Can't use VPN on Android Device

Question

I have a Samsung Galaxy S7 running Android 6.0.1 and a Sophos UTM device with firmware version 9.404-5 (holding off on updates because I was bit by the MTU "feature"). I have almost literally tried every combination of IPsec, L2TP, and PPTP configuration that exists on the phone and firewall and cannot get any of them to work. PPTP was my last resort due to what I've heard about it and I can't even get that to connect. For troubleshooting I turned on debugging and disabled every form of protection on the UTM and started allowing all traffic in through the firewall from my phone's IP and out from internal network to my phone -- which was not on wifi. 
 I used to have the Galaxy S6 and was able to use IPsec without issue but the S7's VPN settings are far more limited and I can't figure out a combination of settings that will work. 
 The phone's VPN settings allow for: 
 
 IPSec IKEv2 RSA 
 IPSec IKEv2 PSK 
 IPSec Xauth RSA 
 IPSec Xauth PSK 
 IPSec Hybrid RSA 
 L2TP/IPSec RSA 
 L2TP/IPSec PSK 
 
 The only configurable options on the phone are server ip, user cert, CA cert, server cert, PFS (yes/no), PSK, IPSec identifier, L2TP Secret, username, and password. 
 Unlike the S6 I cannot change the IKE encryption algorithm, IKE authentication algorithm, IKE SA lifetime, etc. 
 
 Any suggestions?

HolgerLehn · Answer

Hi, 
 if you are using a policy with SHA2, this could be a truncating problem. 
 There are two different ways the SHA2 hashes are truncated for L2TP / IPSec RemoteAccess VPN by various manufacturers. One way is the official RFC defined way to handle SHA2 and the other one is the GOOGLE way. In the GOOGLE way a truncation after 96 bits is happening. 
 If you have connections problems with a mobile device, Check the knowledge base article https://community.sophos.com/kb/en-us/125796 to get further information. Other customers have been able to solve their problem by adapting the policy ("SHA2 256" or "SHA 256 96bit" or by using the command line option. 
 We are working on a solution to support both ways at the same. 
 Greetings 
 Holger