Check specific addresses differently to networks

For a network, as used by IPAM, we care if *any* address in the range
could possibly conflict with an existing address routed by the host.
When given a single address, we only care if *that specific address*
conflicts with an existing route.

net/route.go

@@ -16,7 +16,7 @@ func CheckNetworkFree(subnet *net.IPNet) error {
 	}
 	for _, route := range routes {
 		if route.Dst != nil && overlaps(route.Dst, subnet) {
-			return fmt.Errorf("network %s would overlap with route %s", subnet, route.Dst)
+			return fmt.Errorf("Network %s would overlap with existing route %s", subnet, route.Dst)
 		}
 	}
 	return nil
@@ -26,3 +26,18 @@ func CheckNetworkFree(subnet *net.IPNet) error {
 func overlaps(n1, n2 *net.IPNet) bool {
 	return n1.Contains(n2.IP) || n2.Contains(n1.IP)
 }
+
+// For a specific address, we only care if it is actually *inside* an
+// existing route, because weave-local traffic never hits IP routing.
+func CheckAddressOverlap(addr net.IP) error {
+	routes, err := netlink.RouteList(nil, netlink.FAMILY_V4)
+	if err != nil {
+		return err
+	}
+	for _, route := range routes {
+		if route.Dst != nil && route.Dst.Contains(addr) {
+			return fmt.Errorf("Address %s would overlap with existing route %s", addr, route.Dst)
+		}
+	}
+	return nil
+}

prog/netcheck/netcheck.go

@@ -1,4 +1,4 @@
-/* netcheck: check whether a given network overlaps with any existing routes */
+/* netcheck: check whether a given network or address overlaps with any existing routes */
 package main
 
 import (
@@ -19,12 +19,17 @@ func main() {
 		os.Exit(0)
 	}
 
-	ipRangeStr := os.Args[1]
-	_, ipnet, err := net.ParseCIDR(ipRangeStr)
+	cidrStr := os.Args[1]
+	addr, ipnet, err := net.ParseCIDR(cidrStr)
 	if err != nil {
 		fatal(err)
 	}
-	if err := weavenet.CheckNetworkFree(ipnet); err != nil {
+	if ipnet.IP.Equal(addr) {
+		err = weavenet.CheckNetworkFree(ipnet)
+	} else {
+		err = weavenet.CheckAddressOverlap(addr)
+	}
+	if err != nil {
 		fatal(err)
 	}
 	os.Exit(0)