// Tutorial //

How to Set Up SSH Keys on Ubuntu 20.04

Published on April 23, 2020
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By Brian Boucheron
Developer and author at DigitalOcean.
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How to Set Up SSH Keys on Ubuntu 20.04
Not using Ubuntu 20.04?Choose a different version or distribution.
Ubuntu 20.04

Introduction

SSH, or secure shell, is an encrypted protocol used to administer and communicate with servers. When working with an Ubuntu server, chances are you will spend most of your time in a terminal session connected to your server through SSH.

In this guide, we’ll focus on setting up SSH keys for an Ubuntu 20.04 installation. SSH keys provide a secure way of logging into your server and are recommended for all users.

Step 1 — Creating the Key Pair

The first step is to create a key pair on the client machine (usually your computer):

  1. ssh-keygen

By default recent versions of ssh-keygen will create a 3072-bit RSA key pair, which is secure enough for most use cases (you may optionally pass in the -b 4096 flag to create a larger 4096-bit key).

After entering the command, you should see the following output:

Output
Generating public/private rsa key pair. Enter file in which to save the key (/your_home/.ssh/id_rsa):

Press enter to save the key pair into the .ssh/ subdirectory in your home directory, or specify an alternate path.

If you had previously generated an SSH key pair, you may see the following prompt:

Output
/home/your_home/.ssh/id_rsa already exists. Overwrite (y/n)?

If you choose to overwrite the key on disk, you will not be able to authenticate using the previous key anymore. Be very careful when selecting yes, as this is a destructive process that cannot be reversed.

You should then see the following prompt:

Output
Enter passphrase (empty for no passphrase):

Here you optionally may enter a secure passphrase, which is highly recommended. A passphrase adds an additional layer of security to prevent unauthorized users from logging in. To learn more about security, consult our tutorial on How To Configure SSH Key-Based Authentication on a Linux Server.

You should then see the output similar to the following:

Output
Your identification has been saved in /your_home/.ssh/id_rsa Your public key has been saved in /your_home/.ssh/id_rsa.pub The key fingerprint is: SHA256:/hk7MJ5n5aiqdfTVUZr+2Qt+qCiS7BIm5Iv0dxrc3ks user@host The key's randomart image is: +---[RSA 3072]----+ | .| | + | | + | | . o . | |o S . o | | + o. .oo. .. .o| |o = oooooEo+ ...o| |.. o *o+=.*+o....| | =+=ooB=o.... | +----[SHA256]-----+

You now have a public and private key that you can use to authenticate. The next step is to place the public key on your server so that you can use SSH-key-based authentication to log in.

Step 2 — Copying the Public Key to Your Ubuntu Server

The quickest way to copy your public key to the Ubuntu host is to use a utility called ssh-copy-id. Due to its simplicity, this method is highly recommended if available. If you do not have ssh-copy-id available to you on your client machine, you may use one of the two alternate methods provided in this section (copying via password-based SSH, or manually copying the key).

Copying the Public Key Using ssh-copy-id

The ssh-copy-id tool is included by default in many operating systems, so you may have it available on your local system. For this method to work, you must already have password-based SSH access to your server.

To use the utility, you specify the remote host that you would like to connect to, and the user account that you have password-based SSH access to. This is the account to which your public SSH key will be copied.

The syntax is:

  1. ssh-copy-id username@remote_host

You may see the following message:

Output
The authenticity of host '203.0.113.1 (203.0.113.1)' can't be established. ECDSA key fingerprint is fd:fd:d4:f9:77:fe:73:84:e1:55:00:ad:d6:6d:22:fe. Are you sure you want to continue connecting (yes/no)? yes

This means that your local computer does not recognize the remote host. This will happen the first time you connect to a new host. Type “yes” and press ENTER to continue.

Next, the utility will scan your local account for the id_rsa.pub key that we created earlier. When it finds the key, it will prompt you for the password of the remote user’s account:

Output
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed /usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys username@203.0.113.1's password:

Type in the password (your typing will not be displayed, for security purposes) and press ENTER. The utility will connect to the account on the remote host using the password you provided. It will then copy the contents of your ~/.ssh/id_rsa.pub key into a file in the remote account’s home ~/.ssh directory called authorized_keys.

You should see the following output:

Output
Number of key(s) added: 1 Now try logging into the machine, with: "ssh 'username@203.0.113.1'" and check to make sure that only the key(s) you wanted were added.

At this point, your id_rsa.pub key has been uploaded to the remote account. You can continue on to Step 3.

Copying the Public Key Using SSH

If you do not have ssh-copy-id available, but you have password-based SSH access to an account on your server, you can upload your keys using a conventional SSH method.

We can do this by using the cat command to read the contents of the public SSH key on our local computer and piping that through an SSH connection to the remote server.

On the other side, we can make sure that the ~/.ssh directory exists and has the correct permissions under the account we’re using.

We can then output the content we piped over into a file called authorized_keys within this directory. We’ll use the >> redirect symbol to append the content instead of overwriting it. This will let us add keys without destroying previously added keys.

The full command looks like this:

  1. cat ~/.ssh/id_rsa.pub | ssh username@remote_host "mkdir -p ~/.ssh && touch ~/.ssh/authorized_keys && chmod -R go= ~/.ssh && cat >> ~/.ssh/authorized_keys"

You may see the following message:

Output
The authenticity of host '203.0.113.1 (203.0.113.1)' can't be established. ECDSA key fingerprint is fd:fd:d4:f9:77:fe:73:84:e1:55:00:ad:d6:6d:22:fe. Are you sure you want to continue connecting (yes/no)? yes

This means that your local computer does not recognize the remote host. This will happen the first time you connect to a new host. Type yes and press ENTER to continue.

Afterwards, you should be prompted to enter the remote user account password:

Output
username@203.0.113.1's password:

After entering your password, the content of your id_rsa.pub key will be copied to the end of the authorized_keys file of the remote user’s account. Continue on to Step 3 if this was successful.

Copying the Public Key Manually

If you do not have password-based SSH access to your server available, you will have to complete the above process manually.

We will manually append the content of your id_rsa.pub file to the ~/.ssh/authorized_keys file on your remote machine.

To display the content of your id_rsa.pub key, type this into your local computer:

  1. cat ~/.ssh/id_rsa.pub

You will see the key’s content, which should look something like this:

Output
ssh-rsa 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 demo@test

Access your remote host using whichever method you have available.

Once you have access to your account on the remote server, you should make sure the ~/.ssh directory exists. This command will create the directory if necessary, or do nothing if it already exists:

  1. mkdir -p ~/.ssh

Now, you can create or modify the authorized_keys file within this directory. You can add the contents of your id_rsa.pub file to the end of the authorized_keys file, creating it if necessary, using this command:

  1. echo public_key_string >> ~/.ssh/authorized_keys

In the above command, substitute the public_key_string with the output from the cat ~/.ssh/id_rsa.pub command that you executed on your local system. It should start with ssh-rsa AAAA....

Finally, we’ll ensure that the ~/.ssh directory and authorized_keys file have the appropriate permissions set:

  1. chmod -R go= ~/.ssh

This recursively removes all “group” and “other” permissions for the ~/.ssh/ directory.

If you’re using the root account to set up keys for a user account, it’s also important that the ~/.ssh directory belongs to the user and not to root:

  1. chown -R sammy:sammy ~/.ssh

In this tutorial our user is named sammy but you should substitute the appropriate username into the above command.

We can now attempt passwordless authentication with our Ubuntu server.

Step 3 — Authenticating to Your Ubuntu Server Using SSH Keys

If you have successfully completed one of the procedures above, you should be able to log into the remote host without providing the remote account’s password.

The basic process is the same:

  1. ssh username@remote_host

If this is your first time connecting to this host (if you used the last method above), you may see something like this:

Output
The authenticity of host '203.0.113.1 (203.0.113.1)' can't be established. ECDSA key fingerprint is fd:fd:d4:f9:77:fe:73:84:e1:55:00:ad:d6:6d:22:fe. Are you sure you want to continue connecting (yes/no)? yes

This means that your local computer does not recognize the remote host. Type “yes” and then press ENTER to continue.

If you did not supply a passphrase for your private key, you will be logged in immediately. If you supplied a passphrase for the private key when you created the key, you will be prompted to enter it now (note that your keystrokes will not display in the terminal session for security). After authenticating, a new shell session should open for you with the configured account on the Ubuntu server.

If key-based authentication was successful, continue on to learn how to further secure your system by disabling password authentication.

Step 4 — Disabling Password Authentication on Your Server

If you were able to log into your account using SSH without a password, you have successfully configured SSH-key-based authentication to your account. However, your password-based authentication mechanism is still active, meaning that your server is still exposed to brute-force attacks.

Before completing the steps in this section, make sure that you either have SSH-key-based authentication configured for the root account on this server, or preferably, that you have SSH-key-based authentication configured for a non-root account on this server with sudo privileges. This step will lock down password-based logins, so ensuring that you will still be able to get administrative access is crucial.

Once you’ve confirmed that your remote account has administrative privileges, log into your remote server with SSH keys, either as root or with an account with sudo privileges. Then, open up the SSH daemon’s configuration file:

  1. sudo nano /etc/ssh/sshd_config

Inside the file, search for a directive called PasswordAuthentication. This line may be commented out with a # at the beginning of the line. Uncomment the line by removing the #, and set the value to no. This will disable your ability to log in via SSH using account passwords:

/etc/ssh/sshd_config
. . .
PasswordAuthentication no
. . .

Save and close the file when you are finished by pressing CTRL+X, then Y to confirm saving the file, and finally ENTER to exit nano. To actually activate these changes, we need to restart the sshd service:

  1. sudo systemctl restart ssh

As a precaution, open up a new terminal window and test that the SSH service is functioning correctly before closing your current session:

  1. ssh username@remote_host

Once you have verified your SSH service is functioning properly, you can safely close all current server sessions.

The SSH daemon on your Ubuntu server now only responds to SSH-key-based authentication. Password-based logins have been disabled.

Conclusion

You should now have SSH-key-based authentication configured on your server, allowing you to sign in without providing an account password.

If you’d like to learn more about working with SSH, take a look at our SSH Essentials Guide.

We’ve made it super easy to add SSH Keys to your new or existing DigitalOcean virtual machines.

Learn more here


About the authors
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Developer and author at DigitalOcean.

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don’t disable password authentication before you make sure ssh auth is working correctly. I got caught and cant log back in.

This is not helpful at all.

Nice article. Is there a PDF, Sphinx or Word version for offline use ?

Thank you for this tutorial to setup key based authentication. I have one comment, if we disable the Password Authentication for SSH, it will not allow to establish a ssh session on the server as it doesn’t have public keys in the authorized keys for its own users. I solved this by creating keys using ssh-keygen and copying the public key to authorized keys list.

I had some trouble with copying the intended public key when specifying a different path other than id_rsa (for example, I named it blah_key, and it lived at ~/.ssh/blah_key). Every time I attempted to copy via ssh-copy-id utility, it would grab the key under the default path. The following command helped me set the intended key as an authorized key on the server:

ssh-copy-id -i ~/.ssh/mykey user@host

TLDR; if a user chose to write their key to a different path other than the default (id_rsa), here’s how you copy that public key from that path.

this was a bit complex for me, so i followed this and it worked for me https://www.digitalocean.com/docs/droplets/how-to/add-ssh-keys/create-with-putty/

FYI: On Ubuntu 20.04 and if you are using SSH Keys, after you create your keys and provision the DO server, the username for login is root.

For example:

me@my-workstaton:~$ ssh root@138.197.173.34

Once on the DO server, I can create a regular account and I can add the contents of my id_rsa.pub file to the new account.

I then should be able to login my new account on the DO server.

I then add that user account to the sudoer’s file.

Great article however how do I remove the password from the created user? So, I can only SSH into it like root.

im trying to migrate from another host to DO and this is keeping me for days from migrating my websites from the other host. i’ve been trying literally everything and still no success to connect to root in fact yesterday i created a sudo user (as shown in this tutorial) but i couldn’t edit files when connected via ftp client with that user. then i tried to connect to root again, and guess what: “the almighty PERMISSION DENIED”. i’ve been deleting around 20 droplets and created new keys on my computer, then copied the keys to the authorized_keys file, and when i try to login to root the PERMISSION DENIED is haunting me even in my dreams, man! i knew DO is a bit complicated but i never thought i would stuck at this point which had to be so straight forward. is there anything you could help me with this in a step by step basis because i cant even articulate a question because i tried a lot of things and none seems to work. thanks!