[Freeciv-Dev] Re: (PR#6707) Announce server on LAN

["andrearo@xxxxxxxxxxxx" <andrearo@xxxxxxxxxxxx>]

On Wed, 5 Nov 2003, Todd Goodman wrote:
> * Raimar Falke <i-freeciv-lists@xxxxxxxxxxxxx> [031105 12:02]:
> > > Yes, the way he was doing it (SIOCADDMULTI).
> >
> > SIOCADDMULTI isn't mentioned in the patch. Do you suggest that we add
> > this ioctl call to freeciv? How portable is it?
>
> If you're going to use multicast then it's the right way to do it.

Why?

> > > > so that you don't spam the whole internet.
> > >
> > > You don't have to adjust your OS.  The server and clients do it
> > > programatically.  You set TTL to 1 so you don't send anything off LAN.
> > >
> > > There's nothing the user needs to do (unless their OS doesn't support
> > > multicast, in which case it doesn't matter what you do as this feature
> > > wouldn't work...)
> >
> > > There's a pretty good chance that traffic to 224.0.0.1 will never make
> > > it off the local network no matter what you set the TTL to as the router
> > > isn't supposed to forward it regardless.
> >
> > And this IMHO a feature. And equivalent to setting TTL to 1. So what
> > is the advantage of your solution?
>
> It's a feature, but IMNHO it's wrong to try to use multicast to send to
> every host on the local network.
>
> You can set the TTL to a configurable value (default 1) and use a
> different multicast group.
>
> That's the point of multicast, send it to a group of destinations that
> want to receive it, but not to every station on the LAN.
>

I agree with Todd here, the reason to use multicasting is so that only
hosts which have joined the multicasting group shall get the packet.
There's no reason to broadcast the packet to every host on the LAN.

Try this:
andreas@linux:~> export FREECIV_MULTICAST_GROUP="237.0.0.1"

Then start the patched Freeciv, and while looking for LAN servers,
run "netstat --groups" :

andreas@linux:~> netstat --groups
IPv6/IPv4 Group Memberships
Interface       RefCnt Group
--------------- ------ ---------------------
eth0            2      237.0.0.1
lo              1      ipv6-allnodes

As you can see, the interface eth0 has joined the 237.0.0.1 group.
After the client and server are done sending packets to each other, the
interface eth0 leaves the group 237.0.0.1. This is group management is
handled by the operating system kernel, and is invisible to the user.

I can't imagine an OS not supporting multicasting, it's a important part
of IPv4.

 Andreas Røsdal