IIS 上の ASP.NET Web Forms でのアップロードのサイズ制限

<!-- maxRequestLength is in KILOBYTES (default 4096 = 4,194,304 bytes). 5120 KB = 5,242,880
     bytes: comfortably above the 4,000,000-byte image limit plus multipart overhead, so a
     file that is slightly too big still reaches the handler and gets its JSON 413. -->
<httpRuntime targetFramework="4.8" maxRequestLength="5120" />

<requestFiltering>
  <!-- maxAllowedContentLength is in BYTES (default 30,000,000). Same 5,242,880 bytes as
       maxRequestLength above. IIS checks this one first, before ASP.NET runs. -->
  <requestLimits maxAllowedContentLength="5242880" />
</requestFiltering>

// Optional: makes "request too large" from IIS come back as JSON on /rte/ routes.
void Application_EndRequest(object sender, EventArgs e)
{
    RteFiles.RewriteTooLarge(Context);
}

// Call from Application_EndRequest (Global.asax). A body over maxAllowedContentLength is
// rejected by IIS request filtering before any handler runs - HTTP 413.1 with an HTML
// page (404.13 on IIS 7/8) - but EndRequest still runs, so the /rte/ routes can answer
// in JSON instead. The substatus check leaves the handlers' own 413s alone.
public static void RewriteTooLarge(HttpContext context)
{
    HttpResponse response = context.Response;
    bool rejectedByIis = (response.StatusCode == 413 && response.SubStatusCode == 1)
        || (response.StatusCode == 404 && response.SubStatusCode == 13);
    if (rejectedByIis
        && context.Request.AppRelativeCurrentExecutionFilePath.StartsWith("~/rte/", StringComparison.OrdinalIgnoreCase))
    {
        WriteError(context, 413, "The upload is larger than the server allows.");
    }
}

<!-- /rte/ always answers in JSON. With errorMode="Custom" (what remote visitors get by
     default) IIS would otherwise swap its own HTML page in for the 413 that Global.asax
     writes when request filtering rejects an oversized upload. -->
<location path="rte">
  <system.webServer>
    <httpErrors existingResponse="PassThrough" />
  </system.webServer>
</location>

head -c 4500000 /dev/zero > four.jpg     # then make it start like a JPEG:
printf '\xff\xd8\xff' | cat - four.jpg > big.jpg
curl -F "file=@big.jpg" http://localhost:5401/rte/upload
{"error":"The image is larger than the 4 MB (4,000,000 bytes) limit."}

head -c 6000000 /dev/urandom > huge.jpg
curl -F "file=@huge.jpg" http://localhost:5401/rte/upload
{"error":"The upload is larger than the server allows."}