I have a SIM900 GSM module that I use to send GET and POST requests to servers.
Recently I rented a host for this purpose. I wrote a simple page using asp.net webforms to parse incoming data from the GSM module, everything was working until a few days ago I noticed that I no longer can receive data from my gsm module.
After investigating further I found out that the host I rented keeps returning HTTP 400 errors to my GSM module. These responses are not from IIS but from Microsoft-HTTPAPI/2.0. The request header is this:
GET /test/data?meow HTTP/1.1
Host : www.whatever.com
Connection : keep-alive
And this is the server response(body omitted):
HTTP/1.1 400 Bad Request
Content-Type: text/html; charset=us-ascii
Server: Microsoft-HTTPAPI/2.0
Date: Sun, 11 Oct 2020 12:08:28 GMT
Connection: close
Content-Length: 339
I used Postman (application) to simulate the same request and everything worked just fine.
I also made an exact copy of a chrome request header and gave it to the module, but that didn't work either.
Note: I am not using sim900's HTTP commands I am connecting to a certain port(80 in this case) and making a get request manually.
Note 2:I have been given a Plesk panel to manage my website and do not have access to certain server settings.
The request will pass through the http.sys module before entering iis, which will intercept requests that do not comply with the rules, so your response comes from Microsoft-HTTPAPI / 2.0 with a status code of 400. The solution to this can be to modify the registry, but the setting in the registry is based on your application and request, and there is no universal modification method.
How to troubleshoot HTTP 400 errors
Http.sys registry settings for Windows
Another method is to suggest that you use a tool similar to Fiddler to capture the request sent by sim900 and the request sent by postman respectively. After the capture, compare them in detail to find out the differences, and modify the sim900 request to be the same as the postman request, conforming to http .sys rules.