How To Secure Nginx with Let's Encrypt on Debian 10

Introduction

Let’s Encrypt is a Certificate Authority (CA) that provides a straightforward way to obtain and install free TLS/SSL certificates, enabling encrypted HTTPS on web servers. It simplifies the process by providing a software client, Certbot, that attempts to automate most (if not all) of the required steps. Currently, the entire process of obtaining and installing a certificate is fully automated on both Apache and Nginx.

In this tutorial, you will use Certbot to obtain a free SSL certificate for Nginx on Debian 10 and set up your certificate to renew automatically.

This tutorial will use a separate Nginx server block file instead of the default file. We recommend creating new Nginx server block files for each domain because it helps to avoid common mistakes and maintains the default files as a fallback configuration.

Prerequisites

To follow this tutorial, you will need:

• One Debian 10 server, set up by following this initial server setup for Debian 10 tutorial, along with a sudo non-root user and a firewall.

• A fully registered domain name. You can purchase a domain name on Namecheap, get one for free on Freenom, or use the domain registrar of your choice.

• Both of the following DNS records set up for your server. You can follow this introduction to DigitalOcean DNS for details on how to add them.

  • An A record with your_domain pointing to your server’s public IP address.
  • An A record with www.your_domain pointing to your server’s public IP address.

• Nginx installed by following How To Install Nginx on Debian 10. Be sure that you have a server block for your domain. This tutorial will use /etc/nginx/sites-available/your_domain as an example.

Step 1 — Installing Certbot

The first step to using Let’s Encrypt to obtain an SSL certificate is to install the Certbot software on your server.

Installing the python3-certbot-nginx package from the Debian repositories will allow us to install and use Cerbot’s nginx plugin. Working with Python 3 and the python3-certbot-nginx package increases the longevity of our setup: Python 2 will be deprecated by January 2020, so our setup ensures compatibility with Python 3. Debian 10 currently supports both Python 2 and Python 3.

Before installing the python3-certbot-nginx package, update your package list:

sudo apt update

Next, install the dependencies for the python3-certbot-nginx package, which include the python3-acme, python3-certbot, python3-mock, python3-openssl, python3-pkg-resources, python3-pyparsing, and python3-zope.interface packages:

sudo apt install python3-acme python3-certbot python3-mock python3-openssl python3-pkg-resources python3-pyparsing python3-zope.interface

Finally, install the python3-certbot-nginx package:

sudo apt install python3-certbot-nginx

Certbot is now ready to use, but in order for it to configure SSL for Nginx, we need to verify some of Nginx’s configuration.

Step 2 — Confirming Nginx’s Configuration

Certbot needs to be able to find the correct server block in your Nginx configuration for it to be able to automatically configure SSL. Specifically, it does this by looking for a server_name directive that matches your requested domain.

If you followed the server block setup step in the Nginx installation tutorial, you should have a server block for your domain at /etc/nginx/sites-available/your_domain with the server_name directive already set appropriately.

To check, open the server block file for your domain using nano or your favorite text editor:

sudo nano /etc/nginx/sites-available/your_domain

Find the existing server_name line. It should look like this:

...
server_name your_domain www.your_domain;
...

If it does, exit your editor and move on to the next step.

If it doesn’t, update it to match. Then save the file, quit your editor, and verify the syntax of your configuration edits:

sudo nginx -t

If you get an error, reopen the server block file and check for any typos or missing characters. Once your configuration file syntax is correct, reload Nginx to load the new configuration:

sudo systemctl reload nginx

Certbot can now find the correct server block and update it.

Next, let’s update the firewall to allow HTTPS traffic.

Step 3 — Allowing HTTPS Through the Firewall

If you have the ufw firewall enabled, as recommended in the prerequisite guides, you’ll need to adjust the settings to allow for HTTPS traffic.

You can see the current setting by typing:

sudo ufw status

It will probably look like this, meaning that only HTTP traffic is allowed to the web server:

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, allow the Nginx Full profile and delete the redundant Nginx HTTP profile allowance:

sudo ufw allow 'Nginx Full'
sudo ufw delete allow 'Nginx HTTP'

Your status should now look like this:

sudo ufw status

Output
Status: active

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

Next, let’s run Certbot and fetch our certificates.

Step 4 — Obtaining an SSL Certificate

Certbot provides a variety of ways to obtain SSL certificates through plugins. The Nginx plugin will take care of reconfiguring Nginx and reloading the config whenever necessary. To use this plugin, type the following:

sudo certbot --nginx -d your_domain -d www.your_domain

This runs certbot with the --nginx plugin, using -d to specify the names we’d like the certificate to be valid for.

If this is your first time running certbot, you will be prompted to enter an email address and agree to the terms of service. After doing so, certbot will communicate with the Let’s Encrypt server, then run a challenge to verify that you control the domain you’re requesting a certificate for.

If that’s successful, certbot will ask how you’d like to configure your HTTPS settings.

Output
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 then hit ENTER. The configuration will be updated, and Nginx will reload to pick up the new settings. certbot will wrap up with a message telling you the process was successful and where your certificates are stored:

Output
IMPORTANT NOTES:
 - Congratulations! Your certificate and chain have been saved at:
   /etc/letsencrypt/live/your_domain/fullchain.pem
   Your key file has been saved at:
   /etc/letsencrypt/live/your_domain/privkey.pem
   Your cert will expire on 2019-10-08. To obtain a new or tweaked
   version of this certificate in the future, simply run certbot again
   with the "certonly" option. To non-interactively renew *all* of
   your certificates, run "certbot renew"
 - Your account credentials have been saved in your Certbot
   configuration directory at /etc/letsencrypt. You should make a
   secure backup of this folder now. This configuration directory will
   also contain certificates and private keys obtained by Certbot so
   making regular backups of this folder is ideal.
 - If you like Certbot, please consider supporting our work by:

   Donating to ISRG / Let's Encrypt:   https://letsencrypt.org/donate
   Donating to EFF:                    https://eff.org/donate-le

Your certificates are downloaded, installed, and loaded. Try reloading your website using https:// and notice your browser’s security indicator. It should indicate that the site is properly secured, usually with a green lock icon. If you test your server using the SSL Labs Server Test, it will get an A grade.

Let’s finish by testing the renewal process.

Step 5 — Verifying Certbot Auto-Renewal

Let’s Encrypt’s certificates are only valid for ninety days. This is to encourage users to automate their certificate renewal process. The certbot package we installed takes care of this for us by adding a renew script to /etc/cron.d. This script runs twice a day and will automatically renew any certificate that’s within thirty days of expiration.

To test the renewal process, you can do a dry run with certbot:

sudo certbot renew --dry-run

If you see no errors, you’re all set. When necessary, Certbot will renew your certificates and reload Nginx to pick up the changes. If the automated renewal process ever fails, Let’s Encrypt will send a message to the email you specified, warning you when your certificate is about to expire.

Conclusion

In this tutorial, you installed the Let’s Encrypt client certbot, downloaded SSL certificates for your domain, configured Nginx to use these certificates, and set up automatic certificate renewal. If you have further questions about using Certbot, their documentation is a good place to start.