS - Linux SE-VPN 6.1.0-10-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.38-2 (2023-07-27) x86_64 GNU/Linux
SE - version 4.42 Build 9798

The same was on CentOS.

May i ask You for help?
How to troubleshoot?

Thanks in advance.
Rafał

[freak2out]

Same error from Android 13.
Any help?

[freak2out]

Ok, i see. L2TP is no longer available in Android and IKEv2 is not available yet in Softether. Pity.