Bombardier returns error: the server closed connection before returning the first response byte. Make sure the server returns 'Connection: close' response header before closing the connection - 1

[Ushanta]

I downloaded bombardier-windows-amd64.exe from the home page. There's no version info.

I have an ASPNET Core Web API. I am querying a POST method that accepts JSON. The POST responds with Status Code 201 if successful. I am using Bombardier locally to test the app - Windows 10 Pro OS.

I ran Bombardier with the following flags and the results are fine (my requests are large json files):
.\bombardier-windows-amd64.exe -m POST -f <myjsonfile.json> https://localhost:44384/api/run/sample -n 7000 -c 40 -H 'Content-Type: application/json'
Bombarding https://localhost:44384/api/run/sample with 7000 request(s) using 40 connection(s)
7000 / 7000 [====================] 100.00% 409/s 17s
Done!
Statistics Avg Stdev Max
Reqs/sec 409.96 365.59 2105.33
Latency 96.16ms 32.23ms 653.25ms
HTTP codes:
1xx - 0, 2xx - 7000, 3xx - 0, 4xx - 0, 5xx - 0
others - 0
Throughput: 2.84MB/s

Then ran it again but changed the number of requests to 1 and concurrent connections to 1 as well:
.\bombardier-windows-amd64.exe -m POST -f <myjsonfile.json> http://localhost:44384/api/run/sample -n 1 -c 1 -H 'Content-Type: application/json' -p i,p,r
Bombarding http://localhost:44384/api/run/sample with 1 request(s) using 1 connection(s)
1 / 1 [=====================================================] 100.00% 4/s 0s
Done!
Statistics Avg Stdev Max
Reqs/sec 249.75 0.00 499.50
Latency 23.97ms 0.00us 23.97ms
HTTP codes:
1xx - 0, 2xx - 0, 3xx - 0, 4xx - 0, 5xx - 0
others - 1
Errors:
the server closed connection before returning the first response byte. Make sure the server returns 'Connection: close' response header before closing the connection - 1
Throughput: 1.26MB/s

I expected that the second call would be successful and return a 201.

But; it has an error:
the server closed connection before returning the first response byte. Make sure the server returns 'Connection: close' response header before closing the connection - 1
Throughput: 1.26MB/s

Here's the output for 1, 625, 7000 requests:
.\bombardier-windows-amd64.exe -m POST -f <myjsonfile.json> http://localhost:44384/api/run/sample -n 1 -c 1 -H 'Content-Type: application/json' -p i,p,r
Bombarding http://localhost:44384/api/run/sample with 1 request(s) using 1 connection(s)
1 / 1 [===============================================================================================] 100.00% 4/s 0s
Done!
Statistics Avg Stdev Max
Reqs/sec 249.75 0.00 499.50
Latency 23.97ms 0.00us 23.97ms
HTTP codes:
1xx - 0, 2xx - 0, 3xx - 0, 4xx - 0, 5xx - 0
others - 1
Errors:
the server closed connection before returning the first response byte. Make sure the server returns 'Connection: close' response header before closing the connection - 1
Throughput: 1.26MB/s

.\bombardier-windows-amd64.exe -m POST -f <myjsonfile.json> http://localhost:44384/api/run/sample -n 625 -c 5 -H 'Content-Type: application/json' -p i,p,r
Bombarding http://localhost:44384/api/run/sample with 625 request(s) using 5 connection(s)
625 / 625 [=========================================================================================] 100.00% 282/s 2s
Done!
Statistics Avg Stdev Max
Reqs/sec 296.14 84.83 526.34
Latency 16.92ms 3.06ms 40.00ms
HTTP codes:
1xx - 0, 2xx - 0, 3xx - 0, 4xx - 0, 5xx - 0
others - 625
Errors:
the server closed connection before returning the first response byte. Make sure the server returns 'Connection: close' response header before closing the connection - 625
Throughput: 8.91MB/s

.\bombardier-windows-amd64.exe -m POST -f <myjsonfile.json> https://localhost:44384/api/run/sample -n 7000 -c 40 -H 'Content-Type: application/json' -s
Bombarding https://localhost:44384/api/run/sample with 7000 request(s) using 40 connection(s)
7000 / 7000 [======================================================================================] 100.00% 378/s 18s
Done!
Statistics Avg Stdev Max
Reqs/sec 377.67 332.90 1947.40
Latency 103.79ms 27.69ms 448.00ms
HTTP codes:
1xx - 0, 2xx - 7000, 3xx - 0, 4xx - 0, 5xx - 0
others - 0
Throughput: 2.63MB/s

[Ushanta]

It's the URL :(