Hardening SSH Security in Linux

SSH is a popular network protocol used to access remote computers through certain networks or the internet. OpenSSH is an application that provides SSH accessibility for a computer or server. A common method used to start a communication with SSH access is by providing a username and password before a secured connection is established. This method is not really safe because you provide credentials in plain text and transfer them through the network. It also makes it any scumbag individuals be easier to guess your password or perform a brute force attack in order to gain your server access. There are several possible ways that I think can be used for hardening your SSH security.

Disable root access
Change SSH port number
Utilize RSA authentication
Disable plain password login

Disable root access

1) Before you disable the root access, you need to make sure that there is another user account on your remote computer.

2) Create a new user as needed.

$ adduser youruser

3) Edit SSH configuration in /etc/ssh/sshd_config. Set new values for several following properties.

PermitRootLogin no
AllowUsers youruser

AllowUsers property is optional. It is used only for limiting access to certain users. You can add more users by separating it using space in the value.

4) Restart SSH service.

$ service sshd restart

Change SSH Port Number

You can change Port property other than 22.

Utilize RSA Authentication

1) Generate private and public keys in your local computer. I prefer to use PuTTYgen because it can generate not only an OpenSSH-compatible key but also a PuTTY-compatible key. PuTTY is a common application for remote access available for Windows. It needs a certain format of the private key with .PPK extension. PuTTYgen is included in PuTTY installation.

2) Open PuTTYgen. Set the number of bits in the generated key (2048, 4096, etc). Click "Generate".

3) After the process was completed, you can get 4 types of keys. First, in the big text box, it is an OpenSSH-compatible public key. Copy the content, paste it to a text editor (e.g. Notepad) and save it as id_rsa.pub. Second, you possibly need to fill in the key passphrase. It adds more security to your private key. Click "Save public key" and save it as id_rsa.putty.pub. Third, click "Save private key" and save as id_rsa.ppk. It's PuTTY-compatible private key. Fourth, choose the "Conversion" menu then choose the "Export OpenSSH key". It will generate an OpenSSH-compatible private key and save it as id_rsa.

4) Now, back to the OpenSSH configuration in the remote server. Set new values for several following properties.

RSAAuthentication yes
PubkeyAuthentication yes
AuthorizedKeysFile %h/.ssh/authorized_keys

The last property means that the OpenSSH application will check any allowed public keys stored in the user authorized_keys file.

5) Copy your user OpenSSH-compatible public key content and append it to /home/[yur_user]/.ssh/authorized_keys. The .ssh directory should be owned by your user and set its mode to 0700 for security while the authorized_keys mode should be 0600.

6) Back to the local computer and open your PuTTY application. On the Session menu, insert your remote computer address with its user and port. Then, go to Auth menu, choose your PuTTY-compatible private key. Then, back to the Session menu and save your configuration.

Disable plain password login

Before you implement this configuration, you must make sure that the previous process is working. By setting PasswordAuthentication value to "no" will make your server can be accessed only by RSA authentication.

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