From f05225da22b78c4c2980b59b17f0cb783df3f300 Mon Sep 17 00:00:00 2001
From: Bryan Boreham <bjboreham@gmail.com>
Date: Wed, 1 Jul 2015 11:14:37 +0100
Subject: [PATCH] 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              | 17 ++++++++++++++++-
 prog/netcheck/netcheck.go | 13 +++++++++----
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/net/route.go b/net/route.go
index 0d8a1bab..5673117f 100644
--- a/net/route.go
+++ b/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
+}
diff --git a/prog/netcheck/netcheck.go b/prog/netcheck/netcheck.go
index 383f5d9b..85f6003b 100644
--- a/prog/netcheck/netcheck.go
+++ b/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)