IIS: 502 when posting multipart/form-data and using ARR - iis

Question

We have a classic ASP.NET web api that is exposing a file upload endpoint. The file upload accepts a multipart/form-data request that holds the file. The format is:
POST https://dev.appname.com/api/v2/visitors/{{visitorId}}/attachments/upload
X-API-Key: {{apiKey}}
Content-Type: multipart/form-data; boundary=XXX
--XXX
Content-Disposition: form-data; name="image"; filename="filename.png"
Content-Type: image/png
{{ file bytes }}
--XXX--
We use the following rewriting:
<rule name="Rewrite to dev REST API" enabled="true" stopProcessing="true">
<match url="api/(v[0-9]+/.*)" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false">
<add input="{HTTP_HOST}" matchType="Pattern" pattern="(dev|dev-testing).appname.com" ignoreCase="true" negate="false" />
</conditions>
<action type="Rewrite" url="https://restapidev.appname.com/api/{R:1}?tenant={C:1}" appendQueryString="true" />
</rule>
So the url is rewritten from: https://dev.appname.com/api/v2/visitors/{{visitorId}}/attachments/upload to https://restapidev.appname.com/api/v2/visitors/{{visitorId}}/attachments/upload?tenant=dev
This gives us a response 502.
When hitting the https://restapidev.appname.com/api... url directly, the request is successful.
To diagnose the issue, I am writing the request body to a file:
private async Task SaveRequestBody() {
var req = HttpContext.Current.Request;
using var fileStream = File.OpenWrite($"C:\\temp\\{DateTime.Now:yyyy-MM-dd-HH-mm-ss}-UploadAttachment.txt");
await req.GetBufferedInputStream().CopyToAsync(fileStream);
}
I notice that:
When going through the dev.appname... url:
❌ an empty file is written
When going through the restapidev.appname... url
✅ the full request, including the upload is written to a file
When uploading a file without multipart/form-data (Content-Type: image/png, bytes directly posted in the request body)
✅ the full request, including the upload is written to a file
Application logging seems to indicate that the application "hangs" on reading the request stream.
I have enabled failed request tracing, and noticed this:
At 12:36:37.144: GENERAL_READ_ENTITY_START
At 12:38:37.504: GENERAL_READ_ENTITY_END (2 minutes later!!)
BytesReceived="0", ErrorCode="The I/O operation has been aborted because of either a thread exit or an application request. (0x800703e3)"
FYI: There's a warning REWRITE_DISABLED_KERNEL_CACHE in the trace as well, but succesful requests have this warning as well.
I was not able to add the full log here (due to length restrictions on the post).
So our web application seems to "hang" while reading the request body:
only for an multipart/form-data upload
only when routed through ARR
Is there something that could be misconfigured?