Petter Reinholdtsen

A Raspberry Pi based batman-adv Mesh network node
21st October 2013

The last few days I have been experimenting with the batman-adv mesh technology. I want to gain some experience to see if it will fit the Freedombox project, and together with my neighbors try to build a mesh network around the park where I live. Batman-adv is a layer 2 mesh system ("ethernet" in other words), where the mesh network appear as if all the mesh clients are connected to the same switch.

My hardware of choice was the Linksys WRT54GL routers I had lying around, but I've been unable to get them working with batman-adv. So instead, I started playing with a Raspberry Pi, and tried to get it working as a mesh node. My idea is to use it to create a mesh node which function as a switch port, where everything connected to the Raspberry Pi ethernet plug is connected (bridged) to the mesh network. This allow me to hook a wifi base station like the Linksys WRT54GL to the mesh by plugging it into a Raspberry Pi, and allow non-mesh clients to hook up to the mesh. This in turn is useful for Android phones using the Serval Project voip client, allowing every one around the playground to phone and message each other for free. The reason is that Android phones do not see ad-hoc wifi networks (they are filtered away from the GUI view), and can not join the mesh without being rooted. But if they are connected using a normal wifi base station, they can talk to every client on the local network.

To get this working, I've created a debian package meshfx-node and a script build-rpi-mesh-node to create the Raspberry Pi boot image. I'm using Debian Jessie (and not Raspbian), to get more control over the packages available. Unfortunately a huge binary blob need to be inserted into the boot image to get it booting, but I'll ignore that for now. Also, as Debian lack support for the CPU features available in the Raspberry Pi, the system do not use the hardware floating point unit. I hope the routing performance isn't affected by the lack of hardware FPU support.

To create an image, run the following with a sudo enabled user after inserting the target SD card into the build machine:

% wget -O build-rpi-mesh-node \
% sudo bash -x ./build-rpi-mesh-node > build.log 2>&1
% dd if=/root/rpi/rpi_basic_jessie_$(date +%Y%m%d).img of=/dev/mmcblk0 bs=1M

Booting with the resulting SD card on a Raspberry PI with a USB wifi card inserted should give you a mesh node. At least it does for me with a the wifi card I am using. The default mesh settings are the ones used by the Oslo mesh project at Hackeriet, as I mentioned in an earlier blog post about this mesh testing.

The mesh node was not horribly expensive either. I bought everything over the counter in shops nearby. If I had ordered online from the lowest bidder, the price should be significantly lower:

TeknikkmagasinetRaspberry Pi model B349.90
TeknikkmagasinetRaspberry Pi type B case99.90
LefdalJensen Air:Link 25150295.-
Clas OhlsonKingston 16 GB SD card199.-
Total cost943.80

Now my mesh network at home consist of one laptop in the basement connected to my production network, one Raspberry Pi node on the 1th floor that can be seen by my neighbor across the park, and one play-node I use to develop the image building script. And some times I hook up my work horse laptop to the mesh to test it. I look forward to figuring out what kind of latency the batman-adv setup will give, and how much packet loss we will experience around the park. :)

Tags: english, freedombox, mesh network, nuug.

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