[ipv4] Send gratuitous ARPs whenever a new IPv4 address is applied

In a busy network (such as a public cloud), IPv4 addresses may be
recycled rapidly.  When this happens, unidirectional traffic (such as
UDP syslog) will succeed, but bidirectional traffic (such as TCP
connections) may fail due to stale ARP cache entries on other nodes.
The remote ARP cache expiry timeout is likely to exceed iPXE's
connection timeout, meaning that boot attempts can fail before the
problem is automatically resolved.

Fix by sending gratuitous ARPs whenever an IPv4 address is changed, to
attempt to update stale remote ARP cache entries.  Note that this is
not a guaranteed fix, since ARP is an unreliable protocol.

We avoid sending gratuitous ARPs unconditionally, since otherwise any
unrelated settings change (e.g. "set dns 192.168.0.1") would cause
unexpected gratuitous ARPs to be sent.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ipv4.c

@@ -79,11 +79,11 @@ static struct profiler ipv4_rx_profiler __profiler = { .name = "ipv4.rx" };
  * @v address		IPv4 address
  * @v netmask		Subnet mask
  * @v gateway		Gateway address (if any)
- * @ret miniroute	Routing table entry, or NULL
+ * @ret rc		Return status code
  */
-static struct ipv4_miniroute * __malloc
-add_ipv4_miniroute ( struct net_device *netdev, struct in_addr address,
-		     struct in_addr netmask, struct in_addr gateway ) {
+static int add_ipv4_miniroute ( struct net_device *netdev,
+				struct in_addr address, struct in_addr netmask,
+				struct in_addr gateway ) {
 	struct ipv4_miniroute *miniroute;
 
 	DBGC ( netdev, "IPv4 add %s", inet_ntoa ( address ) );
@@ -96,7 +96,7 @@ add_ipv4_miniroute ( struct net_device *netdev, struct in_addr address,
 	miniroute = malloc ( sizeof ( *miniroute ) );
 	if ( ! miniroute ) {
 		DBGC ( netdev, "IPv4 could not add miniroute\n" );
-		return NULL;
+		return -ENOMEM;
 	}
 
 	/* Record routing information */
@@ -114,7 +114,7 @@ add_ipv4_miniroute ( struct net_device *netdev, struct in_addr address,
 		list_add ( &miniroute->list, &ipv4_miniroutes );
 	}
 
-	return miniroute;
+	return 0;
 }
 
 /**
@@ -812,33 +812,69 @@ const struct setting gateway_setting __setting ( SETTING_IP, gateway ) = {
 };
 
 /**
- * Create IPv4 routing table based on configured settings
+ * Send gratuitous ARP, if applicable
  *
+ * @v netdev		Network device
+ * @v address		IPv4 address
+ * @v netmask		Subnet mask
+ * @v gateway		Gateway address (if any)
  * @ret rc		Return status code
  */
-static int ipv4_create_routes ( void ) {
-	struct ipv4_miniroute *miniroute;
-	struct ipv4_miniroute *tmp;
+static int ipv4_gratuitous_arp ( struct net_device *netdev,
+				 struct in_addr address,
+				 struct in_addr netmask __unused,
+				 struct in_addr gateway __unused ) {
+	int rc;
+
+	/* Do nothing if network device already has this IPv4 address */
+	if ( ipv4_has_addr ( netdev, address ) )
+		return 0;
+
+	/* Transmit gratuitous ARP */
+	DBGC ( netdev, "IPv4 sending gratuitous ARP for %s via %s\n",
+	       inet_ntoa ( address ), netdev->name );
+	if ( ( rc = arp_tx_request ( netdev, &ipv4_protocol, &address,
+				     &address ) ) != 0 ) {
+		DBGC ( netdev, "IPv4 could not transmit gratuitous ARP: %s\n",
+		       strerror ( rc ) );
+		/* Treat failures as non-fatal */
+	}
+
+	return 0;
+}
+
+/**
+ * Process IPv4 network device settings
+ *
+ * @v apply		Application method
+ * @ret rc		Return status code
+ */
+static int ipv4_settings ( int ( * apply ) ( struct net_device *netdev,
+					     struct in_addr address,
+					     struct in_addr netmask,
+					     struct in_addr gateway ) ) {
 	struct net_device *netdev;
 	struct settings *settings;
 	struct in_addr address = { 0 };
 	struct in_addr netmask = { 0 };
 	struct in_addr gateway = { 0 };
+	int rc;
 
-	/* Delete all existing routes */
-	list_for_each_entry_safe ( miniroute, tmp, &ipv4_miniroutes, list )
-		del_ipv4_miniroute ( miniroute );
-
-	/* Create a route for each configured network device */
+	/* Process settings for each network device */
 	for_each_netdev ( netdev ) {
+
+		/* Get network device settings */
 		settings = netdev_settings ( netdev );
+
 		/* Get IPv4 address */
 		address.s_addr = 0;
 		fetch_ipv4_setting ( settings, &ip_setting, &address );
 		if ( ! address.s_addr )
 			continue;
+
 		/* Get subnet mask */
 		fetch_ipv4_setting ( settings, &netmask_setting, &netmask );
+
 		/* Calculate default netmask, if necessary */
 		if ( ! netmask.s_addr ) {
 			if ( IN_IS_CLASSA ( address.s_addr ) ) {
@@ -849,18 +885,42 @@ static int ipv4_create_routes ( void ) {
 				netmask.s_addr = INADDR_NET_CLASSC;
 			}
 		}
+
 		/* Get default gateway, if present */
 		fetch_ipv4_setting ( settings, &gateway_setting, &gateway );
-		/* Configure route */
-		miniroute = add_ipv4_miniroute ( netdev, address,
-						 netmask, gateway );
-		if ( ! miniroute )
-			return -ENOMEM;
+
+		/* Apply settings */
+		if ( ( rc = apply ( netdev, address, netmask, gateway ) ) != 0 )
+			return rc;
 	}
 
 	return 0;
 }
 
+/**
+ * Create IPv4 routing table based on configured settings
+ *
+ * @ret rc		Return status code
+ */
+static int ipv4_create_routes ( void ) {
+	struct ipv4_miniroute *miniroute;
+	struct ipv4_miniroute *tmp;
+	int rc;
+
+	/* Send gratuitous ARPs for any new IPv4 addresses */
+	ipv4_settings ( ipv4_gratuitous_arp );
+
+	/* Delete all existing routes */
+	list_for_each_entry_safe ( miniroute, tmp, &ipv4_miniroutes, list )
+		del_ipv4_miniroute ( miniroute );
+
+	/* Create a route for each configured network device */
+	if ( ( rc = ipv4_settings ( add_ipv4_miniroute ) ) != 0 )
+		return rc;
+
+	return 0;
+}
+
 /** IPv4 settings applicator */
 struct settings_applicator ipv4_settings_applicator __settings_applicator = {
 	.apply = ipv4_create_routes,