Category: Linux

Linux related posts

How to Fix SSH error: WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED

If the fingerprint of your remote host changes you will see the following error message appear:

 ~ % ssh -i "mykey.pem" user_id@host.af-south-1.compute.amazonaws.com
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ED25519 key sent by the remote host is
SHA256:S60CvpE17ri+E594StxXBQcNIrga4Nb7uX4s7BPr3dw.
Please contact your system administrator.
Add correct host key in /Users/user_id/.ssh/known_hosts to get rid of this message.
Offending ED25519 key in /Users/user_id/.ssh/known_hosts:2
Host key for my_host.af-south-1.compute.amazonaws.com has changed and you have requested strict checking.
Host key verification failed.

There are many ways to fix this. The easiest of which is simply to delete your “known_hosts” file. This will mean you just need to accept new finger prints on all your SSH hosts. Yes, this is very lazy…

rm ~/.ssh/known_hosts
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Tip: Using the Watch command to poll a URL

If you want to quickly test a URL for changes, then the linux Watch command couple with Curl is a really simple way to hit a URL every n seconds (I use this for blue/green deployment testing to make sure there is no downtime when cutting over):

# Install watch command using homebrew
brew install watch
# Poll andrewbaker.ninja every 1 seconds
watch -n 1 curl andrewbaker.ninja
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How to trigger Scaling Events using Stress-ng Command

If you are testing how your autoscaling policies respond to CPU load then a really simple way to test this is using the “stress” command. Note: this is a very crude mechanism to test and wherever possible you should try and generate synthetic application load.

#!/bin/bash

# DESCRIPTION: After updating from the repo, installs stress-ng, a tool used to create various system load for testing purposes.
yum update -y
# Install stress-ng
sudo apt install stress-ng

# CPU spike: Run a CPU spike for 5 seconds
uptime
stress-ng --cpu 4 --timeout 5s --metrics-brief
uptime

# Disk Test: Start N (2) workers continually writing, reading and removing temporary files:
stress-ng --disk 2 --timeout 5s --metrics-brief

# Memory stress test
# Populate memory. Use mmap N bytes per vm worker, the default is 256MB. 
# You can also specify the size as % of total available memory or in units of 
# Bytes, KBytes, MBytes and GBytes using the suffix b, k, m or g:
# Note: The --vm 2 will start N workers (2 workers) continuously calling 
# mmap/munmap and writing to the allocated memory. Note that this can cause 
# systems to trip the kernel OOM killer on Linux systems if not enough 
# physical memory and swap is not available
stress-ng --vm 2 --vm-bytes 1G --timeout 5s

# Combination Stress
# To run for 5 seconds with 4 cpu stressors, 2 io stressors and 1 vm 
# stressor using 1GB of virtual memory, enter:
stress-ng --cpu 4 --io 2 --vm 1 --vm-bytes 1G --timeout 5s --metrics-brief

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