feat: implement interval jitter

Adds a new argument called 'interval_jitter', expressed as a percentage
number (range 0-100) that has the following effects:

If 0 => no effect.

If >0 => choose a random integer in the
[-(interval * jitter / 100), +(interval * jitter / 100)] range

and add use it as an offset for the sleep interval.

So for example, if our interval is 1000ms and we pick
interval_jitter=20, then the sleep values will be
in the range [800ms, 1200ms]

This has been a requested feature in the original implementation,
and is useful to twarth SSH scanners that have tarpit-detecting logic.

Link: skeeto/endlessh#71

client/client.go

@@ -46,10 +46,11 @@ type Client struct {
 	start     time.Time
 	last      time.Time
 	interval  time.Duration
+	jitter    int64
 	bytesSent int
 }
 
-func NewClient(conn net.Conn, interval time.Duration, maxClients int64) *Client {
+func NewClient(conn net.Conn, interval time.Duration, jitter int64, maxClients int64) *Client {
 	for numCurrentClients >= maxClients {
 		time.Sleep(interval)
 	}
@@ -62,6 +63,7 @@ func NewClient(conn net.Conn, interval time.Duration, maxClients int64) *Client
 		start:     time.Now(),
 		last:      time.Now(),
 		interval:  interval,
+		jitter:    jitter,
 		bytesSent: 0,
 	}
 }
@@ -78,7 +80,7 @@ func (c *Client) Send(bannerMaxLength int64) (int, error) {
 	if time.Now().Before(c.next) {
 		time.Sleep(c.next.Sub(time.Now()))
 	}
-	c.next = time.Now().Add(c.interval)
+	c.next = time.Now().Add(c.Interval())
 	length := rand.Int63n(bannerMaxLength)
 	bytesSent, err := c.conn.Write(randStringBytes(length))
 	if err != nil {
@@ -88,6 +90,16 @@ func (c *Client) Send(bannerMaxLength int64) (int, error) {
 	return bytesSent, nil
 }
 
+func (c *Client) Interval() time.Duration {
+	if c.jitter != 0 {
+		// generate an integer offset in the interval [-jitter, +jitter]
+		offset := time.Duration(rand.Int63n(2*c.jitter+1)-c.jitter) * time.Millisecond
+		glog.V(1).Infof("Jitter debug interval=%v offset=%v\n", c.interval, offset)
+		return c.interval + offset
+	}
+	return c.interval
+}
+
 func (c *Client) MillisecondsSinceLast() int64 {
 	millisecondsSpent := time.Now().Sub(c.last).Milliseconds()
 	c.last = time.Now()

main.go

@@ -67,7 +67,7 @@ func startSending(maxClients int64, bannerMaxLength int64, records chan<- metric
 	return clients
 }
 
-func startAccepting(maxClients int64, connType, connHost, connPort string, interval time.Duration, clients chan<- *client.Client, records chan<- metrics.RecordEntry) {
+func startAccepting(maxClients int64, connType, connHost, connPort string, interval time.Duration, jitter int64, clients chan<- *client.Client, records chan<- metrics.RecordEntry) {
 	go func() {
 		l, err := net.Listen(connType, connHost+":"+connPort)
 		if err != nil {
@@ -84,7 +84,7 @@ func startAccepting(maxClients int64, connType, connHost, connPort string, inter
 				glog.Errorf("Error accepting connection from port %v: %v", connPort, err)
 				os.Exit(1)
 			}
-			c := client.NewClient(conn, interval, maxClients)
+			c := client.NewClient(conn, interval, jitter, maxClients)
 			remoteIpAddr := c.RemoteIpAddr()
 			records <- metrics.RecordEntry{
 				RecordType: metrics.RecordEntryTypeStart,
@@ -113,6 +113,7 @@ var connPorts arrayStrings
 
 func main() {
 	intervalMs := flag.Int("interval_ms", 1000, "Message millisecond delay")
+	intervalJitter := flag.Int64("interval_jitter", 0, "Interval jitter as a percentage. Set to 0 to disable. (default 0)")
 	bannerMaxLength := flag.Int64("line_length", 32, "Maximum banner line length")
 	maxClients := flag.Int64("max_clients", 4096, "Maximum number of clients")
 	connType := flag.String("conn_type", "tcp", "Connection type. Possible values are tcp, tcp4, tcp6")
@@ -154,8 +155,15 @@ func main() {
 	if len(connPorts) == 0 {
 		connPorts = append(connPorts, defaultPort)
 	}
+
+	if *intervalJitter < 0 || *intervalJitter > 100 {
+		glog.Errorf("Error: interval_jitter must be a number between 0 and 100. Got: %v", *intervalJitter)
+		os.Exit(1)
+	}
+	// use float to compute the jitter limit to avoid overflow
+	jitter := int64(float64(*intervalMs) * (float64(*intervalJitter) / 100))
 	for _, connPort := range connPorts {
-		startAccepting(*maxClients, *connType, *connHost, connPort, interval, clients, records)
+		startAccepting(*maxClients, *connType, *connHost, connPort, interval, jitter, clients, records)
 	}
 	for {
 		if *prometheusCleanUnseenSeconds <= 0 {