Sniffing for GSM packets

GSM network works on a frequency 850-900 MHz. In other words we can sniff the packet transfer between the phone and a GSM tower with an rtl-sdr dongle.

How to?
At first you might have to get the dependences:

• apt-get –y install git-core autoconf automake libtool g++ python-dev swig libpcap0.8-dev
• apt-get install gnuradio gnuradio-dev cmake git libboost-all-dev libusb-1.0-0 libusb-1.0-0-dev libfftw3-dev swig python-numpy libpcsclite-dev 

Now you will need to get libosmocore:

• git clone git://git.osmocom.org/libosmocore.git
• cd libosmocore
• autoreconf –i
• ./configure
• make
• make install
• ldconfig
• cd ..

Now you will need airprobe:

• git clone https://github.com/ksnieck/airprobe.git

At the end you might have to install the gsmdecoder:

• cd airprobe/gsmdecode
• ./bootstrap 
• ./configure
• make
• cd ..

At last you might have to install the receiver:

• cd airprobe/gsm-receiver
• ./bootstrap
• ./configure
• make

Now to start the sniffing:

• python gsm_receive_rtl.py -s 1e6

Now open wireshark and set the interface to lo(loopback). And set the filter to gsmtap so that you only get the gsm packets.

Note:- 

If you don't get packets change the frequency with the gnu radio GUI. If you want to know more you can refer to this article. You might have to add a few more dependencies if you are using other distributions of Linux.