We are integrating a 3rd party payment gateway which on a successful transaction sends a [IPN][1] request. So we have built a API endpoint to handle the request which works fine in dev env.

Now the issue is when we use nginx infront of the API Server it gets 301 redirects. When we are using a proxy tunnel like [ngrok][2], which proxies the http request via ngrok servers, the request passes through nginx to our API server successfully but when the IPN POST request is directly served to our nginx it gets a 301 redirect. To dig deeper we intercept the request with a python [SimpleHttpServer][3] leading us to find the difference of http headers:

- without ngrok (direct to nginx)

> ['Host: 35.154.216.72:4040\r\n', 'Accept: */*\r\n', 'Content-Length:
> 1030\r\n', 'Content-Type: application/x-www-form-urlencoded\r\n',
> '**Expect: 100-continue**\r\n']

- With ngrok

> ['Host: 543fdf1c.ngrok.io\r\n', 'Accept: */*\r\n', 'Content-Length:
> 1013\r\n', 'Content-Type: application/x-www-form-urlencoded\r\n',
> 'X-Forwarded-For: 2a01:4f8:192:22ec::2\r\n']

We are assuming the issue lies with the `Expect: 100-continue` headers. So the question is there any way to handle this with Nginx? or what is the industry standard solution for such scenerio?

FYI: Nginx conf:

location /api/ {

proxy_set_header Expect $http_expect;
client_body_in_file_only on;
proxy_pass_request_headers on;
proxy_http_version 1.1;

proxy_set_header X-Forwarded-Proto https;
proxy_set_header Upgrade $http_upgrade;
#proxy_set_header Connection 'upgrade';
proxy_set_header Connection keep-alive;
proxy_set_header Host $host;
proxy_pass_header X-CSRFToken;
#proxy_pass_header csrftoken;
proxy_cache_bypass $http_upgrade;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_pass http://kk-api;
}
IPN endpoint is `/api/v1/ipn`
[1]: https://en.wikipedia.org/wiki/Instant_payment_notification
[2]: http://www.ngrok.com
[3]: https://docs.python.org/2/library/simplehttpserver.html

For better have a better preview the question is also posted in StackOverflow: https://stackoverflow.com/questions/55810543/nginx-handle-expect-100-continue-header