The first step in setting up the SSH Keys is to create a key pair on the client machine (generally on a computer):
$ ssh-keygen
For starters the ssh-keygen will create an 2048-bit RSA key pair, this will secure enough for most use cases.
As an alternative you can optionally pass in the -b 4096 flag to create a larger 4096-bit key
After entering the command, you should be able to see the following prompt:
Output
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 a custom alternate path. The following prompt should appear:
Output
Enter passphrase ( leave empty for no passphrase):
Optionally you 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 following output:
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:
a9:49:2e:2a:5e:33:3e:a9:de:4e:77:11:58:b6:90:26 username@remote_host
The key's randomart image is:
+--[ RSA 2048]----+
| ..o |
| E o= . |
| o. o |
| .. |
| ..S |
| o o. |
| =o.+. |
|. =++.. |
|o=++. |
+-----------------+
You should now have a public and private key that you can use to authenticate. The next step in the process is to place the public key on your server so that you can use SSH-key-based authentication to log in.
The fastest way to copy your public key to the Ubuntu host is to use a utility called ssh-copy-id. 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).
The ssh-copy-id tool is included by default in many operating systems, you may have it available on your local system. For it to work, you will need to have a password-based SSH access to your server. To use the utility, you need to specify the remote host that you would like to connect to and simply add the user account that you have password SSH access to. This will be the account to which your public SSH key will be copied.
The syntax is:
$ ssh-copy-id username@remote_host
You may see the following message:
Output
The authenticity of host '111.111.11.111 (111.111.11.111)' 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 did not recognize the remote host. This most likely 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@111.111.11.111's password:
Type in the password (note that 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.
After that you should see the following output:
Output
Number of key(s) added: 1
Logging into the machine, with: "ssh 'username@111.111.11.111'"
and check to make sure that only the key you wanted was added.
At this point in the process, your id_rsa.pub key has been uploaded to the remote account.
If you do not have the 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 pipe that through an SSH connection to the remote server. On the other side, we can make sure that the ~/.ssh directory exists under the account we are using and then output the content we piped over into a file called authorized_keys within this directory.
We will 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:
cat ~/.ssh/id_rsa.pub | ssh username@remote_host "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"
You may see the following message:
Output
The authenticity of host '111.111.11.111 (111.111.11.111)' 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@111.111.11.111'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.
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:
cat ~/.ssh/id_rsa.pub
You will see the key's content, which should look something like this:
Output
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQCqql6MzstZYh1TmWWv11q5O3pISj2ZFl9HgH1JLknLLx44+tXfJ7mIrKNxOOwxIxvcBF8PXSYvobFYEZjGIVCEAjrUzLiIxbyCoxVyle
7Q+bqgZ8SeeM8wzytsY+dVGcBxF6N4JS+zVk5eMcV385gG3Y6ON3EG112n6d+SMXY0OEBIcO6x+PnUSGHrSgpBgX7Ks1r7xqFa7heJLLt2wWwkARptX7udSq05paBhcpB0pHtA1Rfz3K2B+ZVIpSDfki9UVKzT8JUmwW6NNzSgxUfQHGwnW7kj4jp4AT0VZk3ADw497M2G
/12N0PPB5CnhHf7ovgy6nL1ikrygTKRFmNZISvAcywB9GVqNAVE+ZHDSCuURNsAInVzgYo9xgJDW8wUw2o8U77+xiFxgI5QSZX3Iq7YLMgeksaO4rBJEa54k8m5wEiEE1nUhLuJ0X/vh2xPff6SQ1BL/zkOhvJCACK6Vb15mDOeCSq54Cr7kvS46itMosi/uS66+PujOO+xt/2FWYepz6ZlN70bRly57Q06J+ZJoc9FfBCbCyYH7U
/ASsmY095ywPsBo1XQ9PqhnN1/YOorJ068foQDNVpm146mUpILVxmq41Cj55YKHEazXGsdBIbXWhcrRf4G2fJLRcGUr9q8/lERo9oxRm5JFX6TCmj6kmiFqv+Ow9gI0x8GvaQ== demo@test
Access your remote host using what method you have available.
Once you have access to your account on the remote server, make sure that the ~/.ssh directory exists. The command will create the directory if necessary, or it will do nothing if it already exists:
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:
In the following 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 must start with ssh-rsa AAAA....
echo public_key_string >> ~/.ssh/authorized_keys
We can now try to access our Ubuntu server with an passwordless authentication.
If you have completed one of the procedures above, you should be able to log into the remote host without the remote account's password.
The basic process is the same as the other time we used it:
$ 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 '111.111.11.111 (111.111.11.111)' 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
If this occurs it 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.
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:
sudo nano /etc/ssh/sshd_config
Inside the file, search for a directive called PasswordAuthentication. This may be commented out. Uncomment the line 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 implement these changes, we need to restart the sshd service:
$ sudo systemctl restart ssh
As a precaution, open up a new terminal window and test that the SSH service is functioning correctly before closing this session:
$ ssh username@remote_host
Once you have verified your SSH service, you can safely close all current server sessions.
The SSH daemon on your Ubuntu server now only responds to SSH keys. Password-based authentication has successfully been disabled.
With the tutorial completed you should now have SSH-key-based authentication configured on your server, this allowing you to sign in without providing an account password.