lillypad lillypad - 1 month ago 29
Python Question

Writing pcap to Buffer with Scapy

I'm having trouble writing a pcap to a file buffer, it's important I do not touch disk for these pcap captures and yes they must be live.

sudo scapy
>>> import io
>>> cap = sniff(timeout=30)
>>> buf = io.BytesIO()
>>> wrpcap(buf, cap)
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "/usr/lib/python2.7/dist-packages/scapy/utils.py", line 524, in wrpcap
with PcapWriter(filename, *args, **kargs) as fdesc:
File "/usr/lib/python2.7/dist-packages/scapy/utils.py", line 682, in __init__
self.f = [open,gzip.open][gz](filename,append and "ab" or "wb", gz and 9 or bufsz)
TypeError: coercing to Unicode: need string or buffer, _io.BytesIO found


This usually happens when you do open(None), is this a bug in the PcapWriter function in Scapy Utils?

I've also tried this with:
buf.seek(0)
before writing and it still fails.

Answer

I got code from scapy (utils.py) and created memwrpcap which can write to io.BytesIO.

buf = io.BytesIO()
memwrpcap(buf, cap)

(after writing it doesn't close buffer and you can move to beginning to read from buffer.)

After that I used standard open() and write() to save data from io.BytesIO and compared this file with file created with wrpcap

diff -c test-std.pcap test-mem.pcap

and it seems they are identical so io.BytesIO has data in pcap format.

Full code - memwrpcam, MemoryPcapWriter and code which I used to test it.

#
# from: scapy/utils.py
#

from scapy.all import *

def memwrpcap(filename, pkt, *args, **kargs):
    """Write a list of packets to a pcap file
    gz: set to 1 to save a gzipped capture
    linktype: force linktype value
    endianness: "<" or ">", force endianness"""

    # use MemoryPcapWriter instead of PcapWriter
    with MemoryPcapWriter(filename, *args, **kargs) as fdesc:
        fdesc.write(pkt)


class MemoryPcapWriter(PcapWriter):
    """A stream PCAP writer with more control than wrpcap()"""
    def __init__(self, filename, linktype=None, gz=False, endianness="", append=False, sync=False):
        """
        linktype: force linktype to a given value. If None, linktype is taken
                  from the first writter packet
        gz: compress the capture on the fly
        endianness: force an endianness (little:"<", big:">"). Default is native
        append: append packets to the capture file instead of truncating it
        sync: do not bufferize writes to the capture file
        """

        self.linktype = linktype
        self.header_present = 0
        self.append=append
        self.gz = gz
        self.endian = endianness
        self.filename=filename
        self.sync=sync
        bufsz=4096
        if sync:
            bufsz=0

        # use filename or file-like object 
        if isinstance(self.filename, str):
            self.f = [open,gzip.open][gz](filename,append and "ab" or "wb", gz and 9 or bufsz)
        else: # file-like object 
            self.f = filename

    def __exit__(self, exc_type, exc_value, tracback):
        self.flush()
        if isinstance(self.filename, str):
            self.close() # don't close file-like object


# --- main ---

#
# run script with sudo
#
# compare results (on Linux)
#    diff -s test-std.pcap test-mem.pcap
#

from scapy.all import *

import io

cap = sniff(timeout=5)

# save to pcap file
wrpcap('test-std.pcap', cap)

# save to buffer
buf = io.BytesIO()
memwrpcap(buf, cap)

# move to beginning and save to file
#print('current position:', buf.tell())
buf.seek(0)
#print('current position:', buf.tell())

with open('test-mem.pcap', 'wb') as fp:
    fp.write(buf.read())
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