I need to calculate the checksum of an IP packet as described in http://www.faqs.org/rfcs/rfc1071.html.
I have already the following code:
#!/usr/bin/python
import struct
data = "45 00 00 47 73 88 40 00 40 06 a2 c4 83 9f 0e 85 83 9f 0e a1"
# a test for the checksum calculation
def _checksum(data):
#calculate the header sum
ip_header_sum = sum(struct.unpack_from("6H", data))
#add the carry
ip_header_sum = (ip_header_sum & 0xFFFF) + (ip_header_sum >> 16 & 0xFFFF)
#invert the sum, python does not support inversion (~a is -a + 1) so we have to do
#little trick: ~a is the same as 0xFFFF & ~a
ip_header_sum = ~ip_header_sum & 0xFFFF
return ip_header_sum #should return 0 if correct
data = data.split()
data = map(lambda x: int(x,16), data)
data = struct.pack("%dB" % len(data), *data)
print " ".join(map(lambda x: "0x%02x" % ord(x), data))
print "Checksum: 0x%04x" % _checksum(data)
It works with a package that I have captured with wireshark and which should have the correct checksum and should therefore evaluate to 0
Unfortunately the result is 0x6524. It is also interesting, that the result is always 0x6524 for every correct packet...
Who spots the error?
Edited to make the error more clear *Edited a second time*
You can use the solution directly from checksum udp calculation python, which results in the expected checksum value of zero.
import struct
data = "45 00 00 47 73 88 40 00 40 06 a2 c4 83 9f 0e 85 83 9f 0e a1"
def carry_around_add(a, b):
c = a + b
return (c & 0xffff) + (c >> 16)
def checksum(msg):
s = 0
for i in range(0, len(msg), 2):
w = ord(msg[i]) + (ord(msg[i+1]) << 8)
s = carry_around_add(s, w)
return ~s & 0xffff
data = data.split()
data = map(lambda x: int(x,16), data)
data = struct.pack("%dB" % len(data), *data)
print ' '.join('%02X' % ord(x) for x in data)
print "Checksum: 0x%04x" % checksum(data)
Results:
45 00 00 47 73 88 40 00 40 06 A2 C4 83 9F 0E 85 83 9F 0E A1
Checksum: 0x0000