server {
    listen 58881 default_server;
    server_name localhost _;
    root /usr/share/nginx/html;
    index background.html;

    location / {
        try_files $uri $uri/ =404;
    }

    # CORS proxy for map tiles. Forwards to the upstream tile server
    # but injects permissive CORS headers so the browser can load
    # tiles via cross-origin img.src requests. The 'resolver' is
    # required by the proxy module to resolve the upstream hostname.
    location ^~ /carto/ {
        resolver 1.1.1.1 8.8.8.8 ipv6=off valid=300s;
        proxy_pass https://a.basemaps.cartocdn.com/;
        proxy_ssl_server_name on;
        proxy_set_header Host a.basemaps.cartocdn.com;
        proxy_set_header User-Agent "Mozilla/5.0 (compatible; tile-proxy)";
        add_header Access-Control-Allow-Origin "*" always;
        add_header Access-Control-Allow-Methods "GET, HEAD, OPTIONS" always;
        add_header Access-Control-Max-Age 86400 always;
    }

    # CORS preflight handler
    location ~* \.(png|jpg)$ {
        if ($request_method = 'OPTIONS') {
            add_header Access-Control-Allow-Origin "*" always;
            add_header Access-Control-Allow-Methods "GET, HEAD, OPTIONS" always;
            add_header Access-Control-Max-Age 86400 always;
            add_header Content-Length 0;
            return 204;
        }
        add_header Access-Control-Allow-Origin "*" always;
    }
}