fix BACnet/IP on OS to bind broadcast to specific port (#489)

* Fixes #346 by binding the broadcast socket to the port specific broadcast address rather than INADDR_ANY or INADDR_BROADCAST. Added additional compile and environment options for those who might need them. 

* Changed the define USE_CLASSADDR to BACNET_IP_BROADCAST_USE_CLASSADDR to help manage namespaces.

* Added BACNET_IP_BROADCAST_BIND_ADDR environment variable to override the default broadcast binding address.

---------

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

src/bacnet/datalink/bip.h

@@ -122,6 +122,10 @@ extern "C" {
     BACNET_STACK_EXPORT
     int bip_get_broadcast_socket(void);
 
+    BACNET_STACK_EXPORT
+    int bip_set_broadcast_binding(
+        const char *ip4_broadcast);
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */

src/bacnet/datalink/dlenv.c

@@ -524,6 +524,7 @@ void dlenv_maintenance_timer(uint16_t elapsed_seconds)
  *   - BACNET_BDT_MASK_1 - dotted IPv4 mask of the BBMD table
  *       entry 1..128 (optional)
  *   - BACNET_IP_NAT_ADDR - dotted IPv4 address of the public facing router
+ *   - BACNET_IP_BROADCAST_BIND_ADDR - dotted IPv4 address to bind broadcasts
  * - BACDL_MSTP: (BACnet MS/TP)
  *   - BACNET_MAX_INFO_FRAMES
  *   - BACNET_MAX_MASTER
@@ -592,6 +593,10 @@ void dlenv_init(void)
             bip_set_port(0xBAC0);
         }
     }
+    pEnv = getenv("BACNET_IP_BROADCAST_BIND_ADDR");
+    if (pEnv) {
+        bip_set_broadcast_binding(pEnv);
+    }
     pEnv = getenv("BACNET_IP_NAT_ADDR");
     if (pEnv) {
         if (bip_get_addr_by_name(pEnv, &addr)) {