StrFixedLenField trailing null stripping

[X-Cli]

Hey,

I am currently writing a packet that contains a padding field, in the middle of a list of fields. This padding length is defined by an arithmetic expression w.r.t. the other fields own sizes and values.

After considering the various options, I thought that a StrFixedLenField was the best fit. StrField is used by the Raw Packet to store arbitrary data, but StrField consumes all remaining data, unless a non-null remain value is defined. In my case, I can compute the size of the padding, and using a remain value equal to the size of the remaining fields seems clumsy.

Unfortunately, when the padding is null-terminated or composed of null-bytes, a call to show() displays mangled packets where the padding trailing null bytes are stripped.
I traced the problem to the following excerpt:

https://github.com/secdev/scapy/blob/master/scapy/fields.py#L503

if type(v) is str:
            v = v.rstrip("\0")

The same display-only alteration is also performed here:
https://github.com/secdev/scapy/blob/master/scapy/fields.py#L524

r = v.rstrip("\0")

Could you explain me the rationale behing this behavior? I can't find any test that breaks when commenting these lines, which otherwise prevent the usage of StrFixedLenField as a "Raw"-like field of fixed size.

Easiest way to reproduce the "bug" is:

from scapy.all import *

class TestPacket(Packet):
  name = 'TestPacket'
  fields_desc = [ StrFixedLenField('data', '', length_from=lambda x: 4)]

TestPacket("tototiti").show()
TestPacket("\Foo\x00"*2).show()
TestPacket("\x00"*8).show()

Thanks.

Cheers,
Florian

[p-l-]

That's not a bug. That's how we (often) want fixed-length strings to be represented, since they would be zero-padded, and the padding is not interesting.

[X-Cli]

Ok. Would you prefer if I wrote a RawFixedLenField in fields.py or within my (contrib) module?

[mwWZ2NYF]

@X-Cli Just in case, have a look at this PR: #446 . The XStrField / XStrLenField / XStrFixedLenField may help you.

[X-Cli]

@plorinquer Excellent! I certainly could use these fields, once they are merged. Thanks!

[gpotter2]

@p-l- This Issue may be closed too :)