SSH Extras

How to setup SSH Key Authentication

To enable SSH key authentication in Linux, you need to generate a public authentication key and append it to the remote host’s ~/.ssh/authorized_keys file.

The following steps describe the process for configuring key authentication login:

Check for an existing SSH key pair.

Before generating a new SSH key pair, first check if you already have an SSH key on your client machine, because you don’t want to overwrite your existing keys.

Run the following ls command to see if existing SSH keys are present:
Terminal window
```
ls -al ~/.ssh/*.pub
```
If there are existing keys, you can either use those and skip the next step or generate a new one.

If you see No such file or directory or no matches found, it means that you do not have an SSH key and you can proceed with the next step and generate a new one.
Generate a new SSH key pair.

The following command will generate a new 4096-bit SSH key pair with your email address as a comment:
Terminal window
```
ssh-keygen -t ed25519 -b 4096 -C "your_email@domain.com"
```
Press Enter to accept the default file location and file name, or append the -f flag to set a custom name, for example:
Terminal window
```
ssh-keygen -t ed25519 -b 4096 -C "your_email@domain.com" -f ~/.ssh/my_ed25519_key
```
Next, the ssh-keygen tool will ask you to type a secure passphrase. Whether you want to use a passphrase is up to you, but we highly recommend you set a strong passphrase. If you don’t want to use a passphrase, just press Enter.

To be sure that the SSH keys are generated, you can list your new private and public keys with:
Terminal window
```
ls ~/.ssh/id_*
```
Copy the public key.

Now that you have generated an SSH key pair, in order to be able to log in to your server you need to copy the public key to the server you want to manage.

The easiest way to copy your public key to your server is to use a command called ssh-copy-id. On your local machine terminal, type:
Terminal window
```
ssh-copy-id remote_username@server_ip_address
```
You will be prompted to enter the remote_username password. Once the user is authenticated, the public key will be appended to the remote user’s authorized_keys file and the connection will be closed.

If for some reason the ssh-copy-id utility is not available on your local computer, you can use the following command to copy the public key:
Terminal window
```
cat ~/.ssh/id_rsa.pub | ssh remote_username@server_ip_address "mkdir ~/.ssh && chmod 700 ~/.ssh && cat >> ~/.ssh/authorized_keys && chmod 600 ~/.ssh/authorized_keys"
```
Log in to your server using SSH keys.

After completing the steps above, you should be able to log in to the remote server. To test it, just try to log in to your server via SSH:
Terminal window
```
ssh -i ~/.ssh/id_ed25519.pub remote_username@server_ip_address
```
If everything went well, you will be logged in after entering your passphrase.