Problem description
When making a HTTP POST request to my Express app (Node.js) the server responds with two "100 Continue" responses and after that a "200 OK" response, when there should only be one "100 Continue".
Postman manages to ignore the second "100 continue" and reports success but another app I'm building with Mule ESB doesn't accept the extra "100 continue" and fails.
The app otherwise handles the POST request without any problems, data is written to the database and so forth.
About the technical enviroment
The Express app is running on a SUSE-server. Nginx serves as a reverse proxy. I manage the multiple Express apps with PM2.
At first i was using a separate Express app, with a proxy package, to act as a reverse proxy. But i switched it out to using Nginx as the reverse proxy, thinking that might have been the issue. But that made no difference.
I have tried the exact same setup with the express reverse proxy on my local machine and that doesn't return "100 Continue" at all, only "200 OK".
I can't figure out why the exact same app returns different responses when it's running on local/server.
Example response
**REQUEST**
POST /<my-endpoint> HTTP/1.1
Host: api.<my-server>.se:443
User-Agent: AHC/1.0
Connection: keep-alive
Accept: */*
Content-Type: application/json; charset=UTF-8
Content-Length: 123
**RESPONSE**
HTTP/1.1 100 Continue
Strict-Transport-Security: max-age=94608000
Date: Thu, 09 May 2019 10:11:34 GMT
Set-Cookie: WASID_HAG=f88cc3f3525ed06b; path=/; domain=.<my-domain>.se
Set-Cookie: WAAK_HAG=41ef118d9c4608f158cfbdcb0140e652; path=/; domain=.<my-domain>.se; secure
Set-Cookie: UPD=6; path=/; domain=.<my-domain>.se
Set-Cookie: RGSC16972=1030; path=/; domain=.<my-domain>.se
HTTP/1.1 100 Continue
Strict-Transport-Security: max-age=94608000
Date: Thu, 09 May 2019 10:11:34 GMT
Set-Cookie: WASID_HAG=f88cc3f3525ed06b; path=/; domain=.<my-domain>.se
Set-Cookie: WAAK_HAG=41ef118d9c4608f158cfbdcb0140e652; path=/; domain=.<my-domain>.se; secure
Set-Cookie: UPD=6; path=/; domain=.<my-domain>.se
Set-Cookie: RGSC16972=1030; path=/; domain=.<my-domain>.se
HTTP/1.1 200 OK
Server: nginx/1.16.0
Date: Thu, 09 May 2019 10:11:34 GMT
Content-Type: application/json; charset=utf-8
Content-Length: 227
Access-Control-Allow-Origin: *
X-DNS-Prefetch-Control: off
X-Frame-Options: SAMEORIGIN
Strict-Transport-Security: max-age=15552000; includeSubDomains
X-Download-Options: noopen
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
ETag: W/"e3-aMSDU6iiOn52HqHZuJKLaZNt38k"
Set-Cookie: WASID_HAG=f88cc3f3525ed06b; path=/; domain=.<my-domain>.se
Set-Cookie: WAAK_HAG=41ef118d9c4608f158cfbdcb0140e652; path=/; domain=.<my-domain>.se; secure
Set-Cookie: UPD=6; path=/; domain=.<my-domain>.se
Set-Cookie: RGSC16972=1030; path=/; domain=.<my-domain>.se
Cache-control: no-store
What could be causing the multiple "100 Continue" responses and how can i prevent them?
Since the app doesn't return any "100 Continue" responses at all on my local machine could there be something with the server/firewall that's causing this?
