net/netns: on Windows, fall back to default interface index when unspecified address is passed to ControlC and bindToInterfaceByRoute is enabled

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

Updates #12979

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

net/netns/netns_windows.go

@@ -86,10 +86,7 @@ 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 err == nil {
 			if canV4 && (addr.Is4() || addr.Is4In6()) {
 				addrV4 := addr.Unmap()
 				ifaceIdxV4, err = getInterfaceIndex(logf, addrV4, defIfaceIdxV4)
@@ -104,6 +101,10 @@ func controlC(logf logger.Logf, network, address string, c syscall.RawConn) (err
 					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
 	}