L2TP Client: Windows

Windows 10

Windows 10 seems very keen on using IPsec with L2TP. To turn this off, in an Administrator Command Prompt do:

SC STOP RASMAN    ; you may have to wait a bit for this to stop
SC QUERY RASMAN   ; ensure it is STOPPED
REG ADD HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Rasman\Parameters /v ProhibitIpSec /t REG_DWORD /d 1 /f
SC START RASMAN

Also, once the connection has been created, ensure CHAP authentication is enabled:

1. Open Network and Sharing Centre
2. Click Change adapter settings
3. Find the L2TP connection, it will have whatever name you gave it
4. Right click the connection and choose Properties
5. Go to the Security Tab
6. Find the 'Authentication' section
7. Select the 'Allow these protocols' radio button
8. Ensure 'Challenge Handshake Authentication Protocol (CHAP)' is checked.

Windows 7

This probably applies to Windows 8 too.

Connecting with Windows 7 is almost the same as the steps for Windows 10 above, except that the default connection settings don't work. You have to edit the connection properties and on the Security tab change:

1. 'Type of VPN:' to 'Layer 2 Tunneling Protocol with IPsec (L2TP/IPSec)' otherwise it only tries PPTP
2. 'Data encryption:' to 'Optional encryption (connect even if no encryption)'

Watch out if you are using IPv6. It seems that Win7 negotiates a non-routable IPv6 address with the LNS. You have to discard this address and manually configure one of your routed IPv6 addresses. ipconfig /release6 is your friend here.