Introduction

Let’s Encrypt is a Certificate Authority (CA) that provides an easy way to obtain and install free TLS/SSL certificates, thereby 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 Ubuntu 16.04 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 some common mistakes and maintains the default files as a fallback configuration as intended. If you want to set up SSL using the default server block, you can follow this Nginx + Let’s Encrypt tutorial instead.

Prerequisites

To follow this tutorial, you will need:

One Ubuntu 16.04 server set up by following this initial server setup for Ubuntu 16.04 tutorial, including a sudo non-root user and a firewall.

A fully registered domain name. This tutorial will use example.com throughout. 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 hostname tutorial for details on how to add them.

An A record with example.com pointing to your server’s public IP address.
An A record with www.example.com pointing to your server’s public IP address.

Nginx installed by following How To Install Nginx on Ubuntu 16.04.

A separate Nginx server block file for your domain, set up by following this Nginx server blocks tutorial for Ubuntu 16.04. This tutorial will use /etc/nginx/sites-available/example.com.

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.
Certbot is in very active development, so the Certbot packages provided by Ubuntu tend to be outdated. However, the Certbot developers maintain a Ubuntu software repository with up-to-date versions, so we’ll use that repository instead.
First, add the repository.

sudo add-apt-repository ppa:certbot/certbot

You’ll need to press ENTER to accept. Then, update the package list to pick up the new repository’s package information.

sudo apt-get update

And finally, install Certbot’s Nginx package with apt-get.

sudo apt-get install python-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 the domain you request a certificate for.
If you followed the prerequisite tutorial on Nginx server blocks, you should have a server block for your domain at /etc/nginx/sites-available/example.com 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/example.com

Find the existing server_name line. It should look like this:
/etc/nginx/sites-available/example.com

. . .
server_name example.com www.example.com;
. . .

If it does, you can 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’s 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, we’ll update our firewall to allow HTTPS traffic.

Step 3 — Allowing HTTPS Through the Firewall

If you have the ufw firewall enabled, as recommended by the prerequisite guides, you’ll need to adjust the settings to allow for HTTPS traffic. Luckily, Nginx registers a few profiles with ufw upon installation.
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:

Status: active

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

To additionally let in HTTPS traffic, we can allow the Nginx Full profile and then delete the redundant Nginx HTTP profile allowance:

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

Your status should look like this now:

sudo ufw status


Status: active

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

We’re now ready to run Certbot and fetch our certificates.

Step 4 — Obtaining an SSL Certificate

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

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

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.

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:

IMPORTANT NOTES:
 - Congratulations! Your certificate and chain have been saved at
   /etc/letsencrypt/live/example.com/fullchain.pem. Your cert will
   expire on 2017-10-23. 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.