Why is my custom header not present sometimes? - http

Question

How do I get my custom header all the way to my Rails application when running behind nginx and Phusion Passenger? It is possible, please see details below, but when I just use the headers pane in Paw it is not passed through.
I am using Paw to test and develop some API endpoints in a Rails application. Everything works as expected in my development environment, which is a Rails 6 application running on macOS using Puma. For security, I use a custom header that contains a personal auth token. When I examine the Rails request object, specifically request.headers, I am able to see all the headers including my custom header and I can authenticate based on its value.
The problem comes when running on my staging system, where I have very little control of the environment. Here, the same Rails application is running under Phusion Passenger behind nginx. When I hit the same endpoint with the same request, just changing the host in the request, the custom header is not present. I verified this by writing all headers to a file for every request in staging.
Where has the header gone? Because the environment is different in staging, I suspect that nginx or Fusion Passenger is receiving the header but not passing it through to my Rails application. I can't verify this since I have no access to the logs other than my Rails application's logs. The application is designed to get requests from an external service, so I send some requests through that service and the header is present. That is very strange, so some headers are being passed through and some are not.
Paw (header defined under headers pane).
I checked with cURL using:
curl -X 'https://example.com/ivr/main_menu' -H 'X_JSW_AUTH_TOKEN':'my_tkn'
With Ruby's Net::HTTP:
uri = URI('https://example.com/ivr/main_menu')
http = Net::HTTP.new(uri.host, uri.port)
http.use_ssl = true
http.verify_mode = OpenSSL::SSL::VERIFY_PEER
req = Net::HTTP::Post.new(uri)
req.add_field "X_JSW_AUTH_TOKEN", "my_tkn"
res = http.request(req)
With HTTPie:
http POST 'https://example.com/ivr/main_menu' 'X_JSW_AUTH_TOKEN':'my_tkn'
With the http gem from httprb:
resp = HTTP.headers(X_JSW_AUTH_TOKEN: "my_tkn").post("https://example.com/ivr/main_menu")

Answer 1

It seems the answer has nothing to do with Paw. There's pretty solid evidence of this when cURL, HTTPie, and Net::HTTP all have the same results as Paw.
The problem is how nginx treats headers with underscores by default by ignoring them. See "Why do HTTP servers forbid underscores in HTTP header names" for more information.
Ruby's HTTP, which was able to provide the header to my application, did so because it replaces underscores ("_") with hyphens ("-") in header names, so these headers made it through to the Rails application. Rails then replaces hyphens in the header names of requests with underscores. So, both the sender (HTTP) and receiver (Rails) were making substitutions behind the scenes. This made it way harder to troubleshoot.
Many thanks to Chris Oliver for the answer.