Question

I am trying to implement a simpley portscanner with Python. It works by creating a number of worker threads which scan ports that are provided in a queue. They save the results in another queue. When all ports are scanned the threads and the application should terminate. And here lies the problem: For small numbers of ports everything works fine, but if I try to scan 200 or more ports, the application will get caught in a deadlock. I have no idea, why.

class ConnectScan(threading.Thread):
    def __init__(self, to_scan, scanned):
        threading.Thread.__init__(self)
        self.to_scan = to_scan
        self.scanned = scanned

    def run(self):
        while True:
            try:
                host, port = self.to_scan.get()
            except Queue.Empty:
                break
            s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
            try:
                s.connect((host, port))
                s.close()
                self.scanned.put((host, port, 'open'))
            except socket.error:
                self.scanned.put((host, port, 'closed'))
            self.to_scan.task_done()


class ConnectScanner(object):
    def scan(self, host, port_from, port_to):
        to_scan = Queue.Queue()
        scanned = Queue.Queue()
        for port in range(port_from, port_to + 1):
            to_scan.put((host, port))
        for i in range(20):
            ConnectScan(to_scan, scanned).start()
        to_scan.join()

Does anybody see what might be wrong? Also I would appreciate some tipps how to debug such threading issues in Python.

Answer 1

It is likely that not all items on the to_scan queue are consumed and that you’re not calling the task_done method enough times to unblock ConnectScanner.

Could it be that an exception is thrown during the runtime of ConnectScan.run that you’re not catching and your threads prematurely terminate?