increase in post request 404 errors and 499 status codes using nginx as reverse proxy against IIS

I’m after some assistance as I’ve run into a couple of issues implementing nginx as a reverse proxy backing onto windows servers. I’ve used the config that works elsewhere with no issues but I’m seeing issues with more failed requests (404s) and nginx specific error codes 499.

We are seeing around 200 post 404 errors in the nginx logs when using the setup and only around 100 of the same errors when running against IIS. The files are there in most cases and the same (get requests anyway) work when hit directly on the windows server.

My first through was that there was something wrong with my setup using the post requests through nginx but it’s only a small percentage so don’t think there’s anything fundamentally wrong with nginx post setup.

The 499 status codes, client closed connection seems a little weird. I can see around 150-300 499 requests for 20k requests. It may be that these were happening anyway and we are just not seeing them being reported in IIS.

Had to remove the nginx site config as wouldn't let me post it!

Really need to start pushing the traffic through these servers as the IIS servers are struggling with the load but just need to get to the bottom of the errors before we can sign the switch over off.

I’ve tried changing the proxy timeout settings, proxy bind address onto private NICs.

Anyone any ideas?