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Amritansh Raghav amritansh at mobilian.com
Wed Jan 17 16:58:11 EST 2001 
Thanks
The reason I didnt want to do it in C was to allow the parser to be extended
easily. I did look at the pack and unpack and they seem to do what I want. I
just need to build the dictionary now

"Donn Cave" <donn at u.washington.edu> wrote in message
news:9452jt$s8e$1 at nntp6.u.washington.edu...
> Quoth "Amritansh Raghav" <amritansh at mobilian.com>:
>
> | So here is what I need to do. I'm trying to write a scripting tool for
our
> | test team. They wish to send arbitrary packets over the network.  What
I'd
> | like to do is allow the testers to specify any packet in a text file -
say
> | something like this:
> | <IP>
> |     "VersionLength"          0        1    # Name of field, Starting
offset,
> | Length in bytes
> |     "TOS"                        1        1
> |     "Length"                     2        2
> |     "Id"                            4        2
> |     "Offset"                      6        2
> |     "TTL"                        8        1
> |     "ProtoId"                    9        1
> |     CheckSum"                10        2
> |     "Source"                   12        4
> |     "Dest"                        16        4
> | </IP>
> |
> | Once they have specified this, the tester should be able to create a
packet
> | an assign values to the field, or receive a packet and parse out its
> | contents.
> | so a script should be able to say
> | p = read()
> | if p["ProtoId"] == 17:
> |     # do something here for UDP packets
> |
> | Solution
> |     I have written a DLL  which can open/close/read/write to an ethernet
> | device. I've got it interfacing to Python. It can accept a buffer and
return
> | a buffer. I've looked at socketmodule.c to get most of that figured out.
I
> | assume what I need to do is build a dictionary whose keys are the field
> | names of a packet and then write a parser to fill in the values into the
> | dictionary. I also need something to parse the text file description.
> |
> | If someone has done something like this before, or can point me to
helpful
> | scripts and source, I would be grateful.
>
> If I understand this, I think at the lowest level the options are few.
> I think since you're already writing C extensions, you might find it
> simpler to make the buffer into a tuple there, but if you want to do
> it in Python, you can use struct.unpack.  See your documentation, or
> print struct.__doc__.
>
> Then it's up to you how to map this tuple to the field names.  I think
> one candidate would be a class ...
>
>    class Packet:
>        def __init__(self, buffer = None):
>            if buffer is None:
>                self.VersionLength = 1
>                ... # default values
>            else:
>                self.unpack(buffer)
>        def unpack(self, buffer):
>            self.VersionLength, ... = struct.unpack('B...', buffer)
>        def pack(self):
>            return struct.pack('B...', self.VersionLength, ...)
>
>    packet = Packet(buffer)
>    packet.ProtoId = 17
>    buffer = packet.pack()
>
> Donn Cave, donn at u.washington.edu
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