A Complete Guide to Securely Connecting Nginx and Curl Using Mutual TLS

How to use TLS, client authentication, and CA certificates in Nginx and Curl

Choose DIY vs smallstep-managed integration

Create a private key and request a certificate for your Nginx server

Before you can teach your server to speak TLS, you will need a certificate issued by a trusted certificate authority (CA). If your organization already runs its own CA and you have a private key and certificate for your Nginx server, along with your CA's root certificate, you can skip to the next step.

If your organization does not yet run its own internal CA, you can read more about creating and running a CA using the open source smallstep software here.

$ step ca certificate "" server.crt server.key

Your certificate and private key will be saved in server.crt and server.key respectively.

Request a copy of your CA root certificate, which will be used to make sure each application can trust certificates presented by other applications.

$ step ca root ca.crt

Your certificate will be saved in ca.crt.

Configure Nginx to authenticate itself with its TLS certificate

We now want to instruct our Nginx server to identify itself using the certificate issued in the last step and to force clients to connect over TLS.

In your Nginx configuration's server block, enable ssl for the listening socket and specify the locations of the server's certificate and private key. We'll also tell Nginx to use TLS protocols and our preferred ciphers:

server { listen 443 ssl; server_name; ssl_certificate server.crt; ssl_certificate_key server.key; ssl_protocols TLSv1.2 TLSv1.3; ssl_ciphers HIGH:!aNULL:!MD5; # ... }

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Configure Nginx to require clients to authenticate with a certificate issued by your CA

To tell Nginx to use mutual TLS and not just one-way TLS, we must instruct it to require client authentication to ensure clients present a certificate from our CA when they connect.

In your server's configuration block, specify the location of your CA root certificate to use for authenticating client certificates. You may choose to make client verification optional so your application can return a 403 message:

server { listen 443 ssl; server_name; # ... ssl_client_certificate /etc/nginx/client_certs/ca.crt; ssl_verify_client optional; # ... location / { if ($ssl_client_verify != SUCCESS) { return 403; } # ... }

That's it! Nginx should now be able to receive TLS connections from clients who authenticate themselves using a certificate issued by your trusted CA.

Create a private key and request a certificate for your Curl client

Request a new certificate from your CA to represent your Curl client.

$ step ca certificate "myuser" client.crt client.key

Your certificate and private key will be saved in client.crt and client.key respectively.

Make a request from Curl using mutual TLS

Now, we need only to configure our Curl client to make authenticated requests using our certificate and private key. The CA root certificate will be used to verify that the client can trust the certificate presented by the server.

Pass your certificate, private key, and root CA certificate to curl to authenticate your request over TLS.

$ curl --cert client.crt --key client.key --cacert ca.crt

Automate certificate renewal

By default, step-ca issues certificates with a 24 hour expiration. Short-lived certificates have many benefits but also require that you renew your certificates each day before they expire. How you renew certificates is often dependent on how you deploy your application. See the step-ca certificate lifecycle management docs for more information.

Let us know you're interested!

Smallstep is building Certificate Manager, a hosted certificate authority offering with integrations that make it easy to manage mututal TLS in technologies like Nginx and Curl.

Certificate Manager is currently in early access (by request only). Please request an invitation and let us know you'd be interested in an integration with Nginx and Curl.

Join Early Access

All documentation content from the Hello mTLS project is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0).

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