Getting Started

Installation

Suitcase may be installed directly from PyPi using pip:

pip install suitcase

This will install the library as well as any libraries depended on by Suitcase.

The library is tested to work on the following python versions:

• Python 2.7
• Python 3.2
• Python 3.3
• Python 3.4

Data Modeling

Suitcase provides a declarative syntax for recording the structure of the messages in your problem domain. The first step is to understand those messages and convert each message layer into its corresponding suitcase.structure.Structure s and suitcase.fields.BaseField s.

Typically, you will end up with one structure for each layer of a protocol stack, if that is your domain. For instance, let’s say that we wanted to parser and/or pack DNS messsages.

With just a little reading, we see that each DNS request and response has a common form which can be declared in Suitcase with the following structure:

import binascii
from suitcase.fields import Payload
from suitcase.fields import UBInt16
from suitcase.fields import BitNum
from suitcase.fields import BitBool
from suitcase.fields import BitField
from suitcase.structure import Structure

class DNSMessage(Structure):
    identification = UBInt16()
    fields = BitField(16,
                      is_reply=BitBool(),  # QR
                      opcode=BitNum(4),
                      truncated=BitBool(),
                      recursion_desired=BitBool(),
                      ra=BitBool(),
                      z=BitBool(),
                      non_authenticated_data_acceptable=BitBool(),
                      cd=BitBool(),
                      rcode=BitNum(4),
                  )
    total_questions = UBInt16()
    total_answers_rrs = UBInt16()
    total_authority_rrs = UBInt16()
    total_additional_rrs = UBInt16()
    data = Payload()  # greedy

Although there is a fair bit here, you can see that it is generally easy to follow and about as simple as the table you can find on the Wikipedia page describing the same message.

Note

This example is based on a quick review of the DNS message format and may not be correct. Please create a github issue if you believe there is a problem with this example.

Using the Model

The structure above provides Suitcase with a wealth of information about the names, sizes, order, and relationship between different elements in a DNSMessage. That knowledge is enough so that Suitcase can now both pack (generate bytes from and object) and parse (generate an object from bytes) for our message.

Parsing

For some input data, I opened up wireshark and then navigated to docs.digi.com in my web browser. Looking through wireshark, I can see the request and copy the hex for the DNS portion of the message:

dns_request_hex = (
    b"28b30100000100000000000004646f63730464696769"
    b"03636f6d0000010001")
dns_request_data = binascii.unhexlify(dns_request_hex)
print(DNSMessage.from_data(dns_request_data))

This generates the following based on the repr of the returned instance of our structure:

DNSMessage (
  identification=10419,
  fields=BitField(
    is_reply=False,
    opcode=0,
    truncated=False,
    recursion_desired=False,
    ra=True,
    z=False,
    non_authenticated_data_acceptable=False,
    cd=False,
    rcode=0,
  ),
  total_questions=1,
  total_answers_rrs=0,
  total_authority_rrs=0,
  total_additional_rrs=0,
  data='\x04docs\x04digi\x03com\x00\x00\x01\x00\x01',
)

Packing

In the same way that a stream of bytes can be used to populate an instance of our structure, we can also build up a DNSMessage and turn it into bytes. This is a little tedious since DNSMessage has quite a few fields, but it is very easy to see what is going on.

m = DNSMessage()
m.identification = 0x1234
m.fields.is_reply = False
m.fields.opcode = 0
m.fields.truncated = False
m.fields.recursion_desired = True
m.fields.ra = False
m.fields.z = False
m.fields.non_authenticated_data_acceptable = False
m.fields.cd = False
m.fields.rcode = 0
m.total_questions = 1
m.total_answers_rrs = 0
m.total_authority_rrs = 0
m.total_additional_rrs = 0
m.data = b"docs.digi.com\x00\x00\x00\x00"

print(repr(m.pack()))

This generates a string like this:

'\x124\x02\x00\x00\x01\x00\x00\x00\x00\x00\x00docs.digi.com\x00\x00\x00\x00'

Working With Streams

In addition to being able to parse and pack messages, Suitcase also includes helpers that make working with stream-based protocols (e.g. TCP, Serial Port, etc.) much easier. See the API documentation for more details.