Set up an automatic SSH login on Linux

Suppose you want to connect to/from a remote Linux machine (that might as well be a virtual machine) via SSH, but without having to type in the password each time.

In the following, I will be referring to:

• the main machine: the machine from which you want to SSH to the remote machine without typing in the password;
• the remote machine: the machine you are SSH-ing into from the main machine.

Create a pair of SSH keys

In order to log in via SSH without typing in the password, the remote machine only needs to know the main machine via its public SSH key.

From the main machine, generate a SSH key pair to use for logging in to the remote machine:

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ssh-keygen -t rsa

Save the key pair as ~/.ssh/id_rsa and ~/.ssh/id_rsa.pub.

Do NOT provide a passphrase if the intention is to use this automatic login for automated tasks (e.g. for scripts where SSH is used indirectly, such as when copying files or pushing to a git repository).

Make the remote know the main machine

This is done by copying the public key of the main machine to the remote machine.

If the ~/.ssh directory does not exist on remote, create it first. Then append the public key that was generated during the previous step to the list of known hosts for remote. From main:

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cat ~/.ssh/id_rsa.pub | ssh user@remote 'cat >> ~/.ssh/authorized_keys'

Test the automatic login

Now, from main, you should be able to SSH to the remote machine without having to type in the password:

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ssh user@remote

Caveat

Creating a SSH key pair without a passphrase means the private key remains unencrypted. If somebody has access to the main machine or manages to exploit a vulnerability and gains access to it, the private SSH key is just out there in plain sight.