A situation where you provision an EC2 server with the selected key-pair provided (and downloaded by you) but you want to use another SSH key-pair to authenticate into the server ?

I got you.

A little backstory:

I am trying to give GitHub Actions an SSH key pair that it can use to connect to and run some commands on a private EC2 server I have provisioned. In order for this to happen, I have to put a public SSH key in the EC2 server and provide the GitHub Actions runner a private key that it can use to connect and authenticate into the server and run my deployment scripts.

I will walk you through the steps:

I am assuming you already have an EC2 server provisioned and its .pem file ready on your local machine.

Here is a twist; you try to login with the command provided by AWS to connect to the instance and you get the error like this:

$ ssh -i "aws-key.pem" ubuntu@ec2-19-30-68-234.eu-central-2.compute.amazonaws.com
Received disconnect from 19-30-68-234 port 22:2: Too many authentication failures
Disconnected from 19-30-68-234 port 22

What this means is that, even when a specific SSH key is to be used in authenticating, the ssh-agent rejects the connection and the SSH client aborts with the above error.

To fix this error, we have to enforce a rule for the ssh-agent to use the specified key to open a connection to the server.

Add this to the ~/.ssh/config file:

Host *
      IdentitiesOnly=yes

Whether you have an already existing config file ready or not, this will work. If you have config file, you must navigate to the Host * portion and add that line to the config there. Save the file.

After this, try authenticating again, It should connect.

Now back to the reson why I am writing this:

  1. generate your alternate SSH key you want to use to authenticate into the server. 
    ssh-keygen -t ed25519 -f ~/.ssh/mysshkey -C me@local.com
  2. There should be two new keys in the .ssh file, mysshkey and mysshkey.pub. The goal here is to move the public key into the EC2 instance’s SSH authorized keys. A neat little command to use is this: 
    cat mysshkey.pub | ssh -i "aws-key.pem" ubuntu@19-30-68-234 'cat >> .ssh/authorized_keys'

    The command takes the output of your public key, opens a connection to the EC2 instance and adds the contents to the authorized_keys file in the EC2 instance.

  3. Connect to the server with the new SSH key 
    ssh -i "mysshkey" ubuntu@19-30-68-234

In this extremely short write-up, I have addressed SSH authentication failure issue with a resolution and I have shown how to add an SSH key to an EC2 server.

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