manipulating hex values

[Stephen Cattaneo]

Thanks to everyone ( Grant, Cliff, and Gabriel) for responding and
helping me.

Cheers,

Steve

-----Original Message-----
From: Grant Edwards [mailto:grante at visi.com] 
Sent: Tuesday, April 01, 2008 7:46 PM
To: python-list at python.org
Subject: Re: manipulating hex values

On 2008-04-01, Stephen Cattaneo <stephen.cattaneo at u4eatech.com> wrote:

>>> I am relatively new to socket programming.  I am attempting to
>>> use raw sockets to spoof my IP address.
>>
>> Don't bother to try...
>
> Is there a better solution to spoofing my IP. then using raw
> sockets 

You'll have to define "spoofing my IP", but I suspect that what
you're trying can't be done by using raw sockets.

> (I'm working on a machine with multiple interfaces and need to
> be able to some how specify which interface that traffic needs
> to be sent/recieved to/from)

That's what routing tables are for.

If you want to send packets using a particular IP, then
configure an interface so that it has that adrress, and then
bind your socket to that address.

> The source of my confusion is that I need to keep my bytes formated 
> correctly.   I am using the below  'raw socket example'
proof-of-concept 
> code as my example.

OK.

> (And yes, I have tried the proof-of-concept.  It works
> correctly.  It is not my code.)

I know.  It's my code. :) I wrote Python's raw socket support
code and the example code that is floating around the 'net.

> dstAddr = "\x01\x02\x03\x04\x05\x06" 
> dstAddr1 = "0x010203040506"
> dstAddr != dstAddr1

Right.

> Follow up question: What is the best to store my bytes up
> until sending the packets?

That depends on what you want to do with them.  Ultimately,
they need to be strings when they're sent out the socket (in
Python a "string" is really just an array of 8-bit bytes).  The
best way to store them is entirely dependent on how you want to
manipulate them before they're sent.

> Perhaps I should use lists of decimal numbers and then 
> before sending convert to hex. 

Again: you're not converting them to hex.  You're converting
them to a python "string" object which is really just an array
of bytes.  Stop saying "hex" when you're talking about a string
(array of bytes).  "hex" is a way to _represent_ a value
textually. It's simply a format used when print a value on
paper or a screen. The value itself isn't hex any more than a
particular instance of Canis lupus familiaris is "English"
because somebody spells it "dog" instead of "chien" or "Hund".

> I.E. dstAddr = [ 1, 2, 3, 4, 5, 6]
> dstAddr.prepareToSend()

I presume you know that list objects don't have a method called
prepareToSend().

> txFrame = struct.pack("!6s6sh",dstAddr,srcAddr,proto) + ethData
>
> Is there a better way to do this?

It's not at all clear what "this" is.  If you want to convert
from a list (or any sequence, really) of integer objects to a
string where each integer is converted into a single byte
within the string, then here are a few ways:

  import operator
  import struct

  intlist = [1,2,3,4,5,6]
  
  s1 = ''.join(chr(b) for b in intlist)
  s2 = reduce(operator.add,[chr(b) for b in intlist])
  s3 = struct.pack("6B",*tuple(intlist))

  print repr(s1)
  print repr(s2)
  print repr(s3)

  print s1==s2
  print s2==s3

When run you get this:

'\x01\x02\x03\x04\x05\x06'
'\x01\x02\x03\x04\x05\x06'
'\x01\x02\x03\x04\x05\x06'
True
True
  
I think the third alternative using struct.pack() is the most
readable, but others will no doubt disagree.

-- 
Grant Edwards                   grante             Yow!  I'm GLAD I
                                  at               remembered to XEROX
all
                               visi.com            my UNDERSHIRTS!!