Let's Encrypt is a free Certificate Authority (CA). It provides a simple way to obtain, install and renew free TLS/SSL certificates. 

Here at Ibmi Media, as part of our Server Management Services, we regularly help our Customers to perform related Let's Encrypt SSL Certificates queries.

In this context, we shall look into how to obtain and install free SSL certificate and Secure Nginx with Let’s Encrypt on Debian 9 server.

How to Install Let's Encrypt on Debian ?

Before proceeding with this Installation procedure, ensure that the following requirements are met:

• Logged in to Debian server with a non-root user with sudo privileges.
• Your domain name should pointing to your server IP address.
• Nginx must installed and configured, as shown in this Guide.
• Have a Nginx server block for your domain, as shown in this Guide.

Here, you will learn how to use Certbot tool to obtain a free SSL certificate for Nginx on Debian.

1. Install Certbot Client

Using Certbot client package, you can easily obtain, install and renew Let's Encrypt SSL certificates. It's useful for configuring web servers to use the SSL certificates. The certbot package is included in the default Debian repositories.

First, we will update the packages list:

$ sudo apt update

Now install Certbot client by executing following command:

$ sudo apt install python-certbot-nginx

You can verify that certbot is installed successfully or not by typing:

$ certbot --version

2. Adjust Firewall

If on server UFW firewall enabled then you need to adjust firewall to allow HHTPS traffic.

You can see the current setting by executing the command:

$ sudo ufw status

Output
Status: active
To                         Action      From
--                         ------      ----
OpenSSH                    ALLOW       Anywhere                  
Nginx HTTP                 ALLOW       Anywhere                  
OpenSSH (v6)               ALLOW       Anywhere (v6)             
Nginx HTTP (v6)            ALLOW       Anywhere (v6)

To let in HTTPS traffic, you need to allow the Nginx Full profile and delete the redundant Nginx HTTP profile allowance:

$ sudo ufw allow 'Nginx Full'
$ sudo ufw delete allow 'Nginx HTTP'
$ sudo ufw delete allow ‘Nginx HTTP’

Now status should look like as below:

Output
Status: active
To                         Action      From
--                         ------      ----
OpenSSH                    ALLOW       Anywhere                  
Nginx Full                 ALLOW       Anywhere                  
OpenSSH (v6)               ALLOW       Anywhere (v6)             
Nginx Full (v6)            ALLOW       Anywhere (v6)

3. Obtain an SSL Certificate

There are many ways to obtain SSL certificates through plugins. Here, we will use certbot client to obtain a SSL certificate:

$ sudo certbot --nginx -d example.com -d www.example.com

Using above command, we are requesting for example.com and www.example.com domains. 

If you are installing certificate first time then it will ask you enter email address and agree terms and conditions. Entered email address will be used for sending email alerts related to SSL renewal and expiration.

Next, If validation got successful it will ask you to configure HTTPS settings:

Please choose whether or not to redirect HTTP traffic to HTTPS, removing HTTP access.
-------------------------------------------------------------------------------
1: No redirect - Make no further changes to the webserver configuration.
2: Redirect - Make all requests redirect to secure HTTPS access. Choose this for
new sites, or if you're confident your site works on HTTPS. You can undo this
change by editing your web server's configuration.
-------------------------------------------------------------------------------
Select the appropriate number [1-2] then [enter] (press 'c' to cancel):

Select your choice and hit Enter to go ahead. Your nginx server block will be updated based on your selected option and will reload Nginx to take new settings effect.

Finally, your domain is secure with Let's Encrypt SSL certificate. You can verify by visiting your site with HTTPS protocol.

4. Auto Renew Let's Encrypt SSL certificate

Let's Encrypt SSL certificates have short-life period of 90 days so you need to renew it before it expire. You can renew SSL certificate before it get expired by running:

$ sudo certbot renew

You can setup automatic process to auto renew Let's Encrypt SSL certificates by adding a cronjob. 

Run following command to open crontab:

$ sudo crontab -e

Next, append the below line at end of file. It will run the command twice a day and renews if the certificate is about to expire:

0 */12 * * * root test -x /usr/bin/certbot -a ! -d /run/systemd/system && perl -e 'sleep int(rand(43200))' && certbot -q renew

Save and close the file.

You also can verify certbot auto-renewal process by running the command:

$ sudo certbot renew --dry-run

[Need urgent assistance in fixing Nginx configuration errors? We can help you. ]