net/netns: on Windows, fall back to default interface index when unsp…

…ecified address is passed to ControlC and bindToInterfaceByRoute is enabled

We were returning an error instead of binding to the default interface.

Updates tailscale#12979

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

net/netns/netns_windows.go

@@ -86,23 +86,24 @@ func controlC(logf logger.Logf, network, address string, c syscall.RawConn) (err
 	var ifaceIdxV4, ifaceIdxV6 uint32
 	if useRoute := bindToInterfaceByRoute.Load() || bindToInterfaceByRouteEnv(); useRoute {
 		addr, err := parseAddress(address)
-		if err != nil {
-			return fmt.Errorf("parseAddress: %w", err)
-		}
-
-		if canV4 && (addr.Is4() || addr.Is4In6()) {
-			addrV4 := addr.Unmap()
-			ifaceIdxV4, err = getInterfaceIndex(logf, addrV4, defIfaceIdxV4)
-			if err != nil {
-				return fmt.Errorf("getInterfaceIndex(%v): %w", addrV4, err)
+		if err == nil {
+			if canV4 && (addr.Is4() || addr.Is4In6()) {
+				addrV4 := addr.Unmap()
+				ifaceIdxV4, err = getInterfaceIndex(logf, addrV4, defIfaceIdxV4)
+				if err != nil {
+					return fmt.Errorf("getInterfaceIndex(%v): %w", addrV4, err)
+				}
 			}
-		}
 
-		if canV6 && addr.Is6() {
-			ifaceIdxV6, err = getInterfaceIndex(logf, addr, defIfaceIdxV6)
-			if err != nil {
-				return fmt.Errorf("getInterfaceIndex(%v): %w", addr, err)
+			if canV6 && addr.Is6() {
+				ifaceIdxV6, err = getInterfaceIndex(logf, addr, defIfaceIdxV6)
+				if err != nil {
+					return fmt.Errorf("getInterfaceIndex(%v): %w", addr, err)
+				}
 			}
+		} else {
+			logf("[unexpected] netns: error parsing address %q: %v", address, err)
+			ifaceIdxV4, ifaceIdxV6 = defIfaceIdxV4, defIfaceIdxV6
 		}
 	} else {
 		ifaceIdxV4, ifaceIdxV6 = defIfaceIdxV4, defIfaceIdxV6