How To Create a Self-Signed SSL Certificate for Nginx on Debian 8

Introduction

TLS, or transport layer security, and its predecessor SSL, which stands for secure sockets layer, are web protocols used to wrap normal traffic in a protected, encrypted wrapper.

Using this technology, servers can send traffic safely between the server and clients without the possibility of the messages being intercepted by outside parties. The certificate system also assists users in verifying the identity of the sites that they are connecting with.

In this guide, we will show you how to set up a self-signed SSL certificate for use with an Nginx web server on Debian 8 server.

Prerequisites

Before you begin, you should have a non-root user configured with sudo privileges. You can learn how to set up such a user account by following our initial server setup for Debian 8.

You will also need to have the Nginx web server installed. If you would like to install an entire LEMP (Linux, Nginx, MySQL, PHP) stack on your server, you can follow our guide on setting up LEMP on Debian 8.

If you just want the Nginx web server, you can instead follow our guide on installing Nginx on Debian 8.

When you have completed the prerequisites, continue below.

Step 1: Create the SSL Certificate

TLS/SSL works by using a combination of a public certificate and a private key. The SSL key is kept secret on the server. It is used to encrypt content sent to clients. The SSL certificate is publicly shared with anyone requesting the content. It can be used to decrypt the content signed by the associated SSL key.

We can create a self-signed key and certificate pair with OpenSSL in a single command:

sudo openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout /etc/ssl/private/nginx-selfsigned.key -out /etc/ssl/certs/nginx-selfsigned.crt

You will be asked a series of questions. Before we go over that, let’s take a look at what is happening in the command we are issuing:

• openssl: This is the basic command line tool for creating and managing OpenSSL certificates, keys, and other files.
• req: This subcommand specifies that we want to use X.509 certificate signing request (CSR) management. The “X.509” is a public key infrastructure standard that SSL and TLS adheres to for its key and certificate management. We want to create a new X.509 cert, so we are using this subcommand.
• -x509: This further modifies the previous subcommand by telling the utility that we want to make a self-signed certificate instead of generating a certificate signing request, as would normally happen.
• -nodes: This tells OpenSSL to skip the option to secure our certificate with a passphrase. We need Nginx to be able to read the file, without user intervention, when the server starts up. A passphrase would prevent this from happening because we would have to enter it after every restart.
• -days 365: This option sets the length of time that the certificate will be considered valid. We set it for one year here.
• -newkey rsa:2048: This specifies that we want to generate a new certificate and a new key at the same time. We did not create the key that is required to sign the certificate in a previous step, so we need to create it along with the certificate. The rsa:2048 portion tells it to make an RSA key that is 2048 bits long.
• -keyout: This line tells OpenSSL where to place the generated private key file that we are creating.
• -out: This tells OpenSSL where to place the certificate that we are creating.

As we stated above, these options will create both a key file and a certificate. We will be asked a few questions about our server in order to embed the information correctly in the certificate.

Fill out the prompts appropriately. The most important line is the one that requests the Common Name (e.g. server FQDN or YOUR name). You need to enter the domain name associated with your server or, more likely, your server’s public IP address.

The entirety of the prompts will look something like this:

Output
Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:New York
Locality Name (eg, city) []:New York City
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Bouncy Castles, Inc.
Organizational Unit Name (eg, section) []:Ministry of Water Slides
Common Name (e.g. server FQDN or YOUR name) []:server_IP_address
Email Address []:admin@your_domain.com

Both of the files you created will be placed in the appropriate subdirectories of the /etc/ssl directory.

While we are using OpenSSL, we should also create a strong Diffie-Hellman group, which is used in negotiating Perfect Forward Secrecy with clients.

We can do this by typing:

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 that we can use in our configuration.

Step 2: Configure Nginx to Use SSL

We have created our key and certificate files under the /etc/ssl directory. Now we just need to modify our Nginx configuration to take advantage of these.

We will make a few adjustments to our configuration.

1. We will create a configuration snippet containing our SSL key and certificate file locations.
2. We will create a configuration snippet containing strong SSL settings that can be used with any certificates in the future.
3. We will adjust our Nginx server blocks to handle SSL requests and use the two snippets above.

This method of configuring Nginx will allow us to keep clean server blocks and put common configuration segments into reusable modules.

Create a Configuration Snippet Pointing to the SSL Key and Certificate

First, let’s create a new Nginx configuration snippet in the /etc/nginx/snippets directory.

To properly distinguish the purpose of this file, let’s call it self-signed.conf:

sudo nano /etc/nginx/snippets/self-signed.conf

Within this file, we just need to set the ssl_certificate directive to our certificate file and the ssl_certificate_key to the associated key. In our case, this will look like this:

ssl_certificate /etc/ssl/certs/nginx-selfsigned.crt;
ssl_certificate_key /etc/ssl/private/nginx-selfsigned.key;

When you’ve added those lines, save and close the file.

Create a Configuration Snippet with Strong Encryption Settings

Next, we will create another snippet that will define some SSL settings. This will set Nginx up with a strong SSL cipher suite and enable some advanced features that will help keep our server secure.

The parameters we will set can be reused in future Nginx configurations, so we will give the file a generic name:

sudo nano /etc/nginx/snippets/ssl-params.conf

To set up Nginx SSL securely, we will be using 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.

For our purposes, we can copy the provided settings in their entirety. We just need to make a few small modifications.

First, we will add our preferred DNS resolver for upstream requests. We will use Google’s for this guide. We will also go ahead and set the ssl_dhparam setting to point to the Diffie-Hellman file we generated earlier.

Finally, you should 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 will not preload the settings, but you can modify that if you are sure you understand the implications:

# 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 8.8.8.8 8.8.4.4 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;

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

Because we are using a self-signed certificate, the SSL stapling will not be used. Nginx will simply output a warning, disable stapling for our self-signed cert, and continue to operate correctly.

Save and close the file when you are finished.

Adjust the Nginx Configuration to Use SSL

Now that we have our snippets, we can adjust our Nginx configuration to enable SSL.

We will assume in this guide that you are using the default server block file in the /etc/nginx/sites-available directory. If you are using a different server block file, substitute its name in the below commands.

Before we go any further, let’s back up our current server block file:

sudo cp /etc/nginx/sites-available/default /etc/nginx/sites-available/default.bak

Now, open the server block file to make adjustments:

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

Inside, your server block probably begins like this:

server {
    listen 80 default_server;
    listen [::]:80 default_server;

    # SSL configuration

    # listen 443 ssl default_server;
    # listen [::]:443 ssl default_server;

    . . .

We will be modifying this configuration so that unencrypted HTTP requests are automatically redirected to encrypted HTTPS. This offers the best security for our sites. If you want to allow both HTTP and HTTPS traffic, use the alternative configuration that follows.

We will be splitting the configuration into two separate blocks. After the two first listen directives, we will add a server_name directive, set to your server’s domain name or, more likely, IP address. We will then set up a redirect to the second server block we will be creating. Afterwards, we will close this short block:

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    server_name server_domain_or_IP;
    return 302 https://$server_name$request_uri;
}

    # SSL configuration

    # listen 443 ssl default_server;
    # listen [::]:443 ssl default_server;

    . . .

Next, we need to start a new server block directly below to contain the remaining configuration. We can uncomment the two listen directives that use port 443. Afterwards, we just need to include the two snippet files we set up:

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    server_name server_domain_or_IP;
    return 302 https://$server_name$request_uri;
}

server {

    # SSL configuration

    listen 443 ssl default_server;
    listen [::]:443 ssl default_server;
    include snippets/self-signed.conf;
    include snippets/ssl-params.conf;

    . . .

Save and close the file when you are finished.

(Alternative Configuration) Allow Both HTTP and HTTPS Traffic

If you want or need to allow both encrypted and unencrypted content, you will have to configure Nginx a bit differently. This is generally not recommended if it can be avoided, but in some situations it may be necessary. Basically, we just compress the two separate server blocks into one block and remove the redirect:

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    listen 443 ssl default_server;
    listen [::]:443 ssl default_server;

    server_name server_domain_or_IP;
    include snippets/self-signed.conf;
    include snippets/ssl-params.conf;

    . . .

Save and close the file when you are finished.

Step 3: Adjust the Firewall

If you have a firewall enabled, you’ll need to adjust the settings to allow for SSL traffic. The required procedure depends on the firewall software you are using. If you do not have a firewall configured currently, feel free to skip forward.

UFW

If you are using ufw, 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
--                         ------      ----
SSH                        ALLOW       Anywhere
WWW                        ALLOW       Anywhere
SSH (v6)                   ALLOW       Anywhere (v6)
WWW (v6)                   ALLOW       Anywhere (v6)

To additionally let in HTTPS traffic, we can allow the “WWW Full” profile and then delete the redundant “WWW” profile allowance:

sudo ufw allow 'WWW Full'
sudo ufw delete allow 'WWW'

Your status should look like this now:

sudo ufw status

Output
Status: active

To                         Action      From
--                         ------      ----
SSH                        ALLOW       Anywhere
WWW Full                   ALLOW       Anywhere
SSH (v6)                   ALLOW       Anywhere (v6)
WWW Full (v6)              ALLOW       Anywhere (v6)

HTTPS requests should now be accepted by your server.

IPTables

If you are using iptables, you can see the current rules by typing:

sudo iptables -S

If you have any rules enabled, they will be displayed. An example configuration might look like this:

Output
-P INPUT DROP
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 80 -j ACCEPT

The commands needed to open SSL traffic will depend on your current rules. For a basic rule set like the one above, you can add SSL access by typing:

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

If we look at the firewall rules again, we should see the new rule:

sudo iptables -S

Output
-P INPUT DROP
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-A INPUT -p tcp -m tcp --dport 443 -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 80 -j ACCEPT

If you are using a program to automatically apply iptables rules at boot, you will want to make sure that you update your configuration with the new rule.

Step 4: Enable the Changes in Nginx

Now that we’ve made our changes and adjusted our firewall, we can restart Nginx to implement our new changes.

First, we should check to make sure that there are no syntax errors in our files. We can do this by typing:

sudo nginx -t

If everything is successful, you will get a result that looks like this:

Output
nginx: [warn] "ssl_stapling" ignored, issuer certificate not found
nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful

Notice the warning in the beginning. As noted earlier, this particular setting throws a warning since our self-signed certificate can’t use SSL stapling. This is expected and our server can still encrypt connections correctly.

If your output matches the above, your configuration file has no syntax errors. We can safely restart Nginx to implement our changes:

sudo systemctl restart nginx

Our server should now be accessible over HTTPS.

Step 5: Test Encryption

Now, we’re ready to test our SSL server.

Open your web browser and type https:// followed by your server’s domain name or IP into the address bar:

https://server_domain_or_IP

Because the certificate we created isn’t signed by one of your browser’s trusted certificate authorities, you will likely see a scary looking warning like the one below:

This is expected and normal. We are only interested in the encryption aspect of our certificate, not the third party validation of our host’s authenticity. Click “ADVANCED” and then the link provided to proceed to your host anyways:

You should be taken to your site. If you look in the browser address bar, you will see some indication of a partial security. This might be a lock with an “x” over it or a triangle with an exclamation point. In this case, this just means that the certificate cannot be validated. It is still encrypting your connection.

If you configured Nginx with two server blocks, automatically redirecting HTTP content to HTTPS, you can also check whether the redirect functions correctly:

http://server_domain_or_IP

If this results in the same icon, this means that your redirect worked correctly.

Step 6: Change to a Permanent Redirect

If your redirect worked correctly and you are sure you want to allow only encrypted traffic, you should modify the Nginx configuration to make the redirect permanent.

Open your server block configuration file again:

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

Find the return 302 and change it to return 301:

server {
	listen 80 default_server;
	listen [::]:80 default_server;
	server_name server_domain_or_IP;
	return 301 https://$server_name$request_uri;
}

. . .

Save and close the file.

Check your configuration for syntax errors:

sudo nginx -t

When you’re ready, restart Nginx to make the redirect permanent:

sudo systemctl restart nginx

Your site should now issue a permanent redirect when accessed over HTTP.

Conclusion

You have configured your Nginx server to use strong encryption for client connections. This will allow you serve requests securely, and will prevent outside parties from reading your traffic.