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#!/usr/local/bin/python2.7
# check wether path mtu to dst is as expected
import os
import threading
from addr import *
from scapy.all import *
#
# we can not use scapy's sr() function as receive side
# ignores the packet we expect to see. Packet is ignored
# due to mismatching sequence numbers. 'bogus_syn' is using
# seq = 1000000, while response sent back by PF has ack,
# which fits regular session opened by 'syn'.
#
class Sniff1(threading.Thread):
filter = None
captured = None
packet = None
def run(self):
self.captured = sniff(iface=LOCAL_IF, filter=self.filter,
count=1, timeout=5)
if self.captured:
self.packet = self.captured[0]
tport=os.getpid() & 0xffff
ip=IP(src=FAKE_NET_ADDR, dst=REMOTE_ADDR)
print "Send SYN packet, receive SYN+ACK"
syn=TCP(sport=tport, dport='echo', seq=1, flags='S', window=(2**16)-1)
synack=sr1(ip/syn, iface=LOCAL_IF, timeout=5)
if synack is None:
print "ERROR: no matching SYN+ACK packet received"
exit(1)
print "Send ACK packet to finish handshake."
ack=TCP(sport=synack.dport, dport=synack.sport, seq=2, flags='A',
ack=synack.seq+1)
send(ip/ack, iface=LOCAL_IF)
print "Connection is established, send bogus SYN, expect challenge ACK"
bogus_syn=TCP(sport=syn.sport, dport=syn.dport, seq=1000000, flags='S',
window=(2**16)-1)
sniffer = Sniff1();
sniffer.filter = "src %s and tcp port %u and dst %s and tcp port %u " \
"and tcp[tcpflags] = tcp-ack" % (ip.dst, syn.dport, ip.src, syn.sport)
sniffer.start()
time.sleep(1)
send(ip/bogus_syn, iface=LOCAL_IF)
sniffer.join(timeout=7)
challenge_ack = sniffer.packet
if challenge_ack is None:
print "ERROR: no matching ACK packet received"
exit(1)
if challenge_ack.getlayer(TCP).seq != (synack.seq + 1):
print "ERROR: expecting seq %d got %d in challange ack" % \
(challenge_ack.getlayer(TCP).seq, (synack.seq + 1))
exit(1)
exit(0)
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