CORS problems in microservices architecture - nginx

Question

After a lot of searches and tries, I have decided to post my problems around CORS.
Context
I have a web site on a local server (NGINX), located on a PI. My web site deals with a microservice through REST API.
My PI is behind my Freebox (French internet box), which redirect a specific port (ex : 28800) to the local # of my pi. FYI, I don't have (and don't want) a fixed IP. And I share my IP with others users. So, I have a range of ports between 20000 and 30000 (approximatively).
And I have a NO-IP hostname, pointed on my freebox IP completed with the specifc port.
To resume:
User -> mysite.no-ip.com ---(Translation NO-IP)---> 91.123.456.678:specifcport ---- (Redirect to my pi) ---> 192.168.0.30:80----(nginx)--->index.html
What I can do:
In my nginx site conf, I set a location /core.
When I do
mysite.no-ip.com/core/blabla, I well obtain the blabla treatment.
No problem!!
Here is my problem:
When I click on the button which launches the blabla action (throught an AJAX POST on mysite.no-ip.comm/core/blabla), I get a CORS failure.
I have tried to modify my nginx configuration, adding headers depends of OPTIONS/GET/POST request type. Nothing appends.
But, I find that my nginx is never touched by the request /core/blabla if this one is send by button. I have a CORS answer thrown by an APACHE Server. And I didn't install any APACHE Server. So, perhaps, it's a server hidden in my Freebox? Don't know how to set it to allow CORS request.
Here is a snippet of my nginx config file
server {
listen 80;
server_name mysite.no-ip.com
location / {
alias /path/to/root/production/folder/here;
}
location /core/ {
if ($request_method = 'OPTIONS') {
add_header 'Access-Control-Allow-Origin' '*';
add_header 'Access-Control-Allow-Credentials' 'true';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
add_header 'Access-Control-Max-Age' 1728000;
add_header 'Content-Type' 'text/plain charset=UTF-8';
add_header 'Content-Length' 0;
return 204;
}
if ($request_method = 'POST') {
add_header 'Access-Control-Allow-Origin' '*';
add_header 'Access-Control-Allow-Credentials' 'true';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
}
proxy_pass http://localhost:8002/;
}
}
Thank you for your help.

Answer 1

Ok, Fix my problem which was solved by a combination of my Internet provider box configuration and addition of some headers on my request.