Trouble converting hex to decimal?

[Earl Eiland]

I'm trying to process the IP packet length field, as recorded by pcap
(Ethereal) and recovered using pcapy. When I slice out those bytes, I
get a value that shows in '\x00' format, rather than '0x00'. Neither
int() nor eval() are working. How do I handle this?

Earl Eiland

[Steve Holden]

Re: Trouble converting hex to decimal?

Earl Eiland wrote:
[color=blue]
> I'm trying to process the IP packet length field, as recorded by pcap
> (Ethereal) and recovered using pcapy. When I slice out those bytes, I
> get a value that shows in '\x00' format, rather than '0x00'. Neither
> int() nor eval() are working. How do I handle this?
>
> Earl Eiland
>[/color]

You could either reconstruct the value yourself from the ord() values of
the individual bytes, or look at the struct module which can help you
with this kind of decoding task.

regards
Steve
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[Alan McIntyre]

Re: Trouble converting hex to decimal?

Earl,

Try this:
[color=blue][color=green][color=darkred]
>>> ord('\x00')[/color][/color][/color]
0

or:
[color=blue][color=green][color=darkred]
>>> import struct
>>> struct.unpack(' b', '\x00')[/color][/color][/color]
(0,)

If you're needing to pull values out of multiple bytes (shorts, longs,
floats, etc.), have a look at the struct module. Here's an example:
[color=blue][color=green][color=darkred]
>>> struct.unpack(' f', '\x00\x00(B')[/color][/color][/color]
(42.0,)

Hope this helps,
Alan

Earl Eiland wrote:[color=blue]
> I'm trying to process the IP packet length field, as recorded by pcap
> (Ethereal) and recovered using pcapy. When I slice out those bytes, I
> get a value that shows in '\x00' format, rather than '0x00'. Neither
> int() nor eval() are working. How do I handle this?
>
> Earl Eiland
>[/color]

[Jorgen Grahn]

Re: Trouble converting hex to decimal?

On Sat, 05 Feb 2005 06:51:32 -0700, Earl Eiland <eee@nmt.edu> wrote:[color=blue]
> I'm trying to process the IP packet length field, as recorded by pcap
> (Ethereal) and recovered using pcapy. When I slice out those bytes, I
> get a value that shows in '\x00' format, rather than '0x00'. Neither
> int() nor eval() are working. How do I handle this?[/color]

From my unpublished protocol analyzing hack:

class Ip:
"IPv4 header"
def __init__(self, frame):
(vi, tos, tlen,
id, ffoff,
ttl, proto, checksum,
source,
dest) = struct.unpack(' ! BBH HH BBH LL', frame[:20])
self.len = 4 * (vi & 0xf)
if proto==6:
self.proto=Tcp
elif proto==17:
self.proto=Udp
elif proto==1:
self.proto=Icmp
self.source = Address(source)
self.dest = Address(dest)

That doesn't take IP options into account (or are they part of Ip.len? I
forget.), but it has the nifty feature that IP.proto tells the caller what
factory function (if any) she should feed the rest of the frame into.

/Jorgen

--
// Jorgen Grahn <jgrahn@ Ph'nglui mglw'nafh Cthulhu
\X/ algonet.se> R'lyeh wgah'nagl fhtagn!