AT Commands HTTP request to JSON source with status 200 but empty response

1
votes

I am working with an Arduino Mega and a SIM900 GSM/GPRS shield to make a get request against my own API.

I am using the following AT Commands, the module executes the request, but I receive a 200 response code without a body/response.

Here is my code:

AT+SAPBR=3,1,"Contype","GPRS"
AT+SAPBR=3,1,"APN","my.apn.com"
AT+SAPBR=1,1
AT+SAPBR=2,1
AT+HTTPPARA="CID",1
AT+HTTPPARA="URL","wbs-api.eu-gb.cf.appdomain.cloud/stats"

AT+HTTPACTION=0
-> 0,200,0 (as output of the command)

As far as I know, the last 0 stands for the size of the response body in bytes.

If I further execute

AT+HTTPREAD
->

Nothing is displayed.

Where has my answer disappeared to and why is it empty?

Note: I'm actually able to successful obtain data performing a request to a different server, but I noticed that the working request returns HTML code and not JSON data.

1
jsonarduinohttpresponseat-commandsim900
Shouldn't the URL start with http://? You also mistyped HTTPPARA. – gre_gor
Hey, it makes No differece if the url starts with http:// or not. It also gives me the empty response. And yes of course, there is a P missing, just a error from copying. – NiklasUllmann
Are you able to perform a GET from any other page? – Roberto Caboni
Yes, but i notized, that the other (working) request returns html code and not a jsons. Could that be a problem? – NiklasUllmann
isn't this a problem of your response page, rather than your script? – Hayden Eastwood

1 Answers

1
votes

TL;DR

Configure the accepted content type with

AT+HTTPPARA="CONTENT","application/json"

Issue analysis

Since you are admittedly able to receive data from different sites, it means that the issue must be something specific to the address you are accessing.

The difference actually resides in the application type, as you are downloading JSON data. The following image shows the Wireshark capture of the response got from the server:

HTTP response

Content-type: application/json

But was the request some way "special"? The following image shows the Wireshark capture of the request sent to the server from my browser:

enter image description here

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8\r\n

So we can understand how that's probably a matter of accepted type.

The solution: content type on SIM900 module

The command for specifying HTTP parameters in Simcom modules (see Simcom AT Commands guide) is AT+HTTPPARA, that you already use in your script. Its syntax is

AT+HTTPPARA=<HTTPParamTag>,<HTTPParamValue>[<UserdataDelimiter>]

There are several allowed values for HTTPParamTag parameter, but one of the options is

"CONTENT"
Used to set the “Content-Type” field in HTTP header

For this reason the command

AT+HTTPPARA="CONTENT","application/json"

should make the trick. Just make sure that your FW version supports it. If not, update the FW version to the most recent one.