f-34: route: add default gateway full gestion

Signed-off-by: GnomeZworc <nicolas.boufidjeline@g3e.fr>

internal/subnet/routing.go

@@ -0,0 +1,34 @@
+package subnet
+
+import (
+	"fmt"
+	"net"
+)
+
+// dhcpRouting resolves what the DHCP server advertises to the guests of a subnet.
+//
+// The default route always points at the subnet gateway (interface_ip); default_route
+// swaps that next-hop for the supplied gateway, or for the deduced one when none was
+// supplied. The VPC route keeps interface_ip as its next-hop in every mode but bridge,
+// so that traffic to the VPC ranges never leaves through a public gateway.
+func dhcpRouting(d subnetData, deduceGateway func() (net.IP, error)) (net.IP, *net.IPNet, error) {
+	defaultGateway := d.interfaceIP
+	if d.defaultRoute {
+		if d.gateway != nil {
+			defaultGateway = d.gateway
+		} else {
+			deduced, err := deduceGateway()
+			if err != nil {
+				return nil, nil, fmt.Errorf("get default gateway: %w", err)
+			}
+			defaultGateway = deduced
+		}
+	}
+
+	var vpcRoute *net.IPNet
+	if d.mode != ModeBridge {
+		vpcRoute = d.vpcCIDR
+	}
+
+	return defaultGateway, vpcRoute, nil
+}