Petter Reinholdtsen

Easier recipe to observe the cell phones around you
24th September 2017

A little more than a month ago I wrote how to observe the SIM card ID (aka IMSI number) of mobile phones talking to nearby mobile phone base stations using Debian GNU/Linux and a cheap USB software defined radio, and thus being able to pinpoint the location of people and equipment (like cars and trains) with an accuracy of a few kilometer. Since then we have worked to make the procedure even simpler, and it is now possible to do this without any manual frequency tuning and without building your own packages.

The gr-gsm package is now included in Debian testing and unstable, and the IMSI-catcher code no longer require root access to fetch and decode the GSM data collected using gr-gsm.

Here is an updated recipe, using packages built by Debian and a git clone of two python scripts:

  1. Start with a Debian machine running the Buster version (aka testing).
  2. Run 'apt install gr-gsm python-numpy python-scipy python-scapy' as root to install required packages.
  3. Fetch the code decoding GSM packages using 'git clone'.
  4. Insert USB software defined radio supported by GNU Radio.
  5. Enter the IMSI-catcher directory and run 'python scan-and-livemon' to locate the frequency of nearby base stations and start listening for GSM packages on one of them.
  6. Enter the IMSI-catcher directory and run 'python' to display the collected information.

Note, due to a bug somewhere the scan-and-livemon program (actually its underlying program grgsm_scanner) do not work with the HackRF radio. It does work with RTL 8232 and other similar USB radio receivers you can get very cheaply (for example from ebay), so for now the solution is to scan using the RTL radio and only use HackRF for fetching GSM data.

As far as I can tell, a cell phone only show up on one of the frequencies at the time, so if you are going to track and count every cell phone around you, you need to listen to all the frequencies used. To listen to several frequencies, use the --numrecv argument to scan-and-livemon to use several receivers. Further, I am not sure if phones using 3G or 4G will show as talking GSM to base stations, so this approach might not see all phones around you. I typically see 0-400 IMSI numbers an hour when looking around where I live.

I've tried to run the scanner on a Raspberry Pi 2 and 3 running Debian Buster, but the grgsm_livemon_headless process seem to be too CPU intensive to keep up. When GNU Radio print 'O' to stdout, I am told there it is caused by a buffer overflow between the radio and GNU Radio, caused by the program being unable to read the GSM data fast enough. If you see a stream of 'O's from the terminal where you started scan-and-livemon, you need a give the process more CPU power. Perhaps someone are able to optimize the code to a point where it become possible to set up RPi3 based GSM sniffers? I tried using Raspbian instead of Debian, but there seem to be something wrong with GNU Radio on raspbian, causing glibc to abort().

Tags: debian, english, personvern, surveillance.

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