[darkalchemy]

YMMV
Try this, it works with proxying xbt, but not tested with cloudflare:

    location ~ ^/(.*?)/announce$ {
        proxy_pass http://127.0.0.1:2710/$1/announce$is_args$args&ip=$remote_addr;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Host $remote_addr;
        proxy_pass_header Content-Type;
        proxy_pass_header Content-Disposition;
        proxy_pass_header Content-Length;
        proxy_buffering off;
        proxy_buffer_size 128k;
        proxy_busy_buffers_size 256k;
        proxy_buffers 4 256k;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection $http_connection;

        real_ip_recursive on;
    }

nginx variables to explain those used above. You may or may not need all of those headers.