How To Increase the Security and Usability on Debian 9

When you first install a new Debian 9 server, there are a few configuration steps that you should take early on as part of the initial setup. This will increase the security and usability of your server and will give you a solid foundation for subsequent actions.

Logging in as Root
Log into your Debian 9 server from any client machine using SSH with root user's account. If you are on Windows client machine, you can use Putty or any of your favorite ssh client tool to access your linux servers.

ssh root@your_server_ip

Accept the warning about host authenticity if it appears. If you are using password authentication, provide your root password to log in. If you are using an SSH key that is passphrase protected, you may be prompted to enter the passphrase the first time you use the key each session. If this is your first time logging into the server with a password, you may also be prompted to change the root password.

Root User
The root user is the administrative user in a Linux environment that has very broad privileges. Because of the heightened privileges of the root account, you are discouraged from using it on a regular basis. This is because part of the power inherent with the root account is the ability to make very destructive changes, even by accident.

The next step is to set up an alternative user account with a reduced scope of influence for day-to-day work. We'll teach you how to gain increased privileges during the times when you need them.

Creating a New User
Once you are logged in as root, you're ready to add the new user account that we will use to log in from now on.

Note: In some environments, a package called unscd may be installed by default in order to speed up requests to name servers like LDAP. The most recent version currently available in Debian contains a bug that causes certain commands (like the adduser command below) to produce additional output that looks like this:

sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting

These messages are harmless, but if you wish to avoid them, it is safe to remove the unscd package if you do not not plan on using systems like LDAP for user information:

apt remove unscd

This example creates a new user called muhammad, but you should replace it with a username that you like:

adduser muhammad

You will be asked a few questions, starting with the account password.

Enter a strong password and, optionally, fill in any of the additional information if you would like. This is not required and you can just hit ENTER in any field you wish to skip.

Granting Administrative Privileges
Now, we have a new user account with regular account privileges. However, we may sometimes need to do administrative tasks.

To avoid having to log out of our normal user and log back in as the root account, we can set up what is known as "superuser" or root privileges for our normal account. This will allow our normal user to run commands with administrative privileges by putting the word sudo before each command.

To add these privileges to our new user, we need to add the new user to the sudo group. By default, on Debian 9, users who belong to the sudo group are allowed to use the sudo command.

As root, run this command to add your new user to the sudo group (substitute the highlighted word with your new user):

usermod -aG sudo muhammad

Now, when logged in as your regular user, you can type sudo before commands to perform actions with superuser privileges.

Setting Up a Basic Firewall
Debian servers can use firewalls to make sure only connections to certain services are allowed. Although the iptables firewall is installed by default, Debian does not strongly recommend any specific firewall. In this guide, we will install and use the UFW firewall to help set policies and manage exceptions.

We can use the apt package manager to install UFW. Update the local index to retrieve the latest information about available packages and then install the firewall by typing:

apt update
apt install ufw

Firewall profiles allow UFW to manage sets of firewall rules for applications by name. Profiles for some common software are bundled with UFW by default and packages can register additional profiles with UFW during the installation process. OpenSSH, the service allowing us to connect to our server now, has a firewall profile that we can use.

You can see this by typing:

ufw app list

Available applications:

We need to make sure that the firewall allows SSH connections so that we can log back in next time. We can allow these connections by typing:

ufw allow OpenSSH

Afterwards, we can enable the firewall by typing:

ufw enable

Type "y" and press ENTER to proceed. You can see that SSH connections are still allowed by typing:

ufw status

Status: active

To Action From
-- ------ ----
OpenSSH ALLOW Anywhere
OpenSSH (v6) ALLOW Anywhere (v6)

As the firewall is currently blocking all connections except for SSH, if you install and configure additional services, you will need to adjust the firewall settings to allow acceptable traffic in.

Enabling External Access for Your Regular User
Now that we have a regular user for daily use, we need to make sure we can SSH into the account directly.

Note: Until verifying that you can log in and use sudo with your new user, we recommend staying logged in as root. This way, if you have problems, you can troubleshoot and make any necessary changes as root.

The process for configuring SSH access for your new user depends on whether your server's root account uses a password or SSH keys for authentication.

If the Root Account Uses Password Authentication
If you logged in to your root account using a password, then password authentication is enabled for SSH. You can SSH to your new user account by opening up a new terminal session and using SSH with your new username:

ssh muhammad@your_server_ip

After entering your regular user's password, you will be logged in. Remember, if you need to run a command with administrative privileges, type sudo before it like this:

sudo command_to_run

You will be prompted for your regular user password when using sudo for the first time each session (and periodically afterwards).

If the Root Account Uses SSH Key Authentication
If you logged in to your root account using SSH keys, then password authentication is disabled for SSH. You will need to add a copy of your local public key to the new user's ~/.ssh/authorized_keys file to log in successfully.

Since your public key is already in the root account's ~/.ssh/authorized_keys file on the server, we can copy that file and directory structure to our new user account in our existing session with the cp command. Afterwards, we can adjust ownership of the files using the chown command.

Make sure to change the highlighted portions of the command below to match your regular user's name:

cp -r ~/.ssh /home/muhammad
chown -R muhammad:muhammad /home/muhammad/.ssh

Now, open up a new terminal session and using SSH with your new username:

ssh muhammad@your_server_ip

You should be logged in to the new user account without using a password. Remember, if you need to run a command with administrative privileges, type sudo before it like this:

sudo command_to_run

You will be prompted for your regular user password when using sudo for the first time each session (and periodically afterwards).

Completing Optional Configuration
Now that we have a strong baseline configuration, we can consider a few optional steps to make the system more accessible. The following sections cover a few additional tweaks focused on usability.

Installing man Pages
Debian provides extensive manuals for most software in the form of man pages. However, the man command is not always included by default on minimal installations.

Install the man-db package to install the man command and the manual databases:

sudo apt install man-db

Now, to view the manual for a component, you can type:

man command

For example, to view the manual for the top command, type:

man top

Most packages in the Debian repositories include manual pages as part of their installation.

Changing the Default Editor
Debian offers a wide variety of text editors, some of which are included in the base system. Commands with integrated editor support, like visudo and systemctl edit, pass text to the editor command, which is mapped to the system default editor. Setting the default editor according to your preferences can help you configure your system more easily and avoid frustration.

If your preferred editor is not installed by default, use apt to install it first:

sudo apt install your_preferred_editor

Next, you can view the current default and modify the selection using the update-alternatives command:

sudo update-alternatives --config editor

The command displays a table of the editors it knows about with a prompt to change the default:

There are 8 choices for the alternative editor (providing /usr/bin/editor).

Selection Path Priority Status
* 0 /usr/bin/joe 70 auto mode
1 /bin/nano 40 manual mode
2 /usr/bin/jmacs 50 manual mode
3 /usr/bin/joe 70 manual mode
4 /usr/bin/jpico 50 manual mode
5 /usr/bin/jstar 50 manual mode
6 /usr/bin/rjoe 25 manual mode
7 /usr/bin/vim.basic 30 manual mode
8 /usr/bin/vim.tiny 15 manual mode

Press to keep the current choice[*], or type selection number:
The asterisk in the far left column indicates the current selection. To change the default, type the "Selection" number for your preferred editor and press Enter. For example, to use nano as the default editor given the above table, we would choose 1:

Press to keep the current choice[*], or type selection number: 1
update-alternatives: using /bin/nano to provide /usr/bin/editor (editor) in manual mode

From now on, your preferred editor will be used by commands like visudo and systemctl edit, or when the editor command is called.

Wrapping up
You have prepared a solid foundation for your Debian 9. You can install any of the software you need on your server now.

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