Debian Linux Security

Disabling root ssh on Debian

Since the root user is universal for all Linux and Unix systems it was always the preferred bruteforce victim by hackers to access systems. To bruteforce an unprivileged account the hacker must learn the username first and even if succeeding  the attacker stays limited unless using a local exploit.This tutorial shows how to disable the root access through SSH in 2 simple steps.

 How to disable ssh root access on Debian 10 Buster

To disable ssh root access you need to edit the ssh configuration file, on Debian it is /etc/ssh/sshd_config, to edit it using nano text editor run:

nano /etc/ssh/sshd_config

On nano you can press CTRL+W (where) and type PermitRoot to find the following line:

#PermitRootLogin prohibit-password

To disable the root access through ssh just uncomment that line and replace prohibit-password for no like in the following image.

After disabling the root access press CTRL+X and Y to save and exit.

The prohibit-password option prevents password login allowing only login through fall-back actions such as public keys, preventing brute force attacks.

Alternatives to secure your ssh access

Restrict access to Public Key Authentication:

To disable password login allowing only login using a Public key open the /etc/ssh/ssh_config configuration file again by running:

nano /etc/ssh/sshd_config

To disable password login allowing only login using a Public key open the /etc/ssh/ssh_config configuration file again by running:

nano /etc/ssh/sshd_config

Find the line containing PubkeyAuthentication and make sure it says yes like in the example below:

Make sure password authentication is disabled by finding the line containing PasswordAuthentication, if commented uncomment it and make sure it is set as no like in the following image:

Then press CTRL+X and Y to save and exit nano text editor.

Now as the user you want to allow ssh access through you need to generate private and public key pairs. Run:

ssh-keygen

Answer the question sequence leaving the first answer the default by pressing ENTER, set your passphrase, repeat it and the keys will be stored at ~/.ssh/id_rsa

Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa): <Press ENTER>
Enter passphrase (empty for no passphrase): <W
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:34+uXVI4d3ik6ryOAtDKT6RaIFclVLyZUdRlJwfbVGo root@linuxhint
The key's randomart image is:
+---[RSA 2048]----+

 To transfer the key pairs you just created you can use the ssh-copy-id command with the following syntax:

ssh-copy-id <user>@<host>

Change the default ssh port:

Open the /etc/ssh/ssh_config configuration file again by running:

nano /etc/ssh/sshd_config

Let’s say you want to use the port 7645 instead of the default port 22. Add a line like in the example below:

Port 7645

Then press CTRL+X and Y to save and exit.

Restart the ssh service by running:

service sshd restart

Then you should configure iptables to allow communication through port 7645:

iptables -t nat -A PREROUTING -p tcp --dport 22 -j REDIRECT --to-port 7645

You can also use UFW (Uncomplicated Firewall) instead:

ufw allow 7645/tcp

Filtering the ssh port

You can also define rules to accept or reject ssh connections according to specific parameters. The following syntax shows how to accept ssh connections from a specific IP address using iptables:

iptables -A INPUT -p tcp --dport 22 --source <ALLOWED-IP> -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j DROP

The first line of the example above instructs iptables to accept incoming (INPUT) TCP requests  to port 22 from the IP 192.168.1.2. The second line instructs IP tables to drop all connections to port 22. You can also filter the source by mac address like in the example below:

iptables -I INPUT -p tcp --dport 22 -m mac ! --mac-source 02:42:df:a0:d3:8f
-j REJECT

The example above rejects all connections except for the device with mac address 02:42:df:a0:d3:8f.

Using TCP wrappers to filter ssh

Another way to whitelist IP addresses to connect through ssh while rejecting the rest is by editing the directories hosts.deny and hosts.allow located in /etc.

To reject all hosts run:

nano /etc/hosts.deny

Add a last line:

sshd : ALL

Press CTRL+X and Y to save and exit. Now to allow specific hosts through ssh edit the file /etc/hosts.allow, to edit it run:

nano /etc/hosts.allow

Add a line containing:

sshd : <Allowed-IP>

Press CTRL+X to save and exit nano.

Disabling the ssh service

Many domestic users consider ssh useless, if you don’t use it at all you can remove it or you can block or filter the port.

On Debian Linux or based systems like Ubuntu you can remove services using the apt package manager.
To remove the ssh service run:

apt remove ssh

Press Y if requested to finish the removal.

And that’s all about domestic measures to keep ssh safe.

I hope you found this tutorial useful, keep following LinuxHint for more tips and tutorials on Linux and networking.

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About the author

Ivan Vanney

Ivan Vanney

Ivan Vanney has over a decade working as sysadmin. He is s contributor on LinuxHint.com, and the founder of linuxer.info