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How To Secure Nginx with Let's Encrypt on CentOS 7

PostedJanuary 26, 2016 63.7k views Security Let's Encrypt Nginx CentOS


Let's Encrypt is a new 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 only on Apache web servers. However, Let's Encrypt can be used to easily obtain a free SSL certificate, which can be installed manually, regardless of your choice of web server software.

In this tutorial, we will show you how to use the certbot Let's Encrypt client to obtain a free SSL certificate and use it with Nginx on CentOS 7. We will also show you how to automatically renew your SSL certificate. If you're running a different web server, simply follow your web server's documentation to learn how to use the certificate with your setup.

Nginx with Let's Encrypt TLS/SSL Certificate and Auto-renewal


Before following this tutorial, you'll need a few things.

You should have an CentOS 7 server with a non-root user who has sudo privileges. You can learn how to set up such a user account by following steps 1-3 in our initial server setup for CentOS 7 tutorial.

You must own or control the registered domain name that you wish to use the certificate with. If you do not already have a registered domain name, you may register one with one of the many domain name registrars out there (e.g. Namecheap, GoDaddy, etc.).

If you haven't already, be sure to create an A Record that points your domain to the public IP address of your server. This is required because of how Let's Encrypt validates that you own the domain it is issuing a certificate for. For example, if you want to obtain a certificate for example.com, that domain must resolve to your server for the validation process to work. Our setup will use example.com and www.example.com as the domain names, so both DNS records are required.

Once you have all of the prerequisites out of the way, let's move on to installing the Let's Encrypt client software.

Step 1 — Install the Certbot Let's Encrypt Client

The first step to using Let's Encrypt to obtain an SSL certificate is to install the certbot software on your server. Currently, the best way to install this is through the EPEL repository.

Enable access to the EPEL repository on your server by typing:

  • sudo yum install epel-release

Once the repository has been enabled, you can obtain the certbot package by typing:

  • sudo yum install certbot

The certbot Let's Encrypt client should now be installed and ready to use.

Step 2 — Obtain a Certificate

Let's Encrypt provides a variety of ways to obtain SSL certificates, through various plugins. Unlike the Apache plugin, which is covered in a different tutorial, most of the plugins will only help you with obtaining a certificate which you must manually configure your web server to use. Plugins that only obtain certificates, and don't install them, are referred to as "authenticators" because they are used to authenticate whether a server should be issued a certificate.

We'll show you how to use the Webroot plugin to obtain an SSL certificate.

How To Use the Webroot Plugin

The Webroot plugin works by placing a special file in the /.well-known directory within your document root, which can be opened (through your web server) by the Let's Encrypt service for validation. Depending on your configuration, you may need to explicitly allow access to the /.well-known directory.

If you haven't installed Nginx yet, you can do so now. The EPEL repository should already be enabled from the previous section, so you can install Nginx by typing:

  • sudo yum install nginx

To ensure that the directory is accessible to Let's Encrypt for validation, let's make a quick change to our default Nginx server block. The default Nginx configuration file allows us to easily add directives to the port 80 server block by adding files in the /etc/nginx/default.d directory.

If you're using the default configuration, create a new file called le-well-known.conf and open it for editing with this command:

  • sudo vi /etc/nginx/default.d/le-well-known.conf

Then paste in these lines:

location ~ /.well-known {
        allow all;

Save and exit.

Check the configuration for syntax errors by typing:

  • sudo nginx -t

If no errors were reported, start or restart Nginx with this command:

  • sudo systemctl restart nginx

If you aren't using the default server block, you will need to look up what your document root is set to by looking for the root directive in your default Nginx server block. This is the value that Let's Encrypt requires, as webroot-path, when using the Webroot plugin. The default root is /usr/share/nginx/html.

Next, make sure port 80 and 443 are open in your firewall. If you are not running a firewall, you can skip ahead.

If you have a firewalld firewall running, you can open these ports by typing:

  • sudo firewall-cmd --add-service=http
  • sudo firewall-cmd --add-service=https
  • sudo firewall-cmd --runtime-to-permanent

If have an iptables firewall running, the commands you need to run are highly dependent on your current rule set. For a basic rule set, you can add HTTP and HTTPS access by typing:

  • sudo iptables -I INPUT -p tcp -m tcp --dport 80 -j ACCEPT
  • sudo iptables -I INPUT -p tcp -m tcp --dport 443 -j ACCEPT

Now that we know our webroot-path, we can use the Webroot plugin to request an SSL certificate with these commands. Here, we are also specifying our domain names with the -d option. If you want a single cert to work with multiple domain names (e.g. example.com and www.example.com), be sure to include all of them. Also, make sure that you replace the highlighted parts with the appropriate webroot path and domain name(s):

  • sudo certbot certonly -a webroot --webroot-path=/usr/share/nginx/html -d example.com -d www.example.com

After certbot initializes, you will be prompted for some information. The exact prompts may vary depending on if you've used certbot before, but we'll step you through the first time.

At the prompt, enter an email address that will be used for notices and lost key recovery:

Email prompt

Then you must agree to the Let's Encrypt Subscribe Agreement. Select Agree:

Let's Encrypt Subscriber's Agreement

If everything was successful, you should see an output message that looks something like this:

IMPORTANT NOTES: - Congratulations! Your certificate and chain have been saved at /etc/letsencrypt/live/example.com/fullchain.pem. Your cert will expire on 2016-03-15. To obtain a new version of the certificate in the future, simply run Let's Encrypt again. - If you lose your account credentials, you can recover through e-mails sent to sammy@digitalocean.com - Your account credentials have been saved in your Let's Encrypt 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 Let's Encrypt so making regular backups of this folder is ideal. - If like Let's Encrypt, please consider supporting our work by: Donating to ISRG / Let's Encrypt: https://letsencrypt.org/donate Donating to EFF: https://eff.org/donate-le

You will want to note the path and expiration date of your certificate, which was highlighted in the example output.

Note: If you receive an error like Failed to connect to host for DVSNI challenge, recheck your server's firewall to make sure it is configured to allow TCP traffic on port 80 and 443.

Note: If your domain is routing through a DNS service like CloudFlare, you will need to temporarily disable it until you have obtained the certificate.

Certificate Files

After obtaining the cert, you will have the following PEM-encoded files:

  • cert.pem: Your domain's certificate
  • chain.pem: The Let's Encrypt chain certificate
  • fullchain.pem: cert.pem and chain.pem combined
  • privkey.pem: Your certificate's private key

It's important that you are aware of the location of the certificate files that were just created, so you can use them in your web server configuration. The files themselves are placed in a subdirectory in /etc/letsencrypt/archive. However, the certbot Let's Encrypt client creates symbolic links to the most recent certificate files in the /etc/letsencrypt/live/your_domain_name directory. Because the links will always point to the most recent certificate files, this is the path that you should use to refer to your certificate files.

You can check that the files exist by running this command (substituting in your domain name):

  • sudo ls -l /etc/letsencrypt/live/your_domain_name

The output should be the four previously mentioned certificate files. In a moment, you will configure your web server to use fullchain.pem as the certificate file, and privkey.pem as the certificate key file.

Generate Strong Diffie-Hellman Group

To further increase security, you should also generate a strong Diffie-Hellman group. To generate a 2048-bit group, use this command:

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

This may take a few minutes but when it's done you will have a strong DH group at /etc/ssl/certs/dhparam.pem.

Step 3 — Configure TLS/SSL on Web Server (Nginx)

Now you must edit the Nginx configuration to use the Let's Encrypt certificate files. The default Nginx configuration on CentOS is pretty open-ended but we will create a new server block that uses SSL/TLS and listens on port 443. Then we'll configure the default (HTTP on port 80) server block to redirect to the HTTPS-enabled server block.

By default, additional server block configuration can be placed in /etc/nginx/conf.d. Create a new file called ssl.conf and open it for editing with this command:

  • sudo vi /etc/nginx/conf.d/ssl.conf

Then paste this configuration in. Be sure to change every instance of example.com, all four, with your own domain name:

server {
        listen 443 http2 ssl;

        server_name example.com www.example.com;

        ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;

        # from https://cipherli.st/                                            #
        # and https://raymii.org/s/tutorials/Strong_SSL_Security_On_nginx.html #

        ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
        ssl_prefer_server_ciphers on;
        ssl_ciphers "EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH";
        ssl_ecdh_curve secp384r1;
        ssl_session_cache shared:SSL:10m;
        ssl_session_tickets off;
        ssl_stapling on;
        ssl_stapling_verify on;
        resolver valid=300s;
        resolver_timeout 5s;
        # Disable preloading HSTS for now.  You can use the commented out header line that includes
        # the "preload" directive if you understand the implications.
        #add_header Strict-Transport-Security "max-age=63072000; includeSubdomains; preload";
        add_header Strict-Transport-Security "max-age=63072000; includeSubdomains";
        add_header X-Frame-Options DENY;
        add_header X-Content-Type-Options nosniff;

        # END https://cipherli.st/ BLOCK #

        ssl_dhparam /etc/ssl/certs/dhparam.pem;

        location ~ /.well-known {
                allow all;

        # The rest of your server block
        root /usr/share/nginx/html;
        index index.html index.htm;

        location / {
                # First attempt to serve request as file, then
                # as directory, then fall back to displaying a 404.
                try_files $uri $uri/ =404;

To set up Nginx SSL securely, in the configuration above we used the recommendations by Remy van Elst on the Cipherli.st site. This site is designed to provide easy-to-consume encryption settings for popular software. You can read more about his decisions regarding the Nginx choices here.

Note: The default suggested settings on Cipherli.st offer strong security. Sometimes, this comes at the cost of greater client compatibility. If you need to support older clients, there is an alternative list that can be accessed by clicking the link on the link labeled "Yes, give me a ciphersuite that works with legacy / old software."

The compatibility list can be used instead of the default suggestions in the configuration above between the two comment blocks. The choice of which config you use will depend largely on what you need to support.

There are a few pieces of the configuration that you may wish to modify. First, we added our preferred DNS resolver for upstream requests. We used Google's for this guide, but you can change this if you have other preferences.

Finally, you should take take a moment to read up on HTTP Strict Transport Security, or HSTS, and specifically about the "preload" functionality. Preloading HSTS provides increased security, but can have far reaching consequences if accidentally enabled or enabled incorrectly. In this guide, we did not preload the settings, but you can modify that if you are sure you understand the implications.

Modify the rest of the server block to suit your needs.

Save and exit. This configures Nginx to use SSL, and tells it to use the Let's Encrypt SSL certificate that we obtained earlier. Also, the SSL options specified here ensure that only the most secure protocols and ciphers will be used. Note that this example configuration simply serves the default Nginx page, so you may want to modify it to meet your needs.

Next, we'll configure Nginx to redirect HTTP requests on port 80 to HTTPS on port 443.

The default Nginx configuration file allows us to easily add directives to the port 80 server block by adding files in the /etc/nginx/default.d directory. Create a new file called ssl-redirect.conf and open it for editing with this command:

  • sudo vi /etc/nginx/default.d/ssl-redirect.conf

Then paste in this line:

    return 301 https://$host$request_uri;

Save and exit. This configures the HTTP on port 80 (default) server block to redirect incoming requests to HTTPS.

Now, check the configuration for syntax errors:

  • sudo nginx -t

If no errors are found, restart Nginx:

  • sudo systemctl restart nginx

You will also want to enable Nginx, so it starts when your server boots:

  • sudo systemctl enable nginx

The Let's Encrypt TLS/SSL certificate is now in place. At this point, you should test that the TLS/SSL certificate works by visiting your domain via HTTPS in a web browser.

You can use the Qualys SSL Labs Report to see how your server configuration scores:

In a web browser:

This SSL setup should report an A rating.

Step 4 — Set Up Auto Renewal

Let’s Encrypt certificates are valid for 90 days, but it’s recommended that you renew the certificates every 60 days to allow a margin of error. At the time of this writing, automatic renewal is still not available as a feature of the client itself, but you can manually renew your certificates by running the certbot Let’s Encrypt client with the renew option.

To trigger the renewal process for all installed domains, run this command:

  • sudo certbot renew

Because we recently installed the certificate, the command will only check for the expiration date and print a message informing that the certificate is not due to renewal yet. The output should look similar to this:

Saving debug log to /var/log/letsencrypt/example.com ------------------------------------------------------------------------------- Processing /etc/letsencrypt/renewal/example.com.conf ------------------------------------------------------------------------------- Cert not yet due for renewal The following certs are not due for renewal yet: /etc/letsencrypt/live/example.com/fullchain.pem (skipped) No renewals were attempted.

Notice that if you created a bundled certificate with multiple domains, only the base domain name will be shown in the output, but the renewal should be valid for all domains included in this certificate.

A practical way to ensure your certificates won’t get outdated is to create a cron job that will periodically execute the automatic renewal command for you. Since the renewal first checks for the expiration date and only executes the renewal if the certificate is less than 30 days away from expiration, it is safe to create a cron job that runs every week or even every day, for instance.

Let's edit the crontab to create a new job that will run the renewal command every week. To edit the crontab for the root user, run:

  • sudo crontab -e

Add the following lines:

crontab entry
30 2 * * 1 /usr/bin/certbot renew >> /var/log/le-renew.log 35 2 * * 1 /usr/bin/systemctl reload nginx

Save and exit. This will create a new cron job that will execute the certbot renew command every Monday at 2:30 am, and reload Nginx at 2:35am (so the renewed certificate will be used). The output produced by the command will be piped to a log file located at /var/log/le-renewal.log.

For more information on how to create and schedule cron jobs, you can check our How to Use Cron to Automate Tasks in a VPS guide.


That's it! Your web server is now using a free Let's Encrypt TLS/SSL certificate to securely serve HTTPS content.


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