Hanif Jetha
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 18.04 installation. SSH keys provide a secure way of logging into your server and are recommended for all users.
The first step is to create a key pair on the client machine (usually your local computer):
- ssh-keygen
By default ssh-keygen
will create a 2048-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 receive the following output:
OutputGenerating 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’ve previously generated an SSH key pair, you may receive 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.
The next prompt will ask you to enter a secure passphrase:
OutputEnter passphrase (empty for no passphrase):
Here you have the option to enter a secure passphrase, which is highly recommended. A passphrase adds a 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.
All together, the ssh-keygen
command will return output like the following:
OutputYour 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 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.
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).
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.
Note: 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 the following:
- ssh-copy-id username@remote_host
You may receive the following message:
OutputThe 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. Write “yes” and press ENTER
to continue.
Next, the utility will scan your local account for the id_rsa.pub
key that you 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:
Write in the password (nothing will 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 receive the following output:
OutputNumber 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.
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. Remember, this will only work if you have password-based SSH access to your server.
You can do this by using the cat
command to read the contents of the public SSH key on your local computer and piping that through an SSH connection to the remote server.
On the other side, you can make sure that the ~/.ssh
directory exists and has the correct permissions under the account you’re using.
You can then output the content you piped over into a file called authorized_keys
within this directory. Use the >>
redirect symbol to append the content instead of overwriting it. This will let you add keys without destroying previously added keys.
The full command displays as the following:
- 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 receive the following message:
OutputThe 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. Write “yes” and press ENTER
to continue.
After, you should be prompted to enter the remote user account’s password:
Outputusername@203.0.113.1's password:
After entering your password, the contents 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 process manually.
This section outlines how to manually append the content of your id_rsa.pub
file to the ~/.ssh/authorized_keys
file on your remote machine.
To display the contents of your id_rsa.pub
run the following command on your local computer:
- cat ~/.ssh/id_rsa.pub
This will return the key’s content in the command’s output:
Outputssh-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:
- 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. For this 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...
:
- echo public_key_string >> ~/.ssh/authorized_keys
Finally, ensure that the ~/.ssh
directory and authorized_keys
file have the appropriate permissions set:
- 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. In this tutorial our user is named sammy but you should substitute the appropriate username into the following command:
- chown -R sammy:sammy ~/.ssh
Now you can attempt passwordless authentication with your Ubuntu server.
If you’ve successfully completed one of the procedures in Step 2, you should be able to log into the remote host without the remote account’s password.
The process is the same:
- ssh username@remote_host
If this is your first time connecting to this host (if you used the manual method), you may receive something like this:
OutputThe 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. Write “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 (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, 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 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:
...
PasswordAuthentication no
...
Save and close the file when you’re finished by pressing CTRL + X
, then Y
and ENTER
to exit nano
. To activate these changes, you 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 the current session:
- ssh username@remote_host
Once you’ve verified that 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 and password-based authentication has been disabled.
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.
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Thanks for this.
Don’t we have to save the private key somewhere?
Step 1: Run
ssh-add
on the client machine to add the SSH key to the agent in order to avoid an error.I have done till 2nd step successfully and following 3rd step. When i tried to login, it says
login incorrect
.How i did that? I opened console and tried this command with sudo and without sudo after login: ssh root@remote-host and then entered password.
thanks
Beware of Step 2 if you are using the manual method to add your SSH key—the paste function has problems with the DO console.
I’m sticking with manual copy key ( accidentally lost my private key in my local machine :( ). But when i copy-paste my new public key into the default terminal in the digital ocean, this content copy just not the same as mine in local machine. I guess i must type every single character in that pub key. Does any one have any suggestions?
@hjet I’m just digging my way through the manual, there might be further commentaries, yet here we go.
nope.no. no way. more like:
The correct syntax I happened to learn here, however, quite some time was wasted to brute-force the correct flags.
Please edit that code block, trifles like that demotivate a lot.
And thanks for the man, since it is truly monumental :)
Fantastic tutorial. I was able to follow the instructions and connect to the server using the key-based authentication. One thing though, is this supposed to work for only one client machine? I used ssh-copy-id without any trouble. I try to follow the same process using a different laptop and i can’t connect to the server since it is now asking for a key. I changed the sshd config to allow password authentication but it appears it now defaults to key-based authentication and i’m unable to use ssh-copy-id from the 2nd client (since it doesn’t have they key yet). Should be a looking at another process for setting up the 2nd client? I really hope i don’t have to manually copy the key…