Document use of Landrush behind firewall or VPN

Even with Landrush there might be issues within VPN's or behind firewalls. All DNS queries of guest will be directed to Landrush. Landrush itself passes queries which it does not know how to answer on to a (configurable) upstream sever. This is per default Google's 8.8.8.8. If the VPN/firewall blocks this upstream DNS server, the guest pretty much looses access to the internet.

Tasks:

• How to troubleshoot Landrush (logs, error msg, etc.)
• How to configurer upstream DNS server