proxy_ssl_certificate not working as expected

[elanh]

Hello,

I'm using nginx as a proxy to a backend server. 
The backend server is also using nginx and enforcing client certificate
authentication using the ssl_client_certificate and ssl_verify_client
directives.

In my nginx server I set the following:

    location  /proxy {
        proxy_pass                 https://www.backend.com;

        proxy_set_header       X-Forwarded-Host $host;
        proxy_set_header       X-Forwarded-Server $host;
        proxy_set_header       X-Forwarded-For $proxy_add_x_forwarded_for;

        proxy_ssl_certificate         /etc/nginx/cert/client.crt;
        proxy_ssl_certificate_key  /etc/nginx/cert/client.key;
    }

according to
http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_ssl_certificate.

However, the backend is still responding with a 400 reponse code "No
required SSL certificate was sent".

Note that when issuing requests to the backend server using wget with the
client certificate, I get a valid 200 OK response.

What am I missing in my nginx configuration?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,265281,265281#msg-265281