I have 2 Internet connections each with a router. The routers are both connected as WAN connections to the Fortigate 90D. The connections are from different providers.
I have configured a "Weighted Round Robin" WAN Link Load Balancing Interface and created Policies using this interface.
I use a Ping "Probe Type" for the "Health Check" for each WAN interface.
I specify Primary and Secondary DNS addresses under System -> Network -> DNS
This configuration works "most of the time" but some devices complain of a DNS error and after some hours (8?) many devices on the internal network start to report Internet connection issues. Some devices show this behavior more quickly.
Dropping either WAN link immediately resolves the problem.
A DHCP server is configured on the Internal Interface and the "DNS Server" option is set to "Same as System DNS"
Firmware is: v5.2.9 build 736
My question is: How should DNS be configured for this set-up if the above is not correct. If the above is correct where else should I look to resolve the problem?
Note: Looking at System -> Fortiview -> “All Sessions” shows a good mix of both WAN links being used.