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#!/usr/local/bin/python3
import os
import threading
from addr import *
from scapy.all import *
class Sniff1(threading.Thread):
filter = None
captured = None
packet = None
def __init__(self):
# clear packets buffered by scapy bpf
sniff(iface=LOCAL_IF, timeout=1)
super(Sniff1, self).__init__()
def run(self):
self.captured = sniff(iface=LOCAL_IF, filter=self.filter,
count=1, timeout=3)
if self.captured:
self.packet = self.captured[0]
ip=IP(src=FAKE_NET_ADDR, dst=REMOTE_ADDR)
tport=os.getpid() & 0xffff
print("Send SYN packet, receive SYN+ACK.")
syn=TCP(sport=tport, dport='chargen', seq=1, flags='S', window=(2**16)-1)
synack=sr1(ip/syn, iface=LOCAL_IF, timeout=5)
if synack is None:
print("ERROR: No SYN+ACK from chargen server received.")
exit(1)
print("Send ACK packet, receive chargen data.")
ack=TCP(sport=synack.dport, dport=synack.sport, seq=2, flags='A',
ack=synack.seq+1, window=(2**16)-1)
data=sr1(ip/ack, iface=LOCAL_IF, timeout=5)
if data is None:
print("ERROR: No data from chargen server received.")
exit(1)
print("Fill our receive buffer.")
time.sleep(1)
# sr1 cannot be used, TCP data will not match outgoing ICMP packet
sniffer = Sniff1()
sniffer.filter = \
"ip and src %s and tcp port %u and dst %s and tcp port %u" % \
(ip.dst, syn.dport, ip.src, syn.sport)
sniffer.start()
time.sleep(1)
print("Send ICMP fragmentation needed packet with MTU 1300.")
icmp=ICMP(type="dest-unreach", code="fragmentation-needed",
nexthopmtu=1300)/data
send(IP(src=LOCAL_ADDR, dst=REMOTE_ADDR)/icmp, iface=LOCAL_IF)
print("Path MTU discovery will resend first data with length 1300.")
sniffer.join(timeout=5)
ans = sniffer.packet
if len(ans) == 0:
print("ERROR: No data retransmit from chargen server received.")
exit(1)
data=ans[0]
print("Cleanup the other's socket with a reset packet.")
rst=TCP(sport=synack.dport, dport=synack.sport, seq=2, flags='AR',
ack=synack.seq+1)
send(ip/rst, iface=LOCAL_IF)
len = data.len
print("len=%d" % len)
if len != 1300:
print("ERROR: TCP data packet len is %d, expected 1300." % len)
exit(1)
exit(0)
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