probe if the router is dropping tcp packet for specific source port

probe_tcp_packet_drop.py

import socket
import dpkt
import sys

dst = sys.argv[1]
sport1 = 8080
sport2 = 8081
icmp_socket = sock = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_ICMP)
icmp_socket.settimeout(2)
trace = []
for ttl in range(1, 20):
    def send_syn_and_get_icmp_exceeded_src(sport):
        tcp_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM, socket.IPPROTO_TCP)
        tcp_socket.bind(('', sport))
        tcp_socket.setsockopt(socket.SOL_IP, socket.IP_TTL, ttl)
        tcp_socket.settimeout(2)
        try:
            tcp_socket.connect((dst, 80))
            print('destination reached')
            sys.exit(1)
        except socket.error:
            pass
        finally:
            tcp_socket.close()
        try:
            return socket.inet_ntoa(dpkt.ip.IP(icmp_socket.recv(1024)).src)
        except socket.error:
            return '*'
    if len(trace) >= 4:
        if '*' == trace[-1][1] and '*' == trace[-2][1] and '*' == trace[-3][1]:
            print('filtered by %s' % trace[-4][1])
            sys.exit(0)
    router1_ip = send_syn_and_get_icmp_exceeded_src(sport1)
    router2_ip = send_syn_and_get_icmp_exceeded_src(sport2)
    print('via %15s\t%15s' % (router1_ip, router2_ip))
    trace.append((router1_ip, router2_ip))