I'm running Forefront 2010 SP2 as our VPN gateway, and whilst it's mostly working fine I do have one problem - we have a contractor who occassionally needs to VPN in with his own, non-domain joined PC, but cannot do so - it returns error 787 (The L2TP connection attempt failed because there is no valid machine certificate on your computer for security authentication).
I've looked for an answer as to how to allow/block non-domain PCs from connecting to Forefront but have drawn a blank. Does anyone have any idea how I might allow this (and even better, restrict the non-domain PC to only coming from a specific IP address)?
Many thanks in advance for any advice.