Add DeviceStandby option to only enable the device when nodes are reachable.

This adds a new DeviceStandby option; when it is disabled (the default),
behavior is unchanged. If it is enabled, tinc-up will not be called during
tinc initialization, but will instead be deferred until the first node is
reachable, and it will be closed as soon as no nodes are reachable.

This is useful because it means the device won't be set up until we are fairly
sure there is something listening on the other side. This is more user-friendly,
as one can check on the status of the tinc network connection just by checking
the status of the network interface. Besides, it prevents the OS from thinking
it is connected to some network when it is in fact completely isolated.

doc/tinc.conf.5.in

@@ -193,6 +193,12 @@ instead of
 .Va Device .
 The info pages of the tinc package contain more information
 about configuring the virtual network device.
+.It Va DeviceStandby Li = yes | no Po no Pc
+When disabled,
+.Nm tinc
+calls tinc-up on startup, and tinc-down on shutdown. When enabled,
+.Nm tinc
+will only call tinc-up when at least one node is reachable, and will call tinc-down as soon as no nodes are reachable.
 .It Va DeviceType Li = Ar type Pq platform dependent
 The type of the virtual network device.
 Tinc will normally automatically select the right type of tun/tap interface, and this option should not be used.
@@ -603,12 +609,16 @@ or
 .Bl -tag -width indent
 .It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc-up
 This is the most important script.
-If it is present it will be executed right after the tinc daemon has been started and has connected to the virtual network device.
+If it is present it will be executed right after the tinc daemon has been started and has connected to the virtual network device (or when the first node becomes reachable if
+.Va DeviceStandby
+is used).
 It should be used to set up the corresponding network interface,
 but can also be used to start other things.
 Under Windows you can use the Network Connections control panel instead of creating this script.
 .It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc-down
-This script is started right before the tinc daemon quits.
+This script is started right before the tinc daemon quits (or when the last node becomes unreachable if
+.Va DeviceStandby
+is used).
 .It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /hosts/ Ns Ar HOST Ns Pa -up
 This script is started when the tinc daemon with name
 .Ar HOST

src/graph.c

@@ -204,6 +204,9 @@ static void sssp_bfs(void) {
 static void check_reachability(void) {
 	/* Check reachability status. */
 
+	int reachable_count = 0;
+	int became_reachable_count = 0;
+	int became_unreachable_count = 0;
 	for splay_each(node_t, n, node_tree) {
 		if(n->status.visited != n->status.reachable) {
 			n->status.reachable = !n->status.reachable;
@@ -212,9 +215,13 @@ static void check_reachability(void) {
 			if(n->status.reachable) {
 				logger(DEBUG_TRAFFIC, LOG_DEBUG, "Node %s (%s) became reachable",
 					   n->name, n->hostname);
+				if (n != myself)
+					became_reachable_count++;
 			} else {
 				logger(DEBUG_TRAFFIC, LOG_DEBUG, "Node %s (%s) became unreachable",
 					   n->name, n->hostname);
+				if (n != myself)
+					became_unreachable_count++;
 			}
 
 			if(experimental && OPTION_VERSION(n->options) >= 2)
@@ -277,6 +284,16 @@ static void check_reachability(void) {
 				}
 			}
 		}
+
+		if(n->status.reachable && n != myself)
+			reachable_count++;
+	}
+
+	if (device_standby) {
+		if (reachable_count == 0 && became_unreachable_count > 0)
+			device_disable();
+		else if (reachable_count == became_reachable_count)
+			device_enable();
 	}
 }
 

src/net.h

@@ -137,6 +137,7 @@ extern int udp_sndbuf;
 extern int max_connection_burst;
 extern bool do_prune;
 extern char *myport;
+extern bool device_standby;
 extern bool autoconnect;
 extern bool disablebuggypeers;
 extern int contradicting_add_edge;
@@ -178,6 +179,8 @@ extern void send_packet(struct node_t *, vpn_packet_t *);
 extern void receive_tcppacket(struct connection_t *, const char *, int);
 extern void broadcast_packet(const struct node_t *, vpn_packet_t *);
 extern char *get_name(void);
+extern void device_enable(void);
+extern void device_disable(void);
 extern bool setup_myself_reloadable(void);
 extern bool setup_network(void);
 extern void setup_outgoing_connection(struct outgoing_t *);

src/net_setup.c

@@ -46,6 +46,7 @@
 char *myport;
 static io_t device_io;
 devops_t devops;
+bool device_standby = false;
 
 char *proxyhost;
 char *proxyport;
@@ -726,6 +727,34 @@ static bool add_listen_address(char *address, bool bindto) {
 	return true;
 }
 
+void device_enable(void) {
+	/* Run tinc-up script to further initialize the tap interface */
+
+	char *envp[5] = {NULL};
+	xasprintf(&envp[0], "NETNAME=%s", netname ? : "");
+	xasprintf(&envp[1], "DEVICE=%s", device ? : "");
+	xasprintf(&envp[2], "INTERFACE=%s", iface ? : "");
+	xasprintf(&envp[3], "NAME=%s", myself->name);
+
+	execute_script("tinc-up", envp);
+
+	for(int i = 0; i < 4; i++)
+		free(envp[i]);
+}
+
+void device_disable(void) {
+	char *envp[5] = {NULL};
+	xasprintf(&envp[0], "NETNAME=%s", netname ? : "");
+	xasprintf(&envp[1], "DEVICE=%s", device ? : "");
+	xasprintf(&envp[2], "INTERFACE=%s", iface ? : "");
+	xasprintf(&envp[3], "NAME=%s", myself->name);
+
+	execute_script("tinc-down", envp);
+
+	for(int i = 0; i < 4; i++)
+		free(envp[i]);
+}
+
 /*
   Configure node_t myself and set up the local sockets (listen only)
 */
@@ -918,6 +947,8 @@ static bool setup_myself(void) {
 #endif
 	}
 
+	get_config_bool(lookup_config(config_tree, "DeviceStandby"), &device_standby);
+
 	if(!devops.setup())
 		return false;
 
@@ -1048,18 +1079,8 @@ bool setup_network(void) {
 	if(!init_control())
 		return false;
 
-	/* Run tinc-up script to further initialize the tap interface */
-
-	char *envp[5] = {NULL};
-	xasprintf(&envp[0], "NETNAME=%s", netname ? : "");
-	xasprintf(&envp[1], "DEVICE=%s", device ? : "");
-	xasprintf(&envp[2], "INTERFACE=%s", iface ? : "");
-	xasprintf(&envp[3], "NAME=%s", myself->name);
-
-	execute_script("tinc-up", envp);
-
-	for(int i = 0; i < 4; i++)
-		free(envp[i]);
+	if (!device_standby)
+		device_enable();
 
 	/* Run subnet-up scripts for our own subnets */
 
@@ -1098,25 +1119,17 @@ void close_network_connections(void) {
 		close(listen_socket[i].udp.fd);
 	}
 
-	char *envp[5] = {NULL};
-	xasprintf(&envp[0], "NETNAME=%s", netname ? : "");
-	xasprintf(&envp[1], "DEVICE=%s", device ? : "");
-	xasprintf(&envp[2], "INTERFACE=%s", iface ? : "");
-	xasprintf(&envp[3], "NAME=%s", myself->name);
-
 	exit_requests();
 	exit_edges();
 	exit_subnets();
 	exit_nodes();
 	exit_connections();
 
-	execute_script("tinc-down", envp);
+	if (!device_standby)
+		device_disable();
 
 	if(myport) free(myport);
 
-	for(int i = 0; i < 4; i++)
-		free(envp[i]);
-
 	if (device_fd >= 0)
 		io_del(&device_io);
 	devops.close();