[Tutor] Broadcasting using sockets over adhoc wifi

[Cameron Simpson]

On 11Jun2019 10:35, John Hoeksema <jhoeksem at nd.edu> wrote:
>Summer researcher using Raspbian and Python 3.5.
>
>I'm trying to use a Raspberry Pi 3 B+ to broadcast a message using the
>sockets library to other Pis (same model) over their shared ad-hoc network.
>All of the Pis can ping the others over the ad hoc network. The Pis can
>also communicate using pretty standard client-server code and the python
>socket library <https://docs.python.org/3.4/library/socket.html>. However,
>when I try to *broadcast* a message, the Pis give a "Network is
>unreachable" message (full error down below). A grad student I'm working
>with said that the script he provided me expects the server to be run in
>infrastructure mode, and configuration for ad-hoc mode is required to make
>it work correctly. This is confirmed, as I have successfully run the code
>on a desktop. I have poured over man pages and stackoverflow, and can't
>seem to find resources for how to configure socket broadcasts for ad-hoc
>networks. Any thoughts?

I would start by debugging a bit outside of Python first. And I've never 
used an ad hoc wifi network, but after setup I _imagine_ that it looks 
like a normal local network: an IP address and a network mask, so 
"braodcast" is the usual notion of the IP address with all 1s in the 
local part.

So 1:

Does a manual command line "ping" of the broadcast address work? Eg, if 
the local network were 192.168.3.x/24 and I went:

  ping 192.168.3.255

(or on Linux):

  ping -b 192.168.3.255

I'm assuming your '<broadcast>' below is an actualy broadcast address?  
Or is it a special string? If so, what happens if you hand construct a 
broadcast IP like the *.255 above? Any different?

Does tcpdump show anything useful, either locally or on one of the other 
Pis? Though given "Network is unreachable" I'd guess no packets get sent 
at all.

I repeat my disclaimer: I've not used an ad hoc wifi network.

Cheers,
Cameron Simpson <cs at cskk.id.au>