I am working with SCAPY for dissecting network packets. Some of them have a lot of random padding between strings.
Is there a way to tell SCAPY to just "skip the next 4 bytes after this field"?
Currently I am using a StrFixedLen field for every one of them and it just blows up the output and is very space consuming, and distracting...
I can't use the padded fields because I need to provide a padding byte, but in my case the padding consists of random bytes..
Thanks for any advice!
EDIT: here an example of a packet
x01x07x08x01
x13
ThisIsAString
x02x04x01x01
x10
AnotherOne
The packet is TLV ordered, so the 4-bytes are the Tag, the 5th one is the Legnth of the string, and then there's the Value (string). As there are many tags and I am not and will not be able to distinguish them all, I just want to extract the Length and Value (the string) parts of the packet data. So I want to ignore the 4-byte Tags. Currently I am doing this by declaring a StrLenField which just reads 4 bytes. But this is obviously then shown in the summary of the packet, because it's interpreted as a legitimate field. But I just want to have it ignored, so that SCAPY just skips the 4 bytes of a TLV-field when dissecting.
EDIT 2:
Here is a code snippet of what I am currently doing vs what I want to do:
StrFixedLenField("Padding_1", None, length=4),
FieldLenField("Field_1_Length", None, length_of="Field_1"),
StrLenField("Field_1", "", length_from=lambda x: x.Field_1_Length),
StrFixedLenField("Padding_2", None, length=4),
FieldLenField("Field_2_Length", None, length_of="Field_2"),
StrLenField("Field_2", "", length_from=lambda x: x.Field_2_Length)
Instead of the StrFixedLenField I'd like to have something like skip_bytes(4) so that SCAPY completely ignores these bytes.
Based on this GitHub fork, in which the author added a functionality that is somewhat similar to what I was searching for, I found a solution.
I added a class to fields.py that just inherits from the field class that I want to hide, which in my case it StrFixedLenField:
class StrFixedLenFieldHidden(StrFixedLenField):
pass
Then in packet.py in the definition of _show_or_dump(...) in the for loop which iterates through the fields of the packets, I added this:
for f in self.fields_desc:
if isinstance(f, StrFixedLenFieldHidden):
continue
...
By this, Scapy just ignores the field when doing the summary. In my case it was also important to hide the fields in the PDF dump. This can be done by adding the same lines to the for loop in the do_ps_dump(...) function in packet.py.
Keep in mind that now when running .show() or .show2() Scapy will not show these fields anymore, ALTHOUGH they are still there, so the packet will be properly assembled when sending the packet with sr(...) or whatever.
Related
I see there is way i can get a specific custom section with given name.
var nameSections = WebAssembly.Module.customSections(module, "sec_name");
Is there any way I can get all the custom sections in a given WebAssembly module?
The JavaScript interface at this time does not offer that. The customSection function was discussed and approved here. There is currently an opened issue to address the question you asked here. Generally this issue status at this time is that providing such an API would bring too much complexities (not so much about the sections as a list, but about the content of each). That makes sense to me, because the WebAssembly standard is currently in very active development (many post-MVP features are in progress).
That said, it seems that you are left off to parse the binary by yourself. The binary format is documented here. Basically, after you parse the magic number and the version there is a list of sections: 1 byte ID, one u32 (LEB128 format with allowed trailing zeroes) for the section's byte content length and then are the bytes of the section's content (with that length). After this header and the content bytes, is the next section, if any, till the end.
The custom sections have an id of 0. The content of these sections starts with a name and after it are the section "user" bytes. This name is a vector of bytes. Each vector has a length (again u32) followed by that number of bytes that must be the UTF-8 encoded string.
So the pseudo-code is something like this:
parse the magic
parse the version
loop till there are bytes
read the section id
read the section u32 length
if id = 0 then
read the name u32 length
skip the name bytes
use the user bytes till the end of the section
jump to the end of the section
I am new to rust and creating a POC to convert dbf file to csv. I am reading a .dbf file using rust library dbase.
The issue is, when i crate a sample .dbf file using dbfview the code works fine. But when i use .dbf file which i will be using in real time. I am getting the following error.
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: InvalidFieldType('M')', src/libcore/result.rs:999:5
Here is the code i am using from the given link.
use dbase::FieldValue;
let records = dbase::read("tests/data/line.dbf").unwrap();
for record in records {
for (name, value) in record {
println!("{} -> {:?}", name, value);
match value {
FieldValue::Character(string) => println!("Got string: {}", string),
FieldValue::Numeric(value) => println!("Got numeric value of {}", value),
_ => {}
}
}
}
I think the ^M shows the character appended by windows.
What can i do to handle this error and read the file successfully.
Any help will be much appreciated.
The short answer to your question is no, you will not be able to read this file with dbase-rs (or any current library) and you'll most likely have to rework this file to not contain a memo field.
A deep dive into the DBF file format
The InvalidFieldType error points at a structural feature of the file that your library cannot handle - a Memo field. We're going to deep-dive into the file to figure out why that is, and whether there is anything we can do to fix it.
This is the header definition:
Of particular importance is byte 28 (offset 0000010, byte 0C), which is a bitmask indicating if the table contains a bunch of possible things, most notably:
0x01 if the file comes with an associated .cdx file
0x02 if it contains a memo
0x04 if the file is actually a .dbc file (a database)
At 0x03, your file comes with both an associated .cdx file and contains a memo. As we know (ahead of time) that dbase-rs does not handle that, that's looking increasingly more likely.
Let's keep looking. From here on, each field is 32 bytes long.
Here are your fields:
Bytes 0-10 contain the field name, byte 11 is the type. Due to how the library you wanted to use can only parse certain fields, we only really care about byte 11.
In order of appearance by what the library can parse:
[x] CALL_ID (integer)
[x] CONTACT_ID (integer)
[x] CALL_DATE (DateTime)
[x] SUBJECT (char[])
[ ] NOTES (memo)
The last field is the problematic one. Looking into the library itself, this field type is not supported and will therefore yield an Error, which you are trying to unwrap(). This is the source of your error.
There are two three ways around it:
The "long" way is to patch the library to handle memo fields. This sounds easy, but in practice it really isn't. As the memos are stored in another file (typically a dbt file in the same folder), you're going to have to make that library read both files and reference both. The point of the memo type itself is to store more than 255 bytes of data in a field. You are the only one able to evaluate whether this work is worth the effort.
If your data is less than 255 bytes in size, you can replace that memo field with a char field, and dbfview should allow you to do this
If your field is longer than 255 bytes and you have access to the ability to run sub-processes (i.e. Command::run), you can sneak-convert it using a library that can process Memo fields in another language. this nodeJS library can, but read-only, for example.
I want to create and later send a 5 byte struct like this:
import struct
struct.pack("?i", True, 0x01020304)
>>b'\x01\x00\x00\x00\x04\x03\x02\x01'
but as you see the 1 byte boolean get's padded by 3 bytes or filled up to an integer for some reason.
what I want as a result is:
>>b'\x01\x04\x03\x02\x01'
How can I do this and why does my solution not work? It seems to be correctly used according to the documentation.
My problem was solved by an answer to this question. The = character at the start of the format string tells the pack method to not align the data but to produce a byte string of exactly the specified length. Whether a format character allows alignment or not is specified in this chapter. Somehow I missed that.
I am using Pex from the command line to find input values for test case generation.
I use PexObserve to record certain values during execution.
One of the values that I want to record is an XML-String.
However, when parsing the XML I receive "malformed XML" exceptions, since Pex only writes the first 255 characters into the log.
Is there a way to record the full XML string? or does PexObserve have a different type that will let me record longer texts?
Leaving this here, in case somebody at any point has the same issue.
I've found a solution that helped me.
Unfortunately the 255 character limit is set internally in static readonly fields.
Therefore I needed to use reflection.
My solution works by including the following line in the PUT:
typeof(Microsoft.Pex.Framework.PexObserve.ValueWriterManager).GetField("MaxWrittenElements").SetValue(null, 1000);
Replace the 1000 with any value you like.
BUT: remember that this is a quick-fix solution, that might not work for you.
It may have unwanted side-effects. You're also changing the number of List elements that are written, and perhaps other things.
This is a Python 101 type question, but it had me baffled for a while when I tried to use a package that seemed to convert my string input into bytes.
As you will see below I found the answer for myself, but I felt it was worth recording here because of the time it took me to unearth what was going on. It seems to be generic to Python 3, so I have not referred to the original package I was playing with; it does not seem to be an error (just that the particular package had a .tostring() method that was clearly not producing what I understood as a string...)
My test program goes like this:
import mangler # spoof package
stringThing = """
<Doc>
<Greeting>Hello World</Greeting>
<Greeting>你好</Greeting>
</Doc>
"""
# print out the input
print('This is the string input:')
print(stringThing)
# now make the string into bytes
bytesThing = mangler.tostring(stringThing) # pseudo-code again
# now print it out
print('\nThis is the bytes output:')
print(bytesThing)
The output from this code gives this:
This is the string input:
<Doc>
<Greeting>Hello World</Greeting>
<Greeting>你好</Greeting>
</Doc>
This is the bytes output:
b'\n<Doc>\n <Greeting>Hello World</Greeting>\n <Greeting>\xe4\xbd\xa0\xe5\xa5\xbd</Greeting>\n</Doc>\n'
So, there is a need to be able to convert between bytes and strings, to avoid ending up with non-ascii characters being turned into gobbledegook.
The 'mangler' in the above code sample was doing the equivalent of this:
bytesThing = stringThing.encode(encoding='UTF-8')
There are other ways to write this (notably using bytes(stringThing, encoding='UTF-8'), but the above syntax makes it obvious what is going on, and also what to do to recover the string:
newStringThing = bytesThing.decode(encoding='UTF-8')
When we do this, the original string is recovered.
Note, using str(bytesThing) just transcribes all the gobbledegook without converting it back into Unicode, unless you specifically request UTF-8, viz., str(bytesThing, encoding='UTF-8'). No error is reported if the encoding is not specified.
In python3, there is a bytes() method that is in the same format as encode().
str1 = b'hello world'
str2 = bytes("hello world", encoding="UTF-8")
print(str1 == str2) # Returns True
I didn't read anything about this in the docs, but perhaps I wasn't looking in the right place. This way you can explicitly turn strings into byte streams and have it more readable than using encode and decode, and without having to prefex b in front of quotes.
This is a Python 101 type question,
It's a simple question but one where the answer is not so simple.
In python3, a "bytes" object represents a sequence of bytes, a "string" object represents a sequence of unicode code points.
To convert between from "bytes" to "string" and from "string" back to "bytes" you use the bytes.decode and string.encode functions. These functions take two parameters, an encoding and an error handling policy.
Sadly there are an awful lot of cases where sequences of bytes are used to represent text, but it is not necessarily well-defined what encoding is being used. Take for example filenames on unix-like systems, as far as the kernel is concerned they are a sequence of bytes with a handful of special values, on most modern distros most filenames will be UTF-8 but there is no gaurantee that all filenames will be.
If you want to write robust software then you need to think carefully about those parameters. You need to think carefully about what encoding the bytes are supposed to be in and how you will handle the case where they turn out not to be a valid sequence of bytes for the encoding you thought they should be in. Python defaults to UTF-8 and erroring out on any byte sequence that is not valid UTF-8.
print(bytesThing)
Python uses "repr" as a fallback conversion to string. repr attempts to produce python code that will recreate the object. In the case of a bytes object this means among other things escaping bytes outside the printable ascii range.
TRY THIS:
StringVariable=ByteVariable.decode('UTF-8','ignore')
TO TEST TYPE:
print(type(StringVariable))
Here 'StringVariable' represented as a string. 'ByteVariable' represent as Byte. Its not relevent to question Variables..