Selective download and extraction of data (CAB) - python-3.x

Question

So I have a specific need to download and extract a cab file but the size of each cab file is huge > 200MB. I wanted to selectively download files from the cab as rest of the data is useless.
Done so much so far :
Request 1% of the file from the server. Get the headers and parse them.
Get the files list, their offsets according to This CAB Link.
Send a GET request to server with the Range header set to the file Offset and the Offset+Size.
I am able to get the response but it is in a way "Unreadable" cause it is compressed (LZX:21 - Acc to 7Zip)
Unable to decompress using zlib. Throws invlid header.
Also I did not quite understand nor could trace the CFFOLDER or CFDATA as shown in the example cause its uncompressed.
totalByteArray =b''
eofiles =0
def GetCabMetaData(stream):
global eofiles
cabMetaData={}
try:
cabMetaData["CabFormat"] = stream[0:4].decode('ANSI')
cabMetaData["CabSize"] = struct.unpack("<L",stream[8:12])[0]
cabMetaData["FilesOffset"] = struct.unpack("<L",stream[16:20])[0]
cabMetaData["NoOfFolders"] = struct.unpack("<H",stream[26:28])[0]
cabMetaData["NoOfFiles"] = struct.unpack("<H",stream[28:30])[0]
# skip 30,32,34,35
cabMetaData["Files"]= {}
cabMetaData["Folders"]= {}
baseOffset = cabMetaData["FilesOffset"]
internalOffset = 0
for i in range(0,cabMetaData["NoOfFiles"]):
fileDetails = {}
fileDetails["Size"] = struct.unpack("<L",stream[baseOffset+internalOffset:][:4])[0]
fileDetails["UnpackedStartOffset"] = struct.unpack("<L",stream[baseOffset+internalOffset+4:][:4])[0]
fileDetails["FolderIndex"] = struct.unpack("<H",stream[baseOffset+internalOffset+8:][:2])[0]
fileDetails["Date"] = struct.unpack("<H",stream[baseOffset+internalOffset+10:][:2])[0]
fileDetails["Time"] = struct.unpack("<H",stream[baseOffset+internalOffset+12:][:2])[0]
fileDetails["Attrib"] = struct.unpack("<H",stream[baseOffset+internalOffset+14:][:2])[0]
fileName =''
for j in range(0,len(stream)):
if(chr(stream[baseOffset+internalOffset+16 +j])!='\x00'):
fileName +=chr(stream[baseOffset+internalOffset+16 +j])
else:
break
internalOffset += 16+j+1
cabMetaData["Files"][fileName] = (fileDetails.copy())
eofiles = baseOffset + internalOffset
except Exception as e:
print(e)
pass
print(cabMetaData["CabSize"])
return cabMetaData
def GetFileSize(url):
resp = requests.head(url)
return int(resp.headers["Content-Length"])
def GetCABHeader(url):
global totalByteArray
size = GetFileSize(url)
newSize ="bytes=0-"+ str(int(0.01*size))
totalByteArray = b''
cabHeader= requests.get(url,headers={"Range":newSize},stream=True)
for chunk in cabHeader.iter_content(chunk_size=1024):
totalByteArray += chunk
def DownloadInfFile(baseUrl,InfFileData,InfFileName):
global totalByteArray,eofiles
if(not os.path.exists("infs")):
os.mkdir("infs")
baseCabName = baseUrl[baseUrl.rfind("/"):]
baseCabName = baseCabName.replace(".","_")
if(not os.path.exists("infs\\" + baseCabName)):
os.mkdir("infs\\"+baseCabName)
fileBytes = b''
newRange = "bytes=" + str(eofiles+InfFileData["UnpackedStartOffset"] ) + "-" + str(eofiles+InfFileData["UnpackedStartOffset"]+InfFileData["Size"] )
data = requests.get(baseUrl,headers={"Range":newRange},stream=True)
with open("infs\\"+baseCabName +"\\" + InfFileName ,"wb") as f:
for chunk in data.iter_content(chunk_size=1024):
fileBytes +=chunk
f.write(fileBytes)
f.flush()
print("Saved File " + InfFileName)
pass
def main(url):
GetCABHeader(url)
cabMetaData = GetCabMetaData(totalByteArray)
for fileName,data in cabMetaData["Files"].items():
if(fileName.endswith(".txt")):
DownloadInfFile(url,data,fileName)
main("http://path-to-some-cabinet.cab")
All the file details are correct. I have verified them.
Any guidance will be appreciated. Am I doing it wrong? Another way perhaps?
P.S : Already Looked into This Post

Answer 1

First, the data in the CAB is raw deflate, not zlib-wrapped deflate. So you need to ask zlib's inflate() to decode raw deflate with a negative windowBits value on initialization.
Second, the CAB format does not exactly use standard deflate, in that the 32K sliding window dictionary carries from one block to the next. You'd need to use inflateSetDictionary() to set the dictionary at the start of each block using the last 32K decompressed from the last block.