By default, the Linux server will ask to enter a username and password if someone accesses the server via SSH. However, the previous article explained that you can access the Linux server using the passwordless SSH login method. Now I want my Linux servers to only allow access via SSH key authentication or SSH passwordless login.
Problem
How to allow access to the Linux server only using SSH key authentication?
Solution
You can make the security of your Linux server stronger by restricting access to the Linux server using SSH key authentication. It means the remote server can only being able to accessed the server for those who already use SSH passwordless login, so that if another user wants to access the server, it will be rejected. To allow access to the Linux server only using SSH key authentication, change the configuration in the /etc/ssh/sshd_config file by looking for the line containing PasswordAuthentication and setting it to no, as in the script below:
PasswordAuthentication no
After that, restart the SSH service using the command below:
Ubuntu/Debian
systemctl restart ssh
RockyLinux/AlmaLinux/CentOS
systemctl restart sshd
You should not be able to access the server when you try to connect to it using SSH. This means your SSH configuration is correct. Below is an example of an error that occurs when accessing via Putty:
For example, in the previous article, the sysadmin user on the Ubuntu server could access the RockyLinux server because he had used SSH Passwordless Login as in the image below:
I can not access the RockyLinux server if I access it via the OpenSUSE server, as in the image:
If you want to add another user to be able to access the server, you have to copy the .ssh/id_rsa.pub file and put it into the remote server in the .ssh/authorized_keys file. You can use the help of a user who can access the server to put the file. Look at the image below, where I have included the id_rsa.pub file for the sysadmin user on the OpenSUSE server on the RockyLinux server:
I tried to connect again to the RockyLinux server using the sysadmin user on the OpenSUSE server, I can access the server as in the image below:
Note
Make sure the remote server already contains authorized_keys files from other servers so that it doesn’t make things difficult for you in the future.
References
strongdm.com
tecmint.com
linuxize.com
