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manipulating hex values

P: n/a
Hi all,

I am relatively new to socket programming. I am attempting to use raw
sockets to spoof my IP address. From what I can tell I will have to
build from the Ethernet layer on up. This is fine, but I am having
some trouble with manipulating my hex values.

Seems to me that there are two ways to store hex values:
1. as literal hex - 0x55aa
2. as a string - "\x55aa"

If I want to convert hex to decimal I can use:
int("\x55aa", 16) # note that plain 0x55aa, instead of "\x55aa", will
raise an exception

The Question:
If I want to do any kind of calculation I have found its best to just
convert my values to decimal, do the math, then convert back to hex. In
my bellow code I get
"""byteList.append(int(value,16))
ValueError: invalid literal for int()"""
when attempting to run. I do not understand why this exception is
being raised? It is a for loop iterating over a list of hex strings.
Sorry for the long-ish question. Any help or comments would be appreciated.
----my checksum---
def calcIPCheckSum(ipHeaders):
byteList = []
# convert groups from hex to dec
for value in ipHeaders:
byteList.append(int(value,16)) # Exception raised here!

# 1's compliment
for index in range(len(byteList)):
byteList[index] = abs(byteList[index] - 65535)

# add bytes together shifting the extra byte
total = 0
for value in byteList:
total = total + value
if total 65535:
total = total - 65535

return hex(total)

checksum = calcIPChecksum(["\x45" + "\x00", "\x01\x4a", "\x00\x00",
"\x40"+"\x00", "\x20"+"\x11"])
-------------------------

Cheers,

Steve
Apr 1 '08 #1
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P: n/a
On 2008-04-01, Stephen Cattaneo <st**************@u4eatech.comwrote:
I am relatively new to socket programming. I am attempting to
use raw sockets to spoof my IP address. From what I can tell
I will have to build from the Ethernet layer on up. This is
fine, but I am having some trouble with manipulating my hex
values.

Seems to me that there are two ways to store hex values:
1. as literal hex - 0x55aa
That's a hex represenation of a literal integer object. The
object itself isn't either decimal or hex. It's and integer
object. [It's _probably_ stored in binary, but that's beside
the point.]
2. as a string - "\x55aa"
That is the string "Uaa". Perhaps you meant "\x55\xaa"? If
so, that might or might not have the same bit-pattern as
0x55aa depending on what CPU you're using.
If I want to convert hex to decimal I can use: int("\x55aa", 16)
No you can't'

1) That call doesn't convert hex to decimal. It converts to
an integer object (which almost certainly is stored in 2's
compliment binary). There is nothing in decimal in the
example.

2) "\x55aa" doesn't mean what you think it does. "\x55aa" is
a three byte string consisting of a byte with the value
0x55 and then two ascii 'a' bytes. 0x55 is an ascii 'U'.

I think you meant "0x55aa". That's completely different
from either "\x55aa" or "\x55\xaa".
# note that plain 0x55aa, instead of "\x55aa", will
raise an exception
That's because the above function call requires a string.
0x55aa isn't a string. It's an integer literal. "\x55aa" is a
three byte string that's equal to "Uaa", and it can't be
converted to an integer.
The Question:
If I want to do any kind of calculation I have found its best to just
convert my values to decimal, do the math, then convert back to hex.
First: stop talking (and thinking) about "decimal" stuff.
You're not really doing anything in decimal in any of the code
you've posted so far.
In my bellow code I get
"""byteList.append(int(value,16))
ValueError: invalid literal for int()"""
when attempting to run.
No you don't. You get this:

Traceback (most recent call last):
File "foo.py", line 20, in ?
checksum = calcIPChecksum(["\x45" + "\x00", "\x01\x4a", "\x00\x00", "\x40"+"\x00", "\x20"+"\x11"])
NameError: name 'calcIPChecksum' is not defined

Don't re-type examples in postings. Paste in the actual code
that you're using. That way we don't have to try to guess
about the differences between the real code and what's in the
posting.
I do not understand why this exception is being raised?
If you want to understand why you're getting that exception,
add the line "print value" immediately prior to the
int(value,16) line.
It is a for loop iterating over a list of hex strings. Sorry
for the long-ish question. Any help or comments would be
appreciated.
----my checksum---
def calcIPCheckSum(ipHeaders):
byteList = []
# convert groups from hex to dec
for value in ipHeaders:
byteList.append(int(value,16)) # Exception raised here!

# 1's compliment
for index in range(len(byteList)):
byteList[index] = abs(byteList[index] - 65535)

# add bytes together shifting the extra byte
total = 0
for value in byteList:
total = total + value
if total 65535:
total = total - 65535

return hex(total)

checksum = calcIPChecksum(["\x45" + "\x00", "\x01\x4a", "\x00\x00",
"\x40"+"\x00", "\x20"+"\x11"])
You pretty much lost me on the above algorithm
(hex/decimal/string/integer mixups aside).

FWIW, here's an IP checksum routine:

def calcIPCheckSum(ipHeaders):
total = 0
for w in ipHeaders:
total += w
carries = total >16
sum = (total + carries) & 0xffff
return sum ^ 0xffff

print hex(calcIPCheckSum([0x0100,0xf203,0xf4f5,0xf6f7]))
A slightly more terse version:

import operator
def calcIPCheckSum(ipHeaders):
total = reduce(operator.add,ipHeaders)
carries = total >16
return ((total + carries) ^ 0xffff) & 0xffff
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Apr 1 '08 #2

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