diff --git a/common.go b/common.go
index 2a9a6ea..e9b03ed 100644
--- a/common.go
+++ b/common.go
@@ -57,8 +57,8 @@ func (r routeSlice) Swap(i, j int) {
 }
 
 type router struct {
-	ifaces []net.Interface
-	addrs  []ipAddrs
+	ifaces map[int]net.Interface
+	addrs  map[int]ipAddrs
 	v4, v6 routeSlice
 }
 
@@ -98,9 +98,12 @@ func (r *router) RouteWithSrc(input net.HardwareAddr, src, dst net.IP) (iface *n
 	}
 
 	// Interfaces are 1-indexed, but we store them in a 0-indexed array.
-	ifaceIndex--
+	correspondingIface, ok := r.ifaces[ifaceIndex]
+	if !ok {
+		err = errors.New("Route refereced unknown interface")
+	}
+	iface = &correspondingIface
 
-	iface = &r.ifaces[ifaceIndex]
 	if preferredSrc == nil {
 		switch {
 		case dst.To4() != nil:
diff --git a/netroute_bsd.go b/netroute_bsd.go
index a6bc6cc..76ce8db 100644
--- a/netroute_bsd.go
+++ b/netroute_bsd.go
@@ -53,6 +53,8 @@ const (
 
 func New() (routing.Router, error) {
 	rtr := &router{}
+	rtr.ifaces = make(map[int]net.Interface)
+	rtr.addrs = make(map[int]ipAddrs)
 	tab, err := route.FetchRIB(syscall.AF_UNSPEC, route.RIBTypeRoute, 0)
 	if err != nil {
 		return nil, err
@@ -125,7 +127,7 @@ func New() (routing.Router, error) {
 		return nil, err
 	}
 	for _, iface := range ifaces {
-		rtr.ifaces = append(rtr.ifaces, iface)
+		rtr.ifaces[iface.Index] = iface
 		var addrs ipAddrs
 		ifaceAddrs, err := iface.Addrs()
 		if err != nil {
@@ -145,7 +147,7 @@ func New() (routing.Router, error) {
 				}
 			}
 		}
-		rtr.addrs = append(rtr.addrs, addrs)
+		rtr.addrs[iface.Index] = addrs
 	}
 	return rtr, nil
 }
diff --git a/netroute_linux.go b/netroute_linux.go
index fe769c5..203f285 100644
--- a/netroute_linux.go
+++ b/netroute_linux.go
@@ -22,6 +22,8 @@ import (
 
 func New() (routing.Router, error) {
 	rtr := &router{}
+	rtr.ifaces = make(map[int]net.Interface)
+	rtr.addrs = make(map[int]ipAddrs)
 	tab, err := syscall.NetlinkRIB(syscall.RTM_GETROUTE, syscall.AF_UNSPEC)
 	if err != nil {
 		return nil, err
@@ -83,7 +85,7 @@ loop:
 		return nil, err
 	}
 	for _, iface := range ifaces {
-		rtr.ifaces = append(rtr.ifaces, iface)
+		rtr.ifaces[iface.Index] = iface
 		var addrs ipAddrs
 		ifaceAddrs, err := iface.Addrs()
 		if err != nil {
@@ -103,7 +105,7 @@ loop:
 				}
 			}
 		}
-		rtr.addrs = append(rtr.addrs, addrs)
+		rtr.addrs[iface.Index] = addrs
 	}
 	return rtr, nil
 }