struct: unpack Requires String Argument of Length 16

Question

For my Computer Networking class, I'm trying to implement Traceroute using raw sockets with the ICMP protocol. I need to build a packet and then unpack the response packet using the Python struct class. Here is the code for building the packet:

header = struct.pack("bbHHh", ICMP_ECHO_REQUEST, 0, myChecksum, pid, 1)
data = struct.pack("d", time.time())
packet = header + data

Later, I receive an ICMP packet in the same format with the confirmation. Here is the code for unpacking the packet:

request_code, request_type, checksum, packet_id, \
                sequence, timeSent, data = struct.unpack("bbHHhd", recvPacket)

But I'm getting the following error: struct.error: unpack requires a string argument of length 16.

I don't understand because when I check struct.calcsize() for the format string, it returns 16.

Here is my full program if you would like to run it on your machine

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")

Answer 1

The struct.unpack function requires that the data you pass to it match the format string's length exactly.

If you have a large buffer and you only want to decode part of it, consider using the struct.unpack_from function instead. It takes an additional argument specifying the offset to begin decoding at, and accepts buffers larger than the format string describes:

(request_code, request_type, checksum, packet_id, sequence,
 timeSent, data) = struct.unpack_from("bbHHhd", recvPacket, 0)

You may find this function useful if you want to decode other parts of the packet data after parsing the header.

Answer 2

recvPacket is bigger than your structure. If your structure is the first part of the data, unpack just the bytes of the structure:

pktFormat = 'bbHHhd'
pktSize = struct.calcsize(pktFormat)
... = struct.unpack(pktFormat, recvPacket[:pktSize])