Raspberry Pi ad-hoc networking

Question

I want to try some networking projects with Raspberry Pis, and I need to just send packets between a pair of pis. I would be happy as a first step just being able to ping be

Answer 1

I was unsuccessful with any adapter using the RTL8188CUS chipset. Luckily, I had a number of Ralink RT5370 dongles (from this kit) that support the nl80211 interface and ad-hoc mode.

My solution involves using wpa_supplicant and is configured with 2 files. Ensure that the nl80211 driver is installed:

sudo apt-get install libnl1

Next, create the following wpa_supplicant configuration file called /etc/wpa_supplicant-adhoc.conf on each Pi:

ctrl_interface=DIR=/run/wpa_supplicant GROUP=netdev
update_config=1
ap_scan=2

network={
  ssid="pihoc_wpa"
  mode=1
  frequency=2462
  proto=WPA
  key_mgmt=WPA-NONE
  pairwise=NONE
  group=TKIP
  psk="password"
} 

where you can choose the ssid, frequency (look here for valid values), and psk. Make sure that you are part of the user group net-dev using the command

getent group netdev

and if not, you can add yourself using

sudo usermod -a -G netdev userName

Next, add the following block to the /etc/network/interfaces file on each Pi:

auto wlan0
allow-hotplug wlan0
iface wlan0 inet static
   address 10.10.2.1
   netmask 255.255.255.0
pre-up wpa_supplicant -B -D nl80211 -i wlan0 -c /etc/wpa_supplicant-adhoc.conf

where each Pi has a different address field beginning with 10.10.2.. Also, if your RT5370 adapter is using an interface other than wlan0 (e.g. wlan1, wlan2, etc.), be sure to use that interface name instead.

At this point, the Pis should automatically join the network upon being rebooted. Test the connection by pinging or using ssh, for example run the following from the agent with IP address 10.10.2.1:

ssh 10.10.2.2

to access the agent with IP address 10.10.2.2.

The steps listed here are adapted from this Arch Linux wiki article and this Raspberry Pi forum discussion.