diff --git a/alfred.h b/alfred.h
index f26c80c..621805d 100644
--- a/alfred.h
+++ b/alfred.h
@@ -55,6 +55,11 @@ struct dataset {
 	uint8_t local_data;
 };
 
+struct changed_data_type {
+	uint8_t data_type;
+	struct list_head list;
+};
+
 struct transaction_packet {
 	struct alfred_push_data_v0 *push;
 	struct list_head list;
@@ -118,6 +123,10 @@ struct globals {
 	int unix_sock;
 	const char *unix_path;
 
+	const char *update_command;
+	struct list_head changed_data_types;
+	uint16_t changed_data_type_count; /* maximum is 256 */
+
 	struct timespec if_check;
 
 	struct hashtable_t *data_hash;
@@ -134,6 +143,8 @@ extern const struct in6_addr in6addr_localmcast;
 /* server.c */
 int alfred_server(struct globals *globals);
 int set_best_server(struct globals *globals);
+void changed_data_type(struct globals *globals, uint8_t arg);
+
 /* client.c */
 int alfred_client_request_data(struct globals *globals);
 int alfred_client_set_data(struct globals *globals);
diff --git a/main.c b/main.c
index 02553c0..db1f8b0 100644
--- a/main.c
+++ b/main.c
@@ -61,6 +61,7 @@ static void alfred_usage(void)
 	printf("\n");
 	printf("  -u, --unix-path [path]              path to unix socket used for client-server\n");
 	printf("                                      communication (default: \""ALFRED_SOCK_PATH_DEFAULT"\")\n");
+	printf("  -c, --update-command                command to call on data change\n");
 	printf("  -v, --version                       print the version\n");
 	printf("  -h, --help                          this help\n");
 	printf("\n");
@@ -153,6 +154,7 @@ static struct globals *alfred_init(int argc, char *argv[])
 		{"modeswitch",		required_argument,	NULL,	'M'},
 		{"change-interface",	required_argument,	NULL,	'I'},
 		{"unix-path",		required_argument,	NULL,	'u'},
+		{"update-command",	required_argument,	NULL,	'c'},
 		{"version",		no_argument,		NULL,	'v'},
 		{"verbose",		no_argument,		NULL,	'd'},
 		{NULL,			0,			NULL,	0},
@@ -174,10 +176,13 @@ static struct globals *alfred_init(int argc, char *argv[])
 	globals->mesh_iface = "bat0";
 	globals->unix_path = ALFRED_SOCK_PATH_DEFAULT;
 	globals->verbose = 0;
+	globals->update_command = NULL;
+	INIT_LIST_HEAD(&globals->changed_data_types);
+	globals->changed_data_type_count = 0;
 
 	time_random_seed();
 
-	while ((opt = getopt_long(argc, argv, "ms:r:hi:b:vV:M:I:u:d", long_options,
+	while ((opt = getopt_long(argc, argv, "ms:r:hi:b:vV:M:I:u:dc:", long_options,
 				  &opt_ind)) != -1) {
 		switch (opt) {
 		case 'r':
@@ -237,6 +242,9 @@ static struct globals *alfred_init(int argc, char *argv[])
 		case 'd':
 			globals->verbose++;
 			break;
+		case 'c':
+			globals->update_command = optarg;
+			break;
 		case 'v':
 			printf("%s %s\n", argv[0], SOURCE_VERSION);
 			printf("A.L.F.R.E.D. - Almighty Lightweight Remote Fact Exchange Daemon\n");
@@ -250,6 +258,8 @@ static struct globals *alfred_init(int argc, char *argv[])
 
 	if (signal(SIGPIPE, SIG_IGN) == SIG_ERR)
 		perror("could not register SIGPIPE handler");
+	if (signal(SIGCHLD, SIG_IGN) == SIG_ERR)
+		perror("could not register SIGCHLD handler");
 	return globals;
 }
 
diff --git a/man/alfred.8 b/man/alfred.8
index a8050ab..e21bb55 100644
--- a/man/alfred.8
+++ b/man/alfred.8
@@ -114,6 +114,10 @@ still keeping redundancy (by having multiple masters). Obviously, at least one
 master must be present in the network to let any data exchange happen. Also
 having all nodes in master mode is possible (for maximum decentrality and
 overhead).
+.TP
+\fB\-c\fP, \fB\-\-update-command\fP \fIcommand\fP
+Specify command to execute on data change. It will be called with data-type list
+as arguments.
 .
 .SH EXAMPLES
 Start an alfred server listening on bridge br0 (assuming that this bridge
diff --git a/recv.c b/recv.c
index 02d96c9..59c69b4 100644
--- a/recv.c
+++ b/recv.c
@@ -22,6 +22,7 @@
 #include <errno.h>
 #include <net/ethernet.h>
 #include <netinet/in.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -41,6 +42,7 @@ static int finish_alfred_push_data(struct globals *globals,
 				   struct alfred_push_data_v0 *push)
 {
 	int len, data_len;
+	bool new_entry_created;
 	struct alfred_data *data;
 	struct dataset *dataset;
 	uint8_t *pos;
@@ -57,6 +59,7 @@ static int finish_alfred_push_data(struct globals *globals,
 		if ((int)(data_len + sizeof(*data)) > len)
 			break;
 
+		new_entry_created = false;
 		dataset = hash_find(globals->data_hash, data);
 		if (!dataset) {
 			dataset = malloc(sizeof(*dataset));
@@ -71,6 +74,7 @@ static int finish_alfred_push_data(struct globals *globals,
 				free(dataset);
 				goto err;
 			}
+			new_entry_created = true;
 		}
 		/* don't overwrite our own data */
 		if (dataset->data_source == SOURCE_LOCAL)
@@ -78,6 +82,12 @@ static int finish_alfred_push_data(struct globals *globals,
 
 		clock_gettime(CLOCK_MONOTONIC, &dataset->last_seen);
 
+		/* check that data was changed */
+		if (new_entry_created ||
+		    dataset->data.header.length != data_len ||
+		    memcmp(dataset->buf, data->data, data_len) != 0)
+			changed_data_type(globals, data->header.type);
+
 		/* free old buffer */
 		if (dataset->buf) {
 			free(dataset->buf);
diff --git a/server.c b/server.c
index 83dfc9b..c9c77b9 100644
--- a/server.c
+++ b/server.c
@@ -23,6 +23,7 @@
 #include <inttypes.h>
 #include <net/ethernet.h>
 #include <net/if.h>
+#include <signal.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -136,6 +137,27 @@ int set_best_server(struct globals *globals)
 	return 0;
 }
 
+void changed_data_type(struct globals *globals, uint8_t arg)
+{
+	struct changed_data_type *data_type = NULL;
+
+	if (!globals->update_command)
+		return;
+
+	list_for_each_entry(data_type, &globals->changed_data_types, list) {
+		if (data_type->data_type == arg)
+			return;
+	}
+
+	data_type = malloc(sizeof(*data_type));
+	if (!data_type)
+		return;
+
+	data_type->data_type = arg;
+	list_add(&data_type->list, &globals->changed_data_types);
+	globals->changed_data_type_count++;
+}
+
 static int purge_data(struct globals *globals)
 {
 	struct hash_it_t *hashit = NULL;
@@ -151,6 +173,8 @@ static int purge_data(struct globals *globals)
 		if (diff.tv_sec < ALFRED_DATA_TIMEOUT)
 			continue;
 
+		changed_data_type(globals, dataset->data.header.type);
+
 		hash_remove_bucket(globals->data_hash, hashit);
 		free(dataset->buf);
 		free(dataset);
@@ -259,6 +283,55 @@ static void check_if_sockets(struct globals *globals)
 		check_if_socket(interface);
 }
 
+static void execute_update_command(struct globals *globals)
+{
+	pid_t script_pid;
+	size_t command_len;
+	char *command;
+	struct changed_data_type *data_type, *is;
+	/* data type is uint8_t, so 255 is maximum (3 chars)
+	 * space for appending + terminating null byte
+	 */
+	char buf[5];
+
+	if (!globals->update_command || !globals->changed_data_type_count)
+		return;
+
+	/* length of script + 4 bytes per data type (space +3 chars)
+	 * + 1 for terminating null byte
+	 */
+	command_len = strlen(globals->update_command);
+	command_len += 4 * globals->changed_data_type_count + 1;
+	command = malloc(command_len);
+	if (!command)
+		return;
+
+	strncpy(command, globals->update_command, command_len - 1);
+	command[command_len - 1] = '\0';
+
+	list_for_each_entry_safe(data_type, is, &globals->changed_data_types,
+				 list) {
+		/* append the datatype to command line */
+		snprintf(buf, sizeof(buf), " %d", data_type->data_type);
+		strncat(command, buf, command_len - strlen(command) - 1);
+
+		/* clean the list */
+		list_del(&data_type->list);
+		free(data_type);
+	}
+	globals->changed_data_type_count = 0;
+
+	printf("executing: %s\n", command);
+
+	script_pid = fork();
+	if (script_pid == 0) {
+		system(command);
+		exit(0);
+	}
+
+	free(command);
+}
+
 int alfred_server(struct globals *globals)
 {
 	int maxsock, ret, recvs;
@@ -343,6 +416,7 @@ int alfred_server(struct globals *globals)
 		}
 		purge_data(globals);
 		check_if_sockets(globals);
+		execute_update_command(globals);
 	}
 
 	netsock_close_all(globals);
