对于我的计算机网络课程,我正在尝试使用带有 ICMP 协议的原始套接字来实现 Traceroute。我需要构建一个数据包,然后使用 Python 结构类解压响应数据包。这是构建数据包的代码:
header = struct.pack("bbHHh", ICMP_ECHO_REQUEST, 0, myChecksum, pid, 1)
data = struct.pack("d", time.time())
packet = header + data
后来,我收到了一个格式相同的 ICMP 数据包,并带有确认信息。这是解包数据包的代码:
request_code, request_type, checksum, packet_id, \
sequence, timeSent, data = struct.unpack("bbHHhd", recvPacket)
但我收到以下错误:struct.error: unpack requires a string argument of length 16
.
我不明白,因为当我检查struct.calcsize()
格式字符串时,它返回 16。
这是我的完整程序,如果你想在你的机器上运行它
from socket import *
import socket
import os
import sys
import struct
import time
import select
import binascii
ICMP_ECHO_REQUEST = 8
MAX_HOPS = 30
TIMEOUT = 2.0
TRIES = 2
# The packet that we shall send to each router along the path is the ICMP echo
# request packet, which is exactly what we had used in the ICMP ping exercise.
# We shall use the same packet that we built in the Ping exercise
def checksum(str):
csum = 0
countTo = (len(str) / 2) * 2
count = 0
while count < countTo:
thisVal = ord(str[count+1]) * 256 + ord(str[count])
csum = csum + thisVal
csum = csum & 0xffffffffL
count = count + 2
if countTo < len(str):
csum = csum + ord(str[len(str) - 1])
csum = csum & 0xffffffffL
csum = (csum >> 16) + (csum & 0xffff)
csum = csum + (csum >> 16)
answer = ~csum
answer = answer & 0xffff
answer = answer >> 8 | (answer << 8 & 0xff00)
return answer
def build_packet():
# In the sendOnePing() method of the ICMP Ping exercise ,firstly the header of our
# packet to be sent was made, secondly the checksum was appended to the header and
# then finally the complete packet was sent to the destination.
# Make the header in a similar way to the ping exercise.
# Header is type (8), code (8), checksum (16), id (16), sequence (16)
myChecksum = 0
pid = os.getpid() & 0xFFFF
# Make a dummy header with a 0 checksum.
# struct -- Interpret strings as packed binary data
header = struct.pack("bbHHh", ICMP_ECHO_REQUEST, 0, myChecksum, pid, 1)
#header = struct.pack("!HHHHH", ICMP_ECHO_REQUEST, 0, myChecksum, pid, 1)
data = struct.pack("d", time.time())
# Calculate the checksum on the data and the dummy header.
# Append checksum to the header.
myChecksum = checksum(header + data)
if sys.platform == 'darwin':
myChecksum = socket.htons(myChecksum) & 0xffff
#Convert 16-bit integers from host to network byte order.
else:
myChecksum = htons(myChecksum)
packet = header + data
return packet
def get_route(hostname):
timeLeft = TIMEOUT
for ttl in xrange(1,MAX_HOPS):
for tries in xrange(TRIES):
destAddr = socket.gethostbyname(hostname)
#Fill in start
# Make a raw socket named mySocket
mySocket = socket.socket(AF_INET, SOCK_RAW, getprotobyname("icmp"))
mySocket.bind(("", 12000));
#Fill in end
mySocket.setsockopt(socket.IPPROTO_IP, socket.IP_TTL, struct.pack('I', ttl))
mySocket.settimeout(TIMEOUT)
try:
d = build_packet()
mySocket.sendto(d, (hostname, 0))
t = time.time()
startedSelect = time.time()
whatReady = select.select([mySocket], [], [], timeLeft)
howLongInSelect = (time.time() - startedSelect)
if whatReady[0] == []: # Timeout
print "* * * Request timed out."
recvPacket, addr = mySocket.recvfrom(1024)
print addr
timeReceived = time.time()
timeLeft = timeLeft - howLongInSelect
if timeLeft <= 0:
print "* * * Request timed out."
except socket.timeout:
continue
else:
#Fill in start
# Fetch the icmp type from the IP packet
print struct.calcsize("bbHHhd")
request_code, request_type, checksum, packet_id, \
sequence, timeSent, data = struct.unpack("bbHHhd", recvPacket)
#Fill in end
if request_type == 11:
bytes = struct.calcsize("d")
timeSent = struct.unpack("d", recvPacket[28:28 + bytes])[0]
print " %d rtt=%.0f ms %s" % (ttl,(timeReceived -t)*1000, addr[0])
elif request_type == 3:
bytes = struct.calcsize("d")
timeSent = struct.unpack("d", recvPacket[28:28 + bytes])[0]
print " %d rtt=%.0f ms %s" % (ttl,(timeReceived -t)*1000, addr[0])
elif request_type == 0:
bytes = struct.calcsize("d")
timeSent = struct.unpack("d", recvPacket[28:28 + bytes])[0]
print " %d rtt=%.0f ms %s" % (ttl,(timeReceived -timeSent)*1000, addr[0])
return
else:
print "error"
break
finally:
mySocket.close()
get_route("www.google.com")