Python Pexpect属性错误。 'NoneType'没有属性'sendline'

Question

Writing a script using Pexpect to connect via ssh but it is throwing an attribute error.

import pexpect
PROMPT = ['# ', '>>> ', '> ', '\$ ', '~# ']
def send_command(child, cmd):
    child.sendline(cmd)
    child.expect(PROMPT)
    print child.before, child.after
def connect(user, host, password):
    ssh_newkey = 'Are you sure you want to continue connecting (yes/no)?'
    connStr = 'ssh ' + user + '@' + host
    child = pexpect.spawn(connStr)
    ret = child.expect([ssh_newkey, 'password:'])
    if ret == 0:
        print '[-] Error Connecting'
        return
    elif ret == 1:
        child.sendline('yes')
        ret = child.expect('password:')
        if ret == 0:
            print '[-] Error Connecting'
            return
    child.sendline(password)
    child.expect(PROMPT)
    return child
def main():
    host = 'test.rebex.net'
    user = 'demo'
    password = 'password'
    child = connect(user, host, password)
    send_command(child, 'cat /etc/shadow | grep root')
if __name__ == '__main__':
    main()

I am getting the following error:

[-] Error Connecting
Traceback (most recent call last):
  File "./bruteSSH.py", line 33, in <module>
    main()
  File "./bruteSSH.py", line 31, in main
    send_command(child, 'cat /etc/shadow | grep root')
  File "./bruteSSH.py", line 6, in send_command
    child.sendline(cmd)
AttributeError: 'NoneType' object has no attribute 'sendline'

I believe it has something to do with my child object being a 'NoneType' but I am not able to pin down what I am doing wrong.

Answer 1

First of all your indentation is wrong on the 6 line.

It's causing this error because the child object has not been setup yet properly and connected successfully.

If this is exactly your code then the problem is that "child.sendline()" is executed outside the function whereas child is a local variable inside the function "send_command" so globally the child variable has not yet been defined

Answer 2

You don't return a value on a couple of conditions. That's where you're getting your None and what is causing your error. See the commented lines below:

if ret == 0:
    print '[-] Error Connecting'
    return  # THIS WILL CAUSE YOUR ERROR

elif ret == 1:
    child.sendline('yes')
    ret = child.expect('password:')
    if ret == 0:
        print '[-] Error Connecting'
        return  # THIS WILL ALSO CAUSE YOUR ERROR

But your logic is flawed anyway. Expect returns a 0 or the index of the match if you pass it an array. In your code, your passing it an array. So a return value of a 0 indicates that it successfully matched your first entry-- the "Are you sure" condition. If you match that you'd want to send the "yes". Below is more what I think you're after...

import pexpect
PROMPT = ['# ', '>>> ', '> ', '\$ ', '~# ']
def send_command(child, cmd):
    child.sendline(cmd)
    child.expect(PROMPT)
    print child.before, child.after

def connect(user, host, password):
    ssh_newkey = 'Are you sure you want to continue connecting (yes/no)?'
    connStr = 'ssh ' + user + '@' + host
    child = pexpect.spawn(connStr)
    ret = child.expect(['password:', ssh_newkey])
    if ret == 1:
        child.sendline('yes')
        ret = child.expect('password:')
    if ret != 0:
        print '[-] Error Connecting'
        return  # THIS WILL RETURN A NONE SO YOU SHOULD CHECK FOR IT.  SHOULD EXPLICITLY DO A return None TO MAKE IT CLEARER
    child.sendline(password)
    child.expect(PROMPT)
    return child

def main():
    host = 'localhost'
    user = 'demo'
    password = 'password'
    child = connect(user, host, password)
    if child is not None:
        send_command(child, 'cat /etc/shadow | grep root')
    else:
        print "Problem connecting!"

if __name__ == '__main__':
    main()

Answer 3

The problem is right in front of you. When you encounter an error in the connect function as you are as shown by the "[*] Error Connection" print statement, you return nothing. Only if the connection was successful it returns the child object, but as the connection failed you return a "Null Object" and exit out of your function. You are not able to make a successful connection and hence the child object is never returned to your "child" Variable in your main Function. And you pass this same "Null Object" to your send_command() and hence does not work

import sys

def connect(user, host, password):
    ssh_newkey = 'Are you sure you want to continue connecting (yes/no)?'
    connStr = 'ssh ' + user + '@' + host
    child = pexpect.spawn(connStr)
    ret = child.expect([ssh_newkey, 'password:'])
    if ret == 0:
        print '[-] Error Connecting'
        sys.exit()
    elif ret == 1:
        child.sendline('yes')
        ret = child.expect('password:')
        if ret == 0:
            print '[-] Error Connecting'
            sys.exit()
    child.sendline(password)
    child.expect(PROMPT)
    return child

Now your program will only proceed if the connection was successful. Maybe the expects and password maybe wrong, the overall problem is you are not able to make a successful connection