A simple network client and server to push files and command-line access

import sys
import socket
import getopt
import threading
import subprocess

# define some global variables
listen = False
command = False
upload = False
execute = ""
target = ""
upload_destination = ""
port = 0


# this runs a command and returns the output
def run_command(cmd):
    # trim the newline
    cmd = cmd.rstrip()

    # run the command and get the output back
    try:
        output = subprocess.check_output(cmd, stderr=subprocess.STDOUT,
                                         shell=True)
    except subprocess.CalledProcessError as e:
        output = e.output

    # send the output back to the client
    return output


# this handles incoming client connections
def client_handler(client_socket):
    global upload
    global execute
    global command

    # check for upload
    if len(upload_destination):

        # read in all of the bytes and write to our destination
        file_buffer = ""

        # keep reading data until none is available
        while True:
            data = client_socket.recv(1024)

            if not data:
                break
            else:
                file_buffer += data

        # now we take these bytes and try to write them out
        try:
            file_descriptor = open(upload_destination, "wb")
            file_descriptor.write(file_buffer.encode('utf-8'))
            file_descriptor.close()

            # acknowledge that we wrote the file out
            client_socket.send(
                "Successfully saved file to %s\r\n" % upload_destination)
        except OSError:
            client_socket.send(
                "Failed to save file to %s\r\n" % upload_destination)

    # check for command execution
    if len(execute):
        # run the command
        output = run_command(execute)

        client_socket.send(output)

    # now we go into another loop if a command shell was requested
    if command:

        while True:
            # show a simple prompt
            client_socket.send(" ".encode('utf-8'))

            # now we receive until we see a linefeed (enter key)
            cmd_buffer = b''
            while b"\n" not in cmd_buffer:
                cmd_buffer += client_socket.recv(1024)

            # we have a valid command so execute it and send back the results
            response = run_command(cmd_buffer.decode())

            # send back the response
            client_socket.send(response)


# this is for incoming connections
def server_loop():
    global target
    global port

    # if no target is defined we listen on all interfaces
    if not len(target):
        target = "0.0.0.0"

    server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    server.bind((target, port))
    server.listen(5)

    while True:
        client_socket, addr = server.accept()

        # spin off a thread to handle our new client
        client_thread = threading.Thread(target=client_handler,
                                         args=(client_socket,))
        client_thread.start()


# if we don't listen we are a client... make it so.
def client_sender(buffer):
    client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

    try:
        # connect to our target host
        client.connect((target, port))

        # if we detect input from stdin send it
        # if not we are going to wait for the user to punch some in
        if len(buffer):
            client.send(buffer.encode('utf-8'))

        while True:
            # now wait for data back
            recv_len = 1
            response = b''

            while recv_len:
                data = client.recv(4096)
                recv_len = len(data)
                response += data

                if recv_len  4096:
                    break

            print(response.decode('utf-8'), end=' ')

            # wait for more input
            buffer = input("")
            buffer += "\n"

            # send it off
            client.send(buffer.encode('utf-8'))

    except socket.error as exc:
        # just catch generic errors - you can do your homework to beef this up
        print("[*] Exception! Exiting.")
        print(f"[*] Caught exception socket.error: {exc}")

        # teardown the connection
        client.close()


def usage():
    print("Netcat Replacement")
    print()
    print("Usage: bhpnet.py -t target_host -p port")
    print(
        "-l --listen                - listen on [host]:[port] for incoming "
        "connections")
    print(
        "-e --execute=file_to_run   - execute the given file upon receiving "
        "a connection")
    print("-c --command               - initialize a command shell")
    print(
        "-u --upload=destination    - upon receiving connection upload a file "
        "and write to [destination]")
    print()
    print()
    print("Examples: ")
    print("bhpnet.py -t 192.168.0.1 -p 5555 -l -c")
    print("bhpnet.py -t 192.168.0.1 -p 5555 -l -u=c:\\target.exe")
    print("bhpnet.py -t 192.168.0.1 -p 5555 -l -e=\"cat /etc/passwd\"")
    print("echo 'ABCDEFGHI' | ./bhpnet.py -t 192.168.11.12 -p 135")
    sys.exit(0)


def main():
    global listen
    global port
    global execute
    global command
    global upload_destination
    global target

    if not len(sys.argv[1:]):
        usage()

    # read the commandline options
    try:
        opts, args = getopt.getopt(sys.argv[1:], "hle:t:p:cu:",
                                   ["help", "listen", "execute", "target",
                                    "port", "command", "upload"])
        for o, a in opts:
            if o in ("-h", "--help"):
                usage()
            elif o in ("-l", "--listen"):
                listen = True
            elif o in ("-e", "--execute"):
                execute = a
            elif o in ("-c", "--commandshell"):
                command = True
            elif o in ("-u", "--upload"):
                upload_destination = a
            elif o in ("-t", "--target"):
                target = a
            elif o in ("-p", "--port"):
                port = int(a)
            else:
                assert False, "Unhandled Option"

    except getopt.GetoptError as err:
        print(str(err))
        usage()

    # are we going to listen or just send data from STDIN?
    if not listen and len(target) and port > 0:
        # read in the buffer from the commandline
        # this will block, so send CTRL-D if not sending input
        # to stdin
        buffer = sys.stdin.read()

        # send data off
        client_sender(buffer)

    # we are going to listen and potentially
    # upload things, execute commands and drop a shell back
    # depending on our command line options above
    if listen:
        server_loop()


main()