Implement basic route get functionality

This provides the basic functionality of the iproute2 `route get`
command. It allows you to query netlink for the route to a destination
IP.Net

While implementing I noticed that the `s.Receive()` function was hanging
after the netlink response was recieved, so I added an additional clause
to break from the loop and return the decoded message if the NLM_F_MULTI
flag is not set.

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>

nl/nl_linux.go

@@ -248,6 +248,9 @@ done:
 				continue
 			}
 			res = append(res, m.Data)
+			if m.Header.Flags&syscall.NLM_F_MULTI == 0 {
+				break done
+			}
 		}
 	}
 	return res, nil

route_linux.go

@@ -164,3 +164,62 @@ func RouteList(link Link, family int) ([]Route, error) {
 
 	return res, nil
 }
+
+// RouteGet gets a route to a specific destination from the host system.
+// Equivalent to: 'ip route get'.
+func RouteGet(destination net.IP) ([]Route, error) {
+	req := nl.NewNetlinkRequest(syscall.RTM_GETROUTE, syscall.NLM_F_REQUEST)
+	family := nl.GetIPFamily(destination)
+	var destinationData []byte
+	var bitlen uint8
+	if family == FAMILY_V4 {
+		destinationData = destination.To4()
+		bitlen = 32
+	} else {
+		destinationData = destination.To16()
+		bitlen = 128
+	}
+	msg := &nl.RtMsg{}
+	msg.Family = uint8(family)
+	msg.Dst_len = bitlen
+	req.AddData(msg)
+
+	rtaDst := nl.NewRtAttr(syscall.RTA_DST, destinationData)
+	req.AddData(rtaDst)
+
+	msgs, err := req.Execute(syscall.NETLINK_ROUTE, syscall.RTM_NEWROUTE)
+	if err != nil {
+		return nil, err
+	}
+
+	native := nl.NativeEndian()
+	res := make([]Route, 0)
+	for _, m := range msgs {
+		msg := nl.DeserializeRtMsg(m)
+		attrs, err := nl.ParseRouteAttr(m[msg.Len():])
+		if err != nil {
+			return nil, err
+		}
+
+		route := Route{}
+		for _, attr := range attrs {
+			switch attr.Attr.Type {
+			case syscall.RTA_GATEWAY:
+				route.Gw = net.IP(attr.Value)
+			case syscall.RTA_PREFSRC:
+				route.Src = net.IP(attr.Value)
+			case syscall.RTA_DST:
+				route.Dst = &net.IPNet{
+					IP:   attr.Value,
+					Mask: net.CIDRMask(int(msg.Dst_len), 8*len(attr.Value)),
+				}
+			case syscall.RTA_OIF:
+				routeIndex := int(native.Uint32(attr.Value[0:4]))
+				route.LinkIndex = routeIndex
+			}
+		}
+		res = append(res, route)
+	}
+	return res, nil
+
+}

route_test.go

@@ -34,7 +34,17 @@ func TestRouteAddDel(t *testing.T) {
 		t.Fatal(err)
 	}
 	if len(routes) != 1 {
-		t.Fatal("Link not removed properly")
+		t.Fatal("Link not added properly")
+	}
+
+	dstIP := net.ParseIP("192.168.0.42")
+	routeToDstIP, err := RouteGet(dstIP)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(routeToDstIP) == 0 {
+		t.Fatal("Default route not present")
 	}
 
 	err = RouteDel(&route)