On Fri, 6 Dec 2002, Joshua Graessley wrote:

> I am aware of the issues with broadcast, and I strongly urge people to
> use multicast instead of broadcast for a variety of reasons. All the
> same, I've been asked to address this issue and I wanted to understand
> why FreeBSD doesn't allow broadcast on the loopback interface.
> Conceptually, it sort of makes sense to allow it. Using a broadcast
> should result in everyone on some link receiving your packet. If
> loopback is your only interface that's up, then why not use that? In
> the case of loopback, you are the only one on your link, so you should
> still receive your broadcast.
> Is there a technical reason this was done (i.e. if I set the broadcast
> flag on loopback I'll be chasing down other bugs until my hair turns
> grey or falls out) or is it a conceptual reason (i.e. broadcast, on
> loopback, are you out of your mind?).

With any kind of broadcast media, unless this is specific to Ethernet, or
there are some exceptions, a broadcast packet sent by a station is
received on every port other than the port that the packet came
from.  As far as I know, a station should never receive it's own broadcast
packets unless you have a loop somewhere in your Layer 2 infrastructure.
I believe that the above applies to IP broadcasts (or any other Layer 3
protocols) as well.

> Other platforms out there will handle broadcast on the loopback
> interface. Is it desirable to make changes to the FreeBSD stack to get
> this behavior?

Any examples?  I cannot think of a practical case where this would be
required.  I would think that an application will know if it sent a
broadcast or not, so it shouldn't have to receive that broadcast itself.
Anyone disagree?

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