I've discovered that if you send a request with an empty POST body, meaning no Content-Length header, the GCP Load Balancer (in this case from an Ingress controller through GKE) will reject your request with this error:
$ curl -L -X POST 'http://example.com/fund?amount=0'
<html><head>
<meta http-equiv="content-type" content="text/html;charset=utf-8">
<title>411 Length Required</title>
</head>
<body text=#000000 bgcolor=#ffffff>
<h1>Error: Length Required</h1>
<h2>POST requests require a <code>Content-length</code> header.</h2>
<h2></h2>
</body></html>
Assume I can't change the clients, is there some way to make the LB just accept empty bodies in POST requests?
The workarounds available as of the moment would be adding content headers content-length: 0 if you are to send HTTP POST requests with empty body. Per this RFC 2616 documentation:
If no response body is included, the response MUST include a Content-Length field with a field-value of "0"
Or to set the client to use http/1.1 as default:
For compatibility with HTTP/1.0 applications, HTTP/1.1 requests containing a message-body MUST include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. If a request contains a message-body and a Content-Length is not given, the server SHOULD respond with 400 (bad request) if it cannot determine the length of the message, or with 411 (length required) if it wishes to insist on receiving a valid Content-Length.
Both options require intervention from the client side. Unfortunately, there are no available workarounds/adjustments that can be done from the GCP Load Balancer side at this time.