#!/usr/bin/env python2
import sys,socket,time,threading

IF_DEVICE="eth0"
IP_ADDR="10.0.2.15"
PORT=13531

try:
    SO_BINDTODEVICE=socket.SO_BINDTODEVICE
except:
    SO_BINDTODEVICE=25

class Server(threading.Thread):
    def __init__(self, dev, port):
        threading.Thread.__init__(self)
        self.sk = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
        if dev:
            self.sk.setsockopt(socket.SOL_SOCKET, SO_BINDTODEVICE, dev)
        self.sk.bind(('', port))

    def run(self):
        while True:
            (data, addr) = self.sk.recvfrom(1024)
            print(("recv:%s from %s" % (data, str(addr))))
            if data == 'finish':
                break

# flush rt_cache.
def flush_rt_cache():
    f = open('/proc/sys/net/ipv4/route/flush', 'w')
    f.write('flush') # any message is ok.
    f.close()

# create a rt_cache which is not bound to dev.
# send a dummy packet to ssh port. (any port is ok)
def create_dummy_rt_cache():
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect((IP_ADDR, 22))
    s.send('dummy')
    s.close()

def connect_and_send(msg):
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.setsockopt(socket.SOL_SOCKET, SO_BINDTODEVICE, IF_DEVICE)
    s.connect((IP_ADDR, PORT))
    s.send(msg)
    
if __name__ == '__main__':
    flush_rt_cache()
    create_dummy_rt_cache()
    # start a server thread.
    server = Server(IF_DEVICE, PORT)
    server.start()
    # start clients. but never connect to the server.
    for count in range(10):
        time.sleep(1)
        connect_and_send("count=%d" % count)
        print(("sent:count=%s" % count))
    # revive the connection.
    flush_rt_cache()
    # now, successfully connect to the server.
    connect_and_send('finish')
    server.join()