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Secure Nginx with Let's Encrypt on Ubuntu 18.04

Let’s Encrypt is a free and open certificate authority developed by the Internet Security Research Group (ISRG). Certificates issued by Let’s Encrypt are trusted by almost all browsers today.

In this tutorial, we’ll provide a step by step instructions about how to secure your Nginx with Let’s Encrypt using the certbot tool on Ubuntu 18.04.

Prerequisites

Make sure that you have met the following prerequisites before continuing with this tutorial:

  • You have a domain name pointing to your public server IP. In this tutorial we will use example.com.
  • You have Nginx installed by following this instructions
  • You have a server block for your domain. You can follow this instructions for details on how to create one.

Install Certbot

Update the packages list and install the certbot package:

sudo apt update
sudo apt install certbot

Generate Strong Dh (Diffie-Hellman) Group

Diffie–Hellman key exchange (DH) is a method of securely exchanging cryptographic keys over an unsecured communication channel. We’re going to generate a new set of 2048 bit DH parameters to strengthen the security:

sudo openssl dhparam -out /etc/ssl/certs/dhparam.pem 2048

If you like you can change the size up to 4096 bits but in that case the generation may take more than 30 minutes depending on the system entropy.

Obtaining an SSL certificate

To obtain an SSL certificate for our domain we’re going to use the Webroot plugin that works by creating a temporary file for the requested domain in the ${webroot-path}/.well-known/acme-challenge directory and the Let’s Encrypt validation server makes HTTP requests to validate that the DNS for the requested domain resolves to the server where certbot runs.

To make it more simple we’re going to map all HTTP requests for .well-known/acme-challengeto a single directory, /var/lib/letsencrypt. The following commands will create the directory and make it writable for the Nginx server.

mkdir -p /var/lib/letsencrypt/.well-known
chgrp www-data /var/lib/letsencrypt
chmod g+s /var/lib/letsencrypt

To avoid duplicating code create the following two snippets which we’re going to use in all our Nginx server block files:

/etc/nginx/snippets/letsencrypt.conf
location ^~ /.well-known/acme-challenge/ {
  allow all;
  root /var/lib/letsencrypt/;
  default_type "text/plain";
  try_files $uri =404;
}
/etc/nginx/snippets/ssl.conf
ssl_dhparam /etc/ssl/certs/dhparam.pem;

ssl_session_timeout 1d;
ssl_session_cache shared:SSL:50m;
ssl_session_tickets off;

ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
ssl_ciphers 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:ECDHE-ECDSA-DES-CBC3-SHA:ECDHE-RSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:DES-CBC3-SHA:!DSS';
ssl_prefer_server_ciphers on;

ssl_stapling on;
ssl_stapling_verify on;
resolver 8.8.8.8 8.8.4.4 valid=300s;
resolver_timeout 30s;

add_header Strict-Transport-Security "max-age=15768000; includeSubdomains; preload";
add_header X-Frame-Options SAMEORIGIN;
add_header X-Content-Type-Options nosniff;

The snippet above is using the chippers recomendend by Mozilla, enables OCSP Stapling, HTTP Strict Transport Security (HSTS) and enforces few security‑focused HTTP headers. Once the snippets are created, open the domain server block and include the letsencrypt.confsnippet as shown bellow:

/etc/nginx/sites-available/example.com
server {
  listen 80;
  server_name example.com www.example.com;

  include snippets/letsencrypt.conf;
}

Reload the Nginx configuration for changes to take effect:

sudo systemctl reload nginx

You can now run Certbot with the webroot plugin and obtain the SSL certificate files by issuing:

sudo certbot certonly --agree-tos --email [email protected] --webroot -w /var/lib/letsencrypt/ -d example.com -d www.example.com

If the SSL certificate is successfully obtained, certbot will print the following message:

IMPORTANT NOTES:
 - Congratulations! Your certificate and chain have been saved at:
   /etc/letsencrypt/live/example.com/fullchain.pem
   Your key file has been saved at:
   /etc/letsencrypt/live/example.com/privkey.pem
   Your cert will expire on 2018-07-28. To obtain a new or tweaked
   version of this certificate in the future, simply run certbot
   again. 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
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Now that you have the certificate files, you can edit your domain server block as follows:

/etc/nginx/sites-available/example.com
server {
    listen 80;
    server_name www.example.com example.com;

    include snippets/letsencrypt.conf;
    return 301 https://$host$request_uri;
}

server {
    listen 443 ssl http2;
    server_name www.example.com;

    ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
    ssl_trusted_certificate /etc/letsencrypt/live/example.com/chain.pem;
    include snippets/ssl.conf;

    return 301 https://example.com$request_uri;
}

server {
    listen 443 ssl http2;
    server_name example.com;

    # . . . other code

    ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
    ssl_trusted_certificate /etc/letsencrypt/live/example.com/chain.pem;
    include snippets/ssl.conf;

    # . . . other code
}

With the configuration above we are forcing HTTPS and redirecting from www to non www version.

Finally, reload the Nginx service for changes to take effect:

sudo systemctl reload nginx

SSL certificate auto renewal

Let’s Encrypt’s certificates are valid for 90 days. To automatically renew the certificates before they expire, the certbot package creates a cronjob which runs twice a day and will automatically renew any certificate 30 days before its expiration.

Since we are using the certbot webroot plug-in once the certificate is renewed we also have to reload the nginx service. Append --renew-hook "systemctl reload nginx" to the /etc/cron.d/certbot file so it looks like this:

/etc/cron.d/certbot
0 */12 * * * root test -x /usr/bin/certbot -a \! -d /run/systemd/system && perl -e 'sleep int(rand(3600))' && certbot -q renew --renew-hook "systemctl reload nginx"

To test the renewal process, you can use the certbot --dry-run switch:

sudo certbot renew --dry-run

If there are no errors, it means that the renewal process was successful.


That’s all! If you are facing any problem, feel free to leave a comment.