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Using mTLS with Kong

This guide walks through on how to setup Kong to perform mutual-TLS authentication with an upstream service.

Please note that this guide walks through mTLS configuration between Kong and a Service and not Kong and a client or consumer.

What is mTLS?

Mutual authentication refers to two-way authentication, where the client and server, both can authenticate themselves to the other party.

With mutual TLS authentication, client and server both present TLS certificates to the other party (and can prove their identity using their private key) during the TLS handshake. They can verify the other’s certificate using the their trusted CAs.

mTLS with Kong

Kong 1.3 and above support mutual TLS authentication between Kong and the upstream service.

Let’s take a look at how one can configure it.

Configure Kong to verify upstream server certificate

Kong, by default, does not verify the certificate presented by the upstream service.

To enforce certificate verification, you need to configure the following environment variables on Kong’s container in your deployment:

KONG_NGINX_PROXY_PROXY_SSL_VERIFY="on"
KONG_NGINX_PROXY_PROXY_SSL_VERIFY_DEPTH="3"
KONG_NGINX_PROXY_PROXY_SSL_TRUSTED_CERTIFICATE="/path/to/ca_certs.pem"

These basically translate to NGINX directives to configure NGINX to verify certificates.

Please make sure that the trusted certificates are correctly mounted into Kong’s container and the path to certificate is correctly reflected in the above environment variable.

Configure Kong to present its certificate to the upstream server

In the above section, we achieved one side of mutual authentication, where Kong has been configured to verify the identity of the upstream server.

In this section, we will configure Kong to present its identity to the upstream server.

To configure this, you have two options, depending on your use-case. If you would like Kong to present its client certificate to each and every service that it talks to, you can configure the client certificate at the global level using Nginx directives. If you would like to configure a different certificate for each service that Kong talks to or want to configure Kong to present a client certificate only to a subset of all services that it is configured to communicate with, then you can configure that using an annotation on the Kubernetes Service resource.

Global Nginx directive

You need to configure two Nginx directives for this purpose:

• proxy_ssl_certificate
• proxy_ssl_certificate_key

You can mount the certificate and key pair using secrets into the Kong pod and then set the following two environment variables to set the above two directives:

KONG_NGINX_PROXY_PROXY_SSL_CERTIFICATE="/path/to/client_cert.pem"
KONG_NGINX_PROXY_PROXY_SSL_CERTIFICATE_KEY="/path/to/key.pem"

Once configured, Kong will present its client certificate to every upstream server that it talks to.

Per service annotation

To configure a different client certificate for each service or only for a subset of services, you can do so using the konghq.com/client-cert annotation.

To use the annotation, you first need to create a TLS secret with the client certificate and key in Kubernetes. The secret should be created in the same namespace as your Kubernetes Service to which Kong should authenticate itself.

Once the secret is in place, add the follow annotation on the service:

konghq.com/client-cert: <name-of-secret>

Kong will then use the TLS key-pair to authenticate itself against that service.