Re: Multicast IP addresses

On Fri, Oct 9, 2009 at 10:33 AM, Kelly Jones
<[email protected]> wrote:
> I've known about multicast IP addresses for a long time, but just
> became interested in them. Questions:
>
>  % Is multicast theoretical, or actually being used?
>
>  % What multicast IP addresses broadcast free/test information (time
>  of day, weather, delayed stock quotes, etc)? I think I've figured out
>  how to register myself as a listener to a given multicast IP address,
>  but which IP addresses are actually "broadcasting" anything?
>
>  % What command-line app lets me register myself as a listener and
>  listen to all traffic on a given multicast IP address?
>
>  % Sending data to a multicast IP address is easy: just send a UDP
>  packet to the address. Question: how do multicast IP address users
>  prevent people from spamming a given IP address?
>
>  % Anyone can listen to a multicast IP address w/o the sender knowing
>  it. How do senders transmit non-public data over multicast IP?
>  Encryption?

I'm unaware of the implementation your referring to with accessing
weather data, etc.  For a few years, I've been using Multicast/udpcast
methods to:
Stream audio/video over local networks
Clone large amounts of computers using dd over udpcast

I wasn't aware of the type of public services you reference, and I
don't think it's a good idea to transmit private data over public
networks via multicast.  It wouldn't be hard to use something like SSH
to send and receive data this way, if you wanted to - but, as I
understand it, since anything sent to the multicast address goes to
every endpoint, I'm not sure why you would want to do this.

Any command line app that lets you place a NIC into promiscuous mode
will also allow you to sniff multicast traffic, if you're just wanting
to see raw packets.  If you want to use the data for something useful,
you'll need a client such as VLC or udp-receiver -- anything capable
of decoding the packets.

I was not aware that, without special allowances, multicast traffic
was routable.  I do know it's quite easy to flood a LAN with udpcast
traffic without throttle controls in place on the hardware, so I'd be
interested of seeing at work any multicast traffic on the Internet.
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