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How To Set Up SSH Keys on CentOS 8

May 31, 2021, Written by 0 comment

Introduction

This tutorial will teach you how to set up SSH Keys on a server running CentOS 8. SSH, or secure shell, is an encrypted protocol used to administer and communicate with servers. When working with a CentOS 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 a CentOS 8 server. SSH keys provide a straightforward, secure method of logging into your server and are recommended for all users.

Step 1 — Creating the RSA Key Pair

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 see the following prompt:

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 may optionally enter a secure passphrase, which is highly recommended. A passphrase adds an additional layer of security to your key, to prevent unauthorized users from logging in.

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

Step 2 — Copying the Public Key to Your CentOS Server

The quickest way to copy your public key to the CentOS 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 that follow (copying via password-based SSH, or manually copying the key).

In order to set up SSH Keys on a server running CentOS 8 you will need to follow one of the next three sections. Don’t attempt to copy your key in all of the alternative ways below.

Copying your 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 need only specify the remote host that you would like to connect to and the user account that you have password SSH access to. This is the account to which your public SSH key will be copied:

$ 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 the remote account’s [~/.ssh/authorized_keys] file.

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 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 more 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 any previously added keys.

The full command looks like this:

$ 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 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:

$ 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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 sammy@host

Log in to 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, using this command:

$ 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:

$ 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:

$ 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 key-based authentication with our CentOS server.

Step 3 — Logging In to Your CentOS Server Using SSH Keys

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

The initial process is the same as with password-based authentication:

$ 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 when creating your key pair in step 1, you will be logged in immediately. If you supplied a passphrase you will be prompted to enter it now. After authenticating, a new shell session should open for you with the configured account on the CentOS server.

If key-based authentication was successful, then you have set up SSH Keys on your CentOS 8 server. Continue on to learn how to further secure your system by disabling your SSH server’s password-based authentication.

Step 4 — Disabling Password Authentication on your Server

If you were able to login to 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 vi /etc/ssh/sshd_config

Inside the file, search for a directive called [PasswordAuthentication]. This may be commented out with a # hash. Press [i] to put [vi] into insertion mode, and then 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
...

When you are finished making changes, press [ESC] and then [:wq] to write the changes to the file and quit. To actually implement these changes, we need to restart the [sshd] service:

$ sudo systemctl restart sshd

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

$ ssh username@remote_host

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

The SSH daemon on your CentOS server now only responds to SSH keys. Password-based authentication has successfully been disabled.

Conclusion

You now know how to set up SSH Keys on a server running CentOS 8. Check out our other CentOS 8 guides:

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Original Content by Brian Boucheron and Hanif Jetha and edited by the author of this post according to the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

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