SSH key login

SSH key login

Let’s generate and use SSH keys on our computer. This allows us to ensure our identity better than a password ever could. This allows us to do two main things:

  1. Password-less login: With SSH keys, we can permanently designate our profile on our local computer as safe for our server, allowing us to bypass password verification when logging into our server.
  2. Prevent hacking: Since we no longer need a password to log in, we can simply deactivate password logins on our server altogether, which prevents hacking from people who may be so lucky as to guess our password!

In other words, using an SSH key to login is both safer, faster and easier.

This is especially useful once you start making scripts on your computer that interact with your server. You can upload files in the background, edit your spam filters or anything else from your local computer without having to input your password each time you touch the server.

Generate an SSH key pair

Generating an SSH key is simple. Just run:

ssh-keygen

It will prompt you for several options and you can generally chose the default options in each case. It will ask you to optionally include a password on your SSH key. I generally recommend against this unless you happen to be using a computer where you don’t have root access but someone else does (it does minimize the ease of using an SSH key in our case).

What does this SSH key do?

Now whenever you use ssh to log into a server, you have the public key of this SSH key pair as your identifier. You can tell your server to trust this key and it will automatically allow password-less logins from this computer.

Backing up your key

We will do that momentarily, but first, I recommend you backup your newly generated key if you plan to use it. If we disable logins to this one key and then lose the key, we might be locked out of our server.

I suggest copying your entire ~/.ssh/ directory (user-specific) to a USB drive and storing it securely. You may also copy it to the same place on another computer to use the key there.

Making your server trust your key.

Now that you have generated an SSH key, just run the following:

ssh-copy-id root@yourdomain.com

The command will ask for your server’s root password and log you in briefly. What this does is that it puts your public SSH key fingerprint on your server in a file /root/.ssh/authorized_keys. This file in turn allows approved SSH keys to log in without passwords.

Note that you can also replace root with a username of an account on the server if you had made a non-root user that you’d like to easily log into as well. For the username user, it will also store the key in /home/user/.ssh/authorized_keys.

To test if this has worked, now try logging in normally to your server with ssh:

ssh root@yourdomain.com

It should now let you log in without a password prompt!

If you find that this does not work try running the following, make sure you are in the directory where the keys where created.

chmod 700 ~/.ssh/
chmod 644 ~/.ssh/id_rsa.pub
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/authorized_keys

For whatever reason these files due not have the correct permissions set, as ssh is very picky about correct file permissions this can cause errors. The above will fix these.

Disabling Password Logins for Security

Once we have authorized ssh keys for all the devices we need, we can actually just disable password logins. If you’ve ever looked at your system logs (journalctl -xe) you will find that there are always hundreds of random Chinese computers trying to brute force every server connected to the internet with random passwords. They are usually unsuccessful, but let’s make it impossible for them.

Log into your server and open the /etc/ssh/sshd_config file. Here we can set settings for our SSH daemon that receives SSH requests.

Now find, uncomment or create the following three lines and set them all to no:

PasswordAuthentication no
ChallengeResponseAuthentication no
UsePAM no

Once we’ve done that, we will reload our SSH daemon:

systemctl reload sshd

We’re done!

Now you can log in quickly and password-less-ly to your server, despite the fact that it is now more secure than ever!

With these settings, even if a hacker steals or perfectly guesses an account password, they still cannot log in without an approved SSH key!

What if I lose my SSH key?!

Firstly, don’t do this. Take every precaution that you have a backup.

If this does happen, Vultr and most other VPS providers will have a way out. Log onto their website and select the server you want to log into.

In the image above, to the right of your VPS name are a series of icons. Click on the computer screen-like icon which is the leftmost one.

This will open up a browser window emulating a terminal and you can always login with your password here, since logins here count as being local—they do not use SSH and therefore can indeed validate with your password even if you have disabled it over SSH.

From here, simply reverse the settings we set above and you can log in via SSH with a password and reapprove a newly created SSH key or whatever you want to do.

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