make tcpdump output to stdout and use dpkt to parse the pcap file captured

tcpdump_wrapper.py

@contextlib.contextmanager
def capture(ifname, src, dst):
    events = []
    filter = '(host %s and host %s) or icmp[0] = 11' % (src, dst)
    p = subprocess.Popen(
        ['tcpdump', '-i', ifname, '-w', '-', filter],
        stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)
    try:
        yield events
    finally:
        p.terminate()
    captured_bytes = p.stdout.read()
    fake_file = StringIO(captured_bytes)
    fake_file.__dict__['name'] = ''
    fake_file.__dict__['fileno'] = lambda: None
    for timestamp, eth_frame_bytes in dpkt.pcap.Reader(fake_file):
        eth_frame = dpkt.ethernet.Ethernet(eth_frame_bytes)
        if not hasattr(eth_frame, 'ip'):
            continue
        events.append((timestamp, eth_frame.ip))