Expect: 100-continue with the nginx proxy_pass

[Maxim Dounin]

Hello!

On Wed, Apr 24, 2019 at 03:14:08AM -0400, simanto604 wrote:

> 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?

Most likely, the reason is not "Expect: 100-continue", but the 
Host header, which is also different.

> FYI: Nginx conf: 
> 
>     location /api/ {
> 
>     proxy_set_header Expect $http_expect;

Note that this is simply wrong and likely to break things 
completely as long as there is Expect header in the request.

[...]

-- 
Maxim Dounin
http://mdounin.ru/