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How to Secure Your SSH Connection in CentOS 7

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Michael M.
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SSH is the main protocol used to connect to and administer Linux servers. Because of this, it is one of the most frequently attacked ports when nefarious actors attempt to gain access to your server. In this article, we will discuss a few methods that you can use to make your SSH connection more secure.

Note: Depending on the product you have with OVHcloud, some of these steps may have already been taken. If this is the case, simply proceed to the next step. Additionally, the steps in this article assume that you authenticate to your server using SSH keys. If you use a password instead, certain steps in this article will not work.

Prerequisites

Topics

Creating a New Sudo User

It is always best practice to disallow root authentication over SSH since this is the username people will try to hack into the most. Thus, the first thing we want to do to secure our server is create a new sudo user for SSH. To do so, enter the following command, replacing the red username with the username of your choice: 

# adduser username

Next, we will set a password for the user:

# passwd username

Follow the prompt to create and confirm the password. Now, we want to give our new user sudo privileges so that we can become root and run commands which need administrative privileges. We can do this by entering the following command. 

# usermod -aG wheel username

Last, we want to enable our new user to authenticate using the SSH public key we have already provided to the root user. We can use a simple rsync command to copy the public key over to our new user's authorized_keys file. 

# rsync --archive --chown=username:username ~/.ssh /home/username

Before proceeding to the next step, log out and make sure that you are able to authenticate to the server as the new user using SSH. If you are unable to login as your new user, you will still be able to log in as root; confirm all of the commands have been entered correctly and try to log in as your new user again.

Changing the SSH Daemon Configuration

Since we are using SSH keys and a new user to authenticate to our server, we never want anyone to authenticate using a password or the root username. To accomplish this, we first want to navigate to the configuration file for the OpenSSH daemon. To do so, open the file in a text editor of your choice using the following command:

$ sudo vi /etc/ssh/sshd_config

There are three changes we want to make to this file.  First, we want to change the port on which OpenSSH listens for requests.

Warning: If you have any active firewalls, you will need to allow traffic through the port you choose or you will lock yourself out of your server. If you do lock yourself out of your server, you can regain access through IPMI or KVM. To learn more, please check out our Getting Started with IPMI article.

At the top of the file, you will see a section that looks like this by default:

#Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

Uncomment the "Port" section and choose any valid port number like in the following example. In our example, we are using port 12345.

Port 12345
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

Next scroll down to the # Authentication: portion of the file. You will see five options that will appear as follows by default:

#LoginGraceTime 2m
#PermitRootLogin yes
PermitRootLogin yes
#StrictModes
#MaxAuthTries 6
#MaxSessions 10 

Here we want to change the "yes" next to "PermitRootLogin" to "no." It will appear as follows:

#LoginGraceTime 2m
#PermitRootLogin yes
PermitRootLogin no
#StrictModes
#MaxAuthTries 6
#MaxSessions 10

Now we want to scroll down the sshd_config file a little further to make our final change - disabling password authentication. You will see a section that looks like this by default:

# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no
PasswordAuthentication yes

We want to change the "yes" next to "PasswordAuthentication" to a "no." It will appear as follows:

# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no
PasswordAuthentication no

Save and exit the file. Finally, we need to restart OpenSSH for the changes to take effect. Do this by entering the following command:

$ sudo systemctl restart sshd.service

Let's take a second to review what we did here. We changed the port number that we use to listen for SSH requests. Then, we disabled SSH access for the root user or any user trying to authenticate with a password. If we have done this correctly, the following command will no longer work to log in to the server.

$ ssh username@xxx.xxx.xxx.xxx

To log in now, we are going to have to specify the port number we are using to listen for SSH requests. That means from now on we will need to use the following command, replacing the number next to "-p" with the port number we chose earlier:

$ ssh -p 12345 username@xxx.xxx.xxx.xxx

Make sure that this command works and that the previous one does not. If it does, you are all set to access your server securely through SSH.

Conclusion

With so many bad actors using the internet, it has never been more important to secure any potential entry points to your server. By following this guide, you have aided the security of the most common entry point on Linux servers.

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