Add connection rate limiting.

Tinc now strictly limits incoming connections from the same host to 1 per
second. For incoming connections from multiple hosts short bursts of incoming
connections are allowed (by default 100), but on average also only 1 connection
per second is allowed.

When an incoming connection exceeds the limit, tinc will keep the connection in
a tarpit; the connection will be kept open but it is ignored completely. Only
one connection is in a tarpit at a time to limit the number of useless open
connections.

doc/tinc.conf.5.in

@@ -335,6 +335,11 @@ This only has effect when
 .Va Mode
 is set to
 .Qq switch .
+.It Va MaxConnectionBurst Li = Ar count Pq 100
+This option controls how many connections tinc accepts in quick succession.
+If there are more connections than the given number in a short time interval,
+tinc will reduce the number of accepted connections to only one per second,
+until the burst has passed.
 .It Va MaxTimeout Li = Ar seconds Pq 900
 This is the maximum delay before trying to reconnect to other tinc daemons.
 .It Va Mode Li = router | switch | hub Pq router

doc/tinc.texi

@@ -1101,6 +1101,13 @@ impossible to crack a single key.
 This option controls the amount of time MAC addresses are kept before they are removed.
 This only has effect when Mode is set to "switch".
 
+@cindex MaxConnectionBurst
+@item MaxConnectionBurst = <@var{count}> (100)
+This option controls how many connections tinc accepts in quick succession.
+If there are more connections than the given number in a short time interval,
+tinc will reduce the number of accepted connections to only one per second,
+until the burst has passed.
+
 @cindex Name
 @item Name = <@var{name}> [required]
 This is a symbolic name for this connection.
@@ -1182,7 +1189,6 @@ reordering. Setting this to zero will disable replay tracking completely and
 pass all traffic, but leaves tinc vulnerable to replay-based attacks on your
 traffic.
 
-
 @cindex StrictSubnets
 @item StrictSubnets <yes|no> (no) [experimental]
 When this option is enabled tinc will only use Subnet statements which are

src/net.h

@@ -133,6 +133,7 @@ extern io_t unix_socket;
 extern int keylifetime;
 extern int udp_rcvbuf;
 extern int udp_sndbuf;
+extern int max_connection_burst;
 extern bool do_prune;
 extern char *myport;
 extern int autoconnect;

src/net_setup.c

@@ -711,7 +711,12 @@ static bool setup_myself(void) {
 	get_config_bool(lookup_config(config_tree, "TunnelServer"), &tunnelserver);
 	strictsubnets |= tunnelserver;
 
-
+	if(get_config_int(lookup_config(config_tree, "MaxConnectionBurst"), &max_connection_burst)) {
+		if(max_connection_burst <= 0) {
+			logger(DEBUG_ALWAYS, LOG_ERR, "MaxConnectionBurst cannot be negative!");
+			return false;
+		}
+	}
 
 	if(get_config_int(lookup_config(config_tree, "UDPRcvBuf"), &udp_rcvbuf)) {
 		if(udp_rcvbuf <= 0) {

src/net_socket.c

@@ -45,6 +45,7 @@ int maxtimeout = 900;
 int seconds_till_retry = 5;
 int udp_rcvbuf = 0;
 int udp_sndbuf = 0;
+int max_connection_burst = 100;
 
 listen_socket_t listen_socket[MAXSOCKETS];
 int listen_sockets;
@@ -561,6 +562,47 @@ void handle_new_meta_connection(void *data, int flags) {
 
 	sockaddrunmap(&sa);
 
+	// Check if we get many connections from the same host
+
+	static sockaddr_t prev_sa;
+	static time_t prev_time;
+	static int tarpit = -1;
+
+	if(tarpit >= 0) {
+		closesocket(tarpit);
+		tarpit = -1;
+	}
+
+	if(prev_time == now.tv_sec && !sockaddrcmp_noport(&sa, &prev_sa)) {
+		// if so, keep the connection open but ignore it completely.
+		tarpit = fd;
+		return;
+	}
+
+	memcpy(&prev_sa, &sa, sizeof sa);
+	prev_time = now.tv_sec;
+
+	// Check if we get many connections from different hosts
+
+	static int connection_burst;
+	static int connection_burst_time;
+
+	if(now.tv_sec - connection_burst_time > connection_burst)
+		connection_burst = 0;
+	else
+		connection_burst -= now.tv_sec - connection_burst_time;
+
+	connection_burst_time = now.tv_sec;
+	connection_burst++;
+
+	if(connection_burst >= max_connection_burst) {
+		connection_burst = max_connection_burst;
+		tarpit = fd;
+		return;
+	}
+
+	// Accept the new connection
+
 	c = new_connection();
 	c->name = xstrdup("<unknown>");
 	c->outcipher = myself->connection->outcipher;

src/tincctl.c

@@ -1296,6 +1296,7 @@ const var_t variables[] = {
 	{"KeyExpire", VAR_SERVER},
 	{"LocalDiscovery", VAR_SERVER},
 	{"MACExpire", VAR_SERVER},
+	{"MaxConnectionBurst", VAR_SERVER},
 	{"MaxOutputBufferSize", VAR_SERVER},
 	{"MaxTimeout", VAR_SERVER},
 	{"Mode", VAR_SERVER | VAR_SAFE},