dhcp: Add priority option to dhcp.

Currently, we can not set the metric of routes in dhcp.
It's ok if there is only a network interface.

But if there are multiple network interfaces, and both have a default route,
We need to set the metric of the route to make the traffic
go through the correct network interface.

For host-local and static, we can set the metric with the route.priority option.
But there is no such option for dhcp.

pkg/ipam/ipam_linux.go

@@ -117,6 +117,7 @@ func ConfigureIface(ifName string, res *current.Result) error {
 			Dst:       &r.Dst,
 			LinkIndex: link.Attrs().Index,
 			Gw:        gw,
+			Priority:  r.Priority,
 		}
 
 		if r.Table != nil {

plugins/ipam/dhcp/daemon.go

@@ -109,6 +109,11 @@ func (d *DHCP) Allocate(args *skel.CmdArgs, result *current.Result) error {
 		Gateway: l.Gateway(),
 	}}
 	result.Routes = l.Routes()
+	if conf.IPAM.Priority != 0 {
+		for _, r := range result.Routes {
+			r.Priority = conf.IPAM.Priority
+		}
+	}
 
 	return nil
 }

plugins/ipam/dhcp/main.go

@@ -51,6 +51,8 @@ type IPAMConfig struct {
 	// To override default requesting fields, set `skipDefault` to `false`.
 	// If an field is not optional, but the server failed to provide it, error will be raised.
 	RequestOptions []RequestOption `json:"request"`
+	// The metric of routes
+	Priority int `json:"priority,omitempty"`
 }
 
 // DHCPOption represents a DHCP option. It can be a number, or a string defined in manual dhcp-options(5).