[Interest] Simple device discovery using UDP

[coroberti .]

On Tue, Oct 2, 2018 at 5:02 PM Murphy, Sean <smurphy at walbro.com> wrote:
>
> > I have an app (Desktop) that I want it to know about other running instances
> > on the local network. I figured UDP broadcast was a natural choice. I tried it,
> > but I never saw anything but my own (local) packets. There's quite a few
> > stackoverflow questions, but none used Qt. I'm wondering if it's a wifi/router
> > thing?
>
> A couple of first questions:
> - do you have any sort of firewalls running anywhere?
> - have you tried the Qt Networks examples, and do those work?
>   http://doc.qt.io/qt-5/qtexamples.html#qt-network, maybe the Broadcast
>   Sender & Receiver examples
>
> Sean
>
Hi,
1. As Sean has mentioned, firewall settings that do not allow broadcasts
could be the case.

2. Yet another case could be: QHostAddress::Broadcast.
It's so-called All-Networks-Broadcast (255.255.255.255)
and normally requires alleviated privileges to send to (UNIX root, suid)
and could be guarded.

There's another broadcast address - so-called LAN-broadcast
which is less sensitive and more appropriate for broadcasting.

For example, at network 10.31.4.0/24, it will be 10.31.4.255,
and you can find various rules to calculate it.

3. Instead of using a broadcast address, you can figure our all
addresses on this network
and send unicast requests except to the subnet broadcast address .
At most networks, it will be sending to up to 253 address, whereas
rarely it could come to higher numbers.

Take care,

Kind regards,
Robert
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