I have 3 WAN connections and a remote site with a single WAN address. What I'd like to do is monitor the latency towards the remote site using a single machine (Opsview) on our LAN but through all 3 WAN links. I know I can tag the packets using DSCP but I wasnt able to determine if it's possible to route the traffic via the Fortigate unit accordingly.
You want to configure up policy based routing. In the gui this is found: Router > Static > Policy Routes. You can do this by using TOS rather than DSCP.
Incoming interface: LAN
Source Address: Opsview Machine IP
Destination Address: Remote Site WAN IP
TOS: Assigned Value
Outgoing Interface: WAN1
Gateway Address: Next-hop for that WAN.
Do the same for the other WAN links changing the appropriate values.