Python Paramiko module

Introduction:

Figure [1] Raspberry Pi

A computer by definition is a digital electronic machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically & looks like the image above. As we can see, its not possible to input commands and receive output from the physical device itself so we need some form of input and output device attachments.

This is where a 'computer terminal' comes in, its a device that can be used for entering data into, & transcribing data from a computer. Early computer terminals looked like the image below. You can see some familiar elements from modern day computers such as a keyboard in the image.

Figure [2] Early computer terminal VT100

A later improvement to the computing world was the concept of 'time-sharing' which is the sharing of a computing resource among many users at the same time by means of multiprogramming and multi-tasking.

Main Section:

A typical modern day data center looks like this:

Figure [3] Data center

It has rows and rows of servers with networking gear and power supply units all in a highly redundant configuration to optimize uptime. Each server has the capability of running one or more operating systems on it. It is not feasible to provide a terminal to each of these servers. This is where 'terminal emulator' software comes in.

Terminal emulator (TTY) emulates a terminal, providing input and output allowing users to provide input and receive output without having to directly attach a terminal to the server.

Pseudo-terminals (PTY) - A pair of pseudo-device endpoints (files) establish asynchronous, bidirectional communication (IPC - Inter-process communication) channel (with two ports) between two or more processes. The master provides means by which a terminal emulator process controls the slave. The slave, emulates a hardware text terminal device. PTY are similar to bidirectional pipes.

PTYs are used for implementing remote-login programs such as SSHD, in which data read from the pseudo terminal master is sent across the network to a client program that is connected to a terminal or terminal emulator.

SSH:

SSH (Secure Shell Protocol) is a cryptographic network protocol for operating network services securely over an unsecured network. An SSH client program is typically used for establishing connections to an SSH daemon accepting remote connections.

Figure [4] SSH key based authentication

The command to 'SSH' into a server remotely looks like the follows:
ssh <username>@<ip_addr or hostname>

Once we have logged into a remote machine, we can then execute commands on them. We can even transfer files using SFTP or SCIP protocols.


Working with Paramiko to SSH into devices remotely:

Paramiko is a pure-Python implementation of the SSHv2 protocol, providing both client and server functionality. It provides the foundation for the high-level SSH library Fabric, which is what we recommend you use for common client use-cases such as running remote shell commands or transferring files.

To install Paramiko on a machine with Python (3.4+) and Pip (PyPy) installed with all dependencies:

pip install paramiko[all]

Here's an example from linode documentation of how to connect and run commands on a remote system:

import paramiko

# Update the next three lines with your

# server's information

host = "YOUR_IP_ADDRESS"

username = "YOUR_LIMITED_USER_ACCOUNT"

password = "YOUR_PASSWORD"


client = paramiko.client.SSHClient()

client.set_missing_host_key_policy(paramiko.AutoAddPolicy())

client.connect(host, username=username, password=password)

_stdin, _stdout,_stderr = client.exec_command("df")

print(stdout.read().decode())

client.close()

One of the issues that I came across recently was to configure a FortiMail Virtual Machine on which the operation looks like the following:

config system interface

  edit <port>

  set ip <ip_address> <netmask>

  set allowaccess (http https ping ssh telnet)

end

The problem with running this as separate 'stdin, stdout, stderr = client.exec_command(command)' is that every time there is a new session setup and thus the ip address & allowaccess lines which are actually a sub-menu of the port setting are lost. So the command ended up looking like this:

stdin, stdout, stderr = client.exec_command("config system interface\n edit <port>\n set ip <ip_address> <netmask>\n set allowaccess http https ping ssh telnet")

---

References:

[1] https://en.wikipedia.org/wiki/Computer_terminal

[2] https://en.wikipedia.org/wiki/Time-sharing

[3] https://en.wikipedia.org/wiki/Terminal_emulator

[4] Recommended read -> TTY Demystified - http://www.linusakesson.net/programming/tty/

[5] https://en.wikipedia.org/wiki/Pseudoterminal

[6] https://en.wikipedia.org/wiki/Secure_Shell

[7] https://docs.paramiko.org/en/stable/

[8] https://www.linode.com/docs/guides/use-paramiko-python-to-ssh-into-a-server/

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