Creating a Python Replacement for the Test-Connection cmdlet.

I’ve been working on a PowerShell script to automatically power up and shut down all devices in my home lab. Both in the interest of saving on hydro and to reduce it’s environmental impact. To do this, I’ve been using a low-power Raspberry Pi 3 B+, which will stay powered on to orchestrate these and other scripted activities.

One great PowerShell cmdlet that comes in handy is Test-Connection. Although it can be used for numerous network testing purposes, it’s most commonly used to send an ICMP echo request to a host to see if it responds or not. The cmdlet responds with a simple true or false response, which makes it very handy for scripting purposes. I was really hoping to use this cmdlet because it makes it easy to determine if a device is still online or not and if it’s okay to move on to the next phase of powering things on or shutting things down. For example, I don’t want to power on my management ESXi host until the freenas SAN is online.

The problem I ran into was that not all PowerShell cmdlets have been ported over to PowerShell Core for Linux. Test-Connection is unfortunately one of them due to the way it uses the Microsoft Windows network stack to function. Looking at the GitHub PR, it seems they are getting close to porting it over, but I decided to try my hand at creating a python script with similar basic functionality.

I should note that I’m not a programmer by any stretch of the imagination and it’s been many years since I’ve done any serious coding. This is actually the first bit of python code I’ve written, so I’m sure there are many more efficient ways to achieve what I’ve done here:

import os
import sys

# If no arguments parsed, display the greeting:
if len(sys.argv) == 1:
    print("vswitchzero.com ICMP response script. Specify up to 3 hosts to ping separated by spaces.")
    print("example: python ./pinghost.py 172.16.10.15 192.168.1.1 vc.lab.local")

# If one arg parsed, ping a single host
if len(sys.argv) == 2:
    host1 = sys.argv[1]
    response = os.system("ping -c 1 " + host1 + "> /dev/null")
    if response == 0:
        print host1, 'is responding'
    else:
        print host1, 'is not responding'

# If two args parsed, ping two hosts
if len(sys.argv) == 3:
    host1 = sys.argv[1]
    host2 = sys.argv[2]
    response1 = os.system("ping -c 1 " + host1 + "> /dev/null")
    if response1 == 0:
        print host1, 'is responding'
    else:
        print host1, 'is not responding'
    response2 = os.system("ping -c 1 " + host2 + "> /dev/null")
    if response2 == 0:
        print host2, 'is responding'
    else:
        print host2, 'is not responding'

# If three args parsed, ping three hosts
if len(sys.argv) == 4:
    host1 = sys.argv[1]
    host2 = sys.argv[2]
    host3 = sys.argv[3]
    response1 = os.system("ping -c 1 " + host1 + "> /dev/null")
    if response1 == 0:
        print host1, 'is responding'
    else:
        print host1, 'is not responding'
    response2 = os.system("ping -c 1 " + host2 + "> /dev/null")
    if response2 == 0:
        print host2, 'is responding'
    else:
        print host2, 'is not responding'
 response3 = os.system("ping -c 1 " + host3 + "> /dev/null")
    if response3 == 0:
        print host3, 'is responding'
    else:
        print host3, 'is not responding'

# If more than three args parsed, display an error
if len(sys.argv) > 4:
    print('vswitchzero.com ICMP response script. Specify up to 3 hosts to ping separated by spaces.')
    print('example: python ./pinghost.py 172.16.10.15 192.168.1.1 vc.lab.local')
    print(' ')
    print('ERROR: Too many arguments specified. Use 1-3 IPs or hostnames only. Each should be separated by a space')

The script is simple. You specify up to three space separated IP addresses or host names as command line arguments, and the script will send one ICMP echo request to each of the hosts. Depending on the response, it will output either ‘is responding’ or ‘is not responding’. Below is an example:

pi@raspberrypi:~/scripts $ python pinghost.py vc.lab.local 172.16.10.67 172.16.10.20
vc.lab.local is not responding
172.16.10.67 is responding
172.16.10.20 is not responding

Then using this script, I can create sleep loops in PowerShell to wait for one or more devices to become responsive before proceeding. Here is a simple ‘do until’ loop that utilizes the script:

To wait for a single host to power down, I can do the following:

do
{
    $pingresult = python ~/scripts/pinghost.py 172.16.10.20
    $pingresult
    sleep 5
} until($pingresult -like '*not responding*')

Or to wait for all three hosts in my compute cluster to power up:

do
{
    $pingresult = python ~/scripts/pinghost.py 172.16.10.21 172.16.10.22 172.16.10.23
    $pingresult
    sleep 5
} until($pingresult -NotLike '*not responding*')

It’s not quite as clean as using the Test-Connection cmdlet, but it is functional. Best of all, it was a great ‘intro to python’ for me. I’ll hopefully get a post up soon once I’ve got my startup and shutdown scripts completed.

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