I am in charge of a project in which we have several raspberry distributed around my city, each one connected to a router (TP-LINK TL-MR3020) special to put them USB dongle of 3G connection.
These raspberry consume internet through 3G for simple requests to a server. The problem comes when suddenly with the change of schedule from winter to summer the consumption of 3G data is triggered. We did not know what was happening and we did not have the date collected, but suddenly when we changed from summer to winter time, consumption was normalized to the use of the application (a very low consumption) that we have installed for the use of signings.
The system worked properly connected to Raspberry Pi 3 Model B and Raspbian Stretch Lite as operating system. This system does not have the graphical interface part but it installed an own application with an interface for touch screens. We have ruled out that the consumption of excessive 3G data comes from our application, which is why we attribute it to the raspberry system. It was then when reviewing the data we realized that the consumption of data coincided only with the summer time and was not solved with a simple restart.
We have also ruled out that the raspberry has been attacked, since having a 3G connection there is no remote access by any port (in the case of our operator). In addition, the WiFi of the router is disabled since they are connected by cable from the router + 3G key to the raspberry.
As a solution, we changed the system by default from the system time to the ntp service and we configured it with the schedules that coincide with the Spanish in case it was a problem with the time servers.
Unfortunately we have discovered that these changes have not solved the problem and we do not understand how the raspberry can spend 180 Megabits in a single day when 10 requests will be made every day that should not consume or 5 megabits per request (REST) as much among the 8: 00 and 14:00. In addition, consumption is also at dawn and outside working hours when no one is using the device.
What can I do so that my raspberry does not consume 3G data in an uncontrolled way when nobody is using it? Is it due to some service to maintain the correct schedule? Problems with the ntp service?
As you will understand, I can not walk around all the points several times and consuming raspberry's 3G Internet to update things so if there is a solution to consume the minimum amount of data I would like to have a detailed explanation of why it happens, how to solve it, and what to do so that it does not happen again.