Issue
If you configure the Firebox to use a Domain Name for TCP or Ping Link Monitor in a multi-WAN configuration, the Firebox reports the interface as failed if the Firebox only uses a DNS server on an internal network to resolve the DNS name.
Workaround/Solution
This issue does not occur if you configure the Firebox to use a DNS server that the Firebox connects to over an external network interface.You can still configure the Firebox interface DHCP or Mobile VPN to use an internal DNS server for clients.