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.
Certbot is in very active development, so the Certbot packages provided by Ubuntu tend to be outdated.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
sudo apt-get update
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.
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 servername directive that matches the domain you request a certificate for.
Open the server block file for your domain using vim or your favorite text editor.
sudo vim /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, and quit your editor. In many cases it will appear like: example.com.conf.
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.
Allowing HTTPS Through the Firewall
If you have the ufw firewall enabled, 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'
Check the status to see new rules if you want.
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.
Once you finish the setup. 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.
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.
Problems that may arise:
If your website is behind a CDN (like CloudFlare) or reverse proxy, the TLS-SNI-01 challenge method won't work. If so, you can try using webroot mode instead: https://letsencrypt.readthedocs.io/en/latest/using.html#webroot
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