While I'm no longer working on multicast applications, there seems to be
some interest in the topic, so I'll continue maintaining this page. The
master copy of this page is located at:
Credits: The following is an excerpt from the excellent Multicast-HOWTO
by Juan-Mariano de Goyeneche.
If you want just to send and receive, you must say yes to "IP:
multicasting" when configuring your kernel. If you also want your
Linux box to act as a multicast router (mrouter) you also need to
enable multicast routing in the kernel by selecting "IP:
forwarding/gatewaying", "IP: multicast routing" and "IP: tunneling",
the latter because new versions of mrouted relay on IP tunneling to
send multicast datagrams encapsulated into unicast ones. This is
necessary when establishing tunnels between multicast hosts separated
by unicast-only networks and routers. (The mrouted is a daemon that
implements the multicast routing algorithm -the routing policy- and
instructs the kernel on how to route multicast datagrams).
Some kernel versions label multicast routing as "EXPERIMENTAL", so you
should enable "Prompt for development and/or incomplete code/drivers"
in the "Code maturity level options" section.
If, when running the mrouted, traffic generated in the same network
your Linux box is connected to is correctly forwarded to the other
network, but you can't see the other's network traffic on your local
network, check whether you are receiving ICMP protocol error messages.
Almost sure you forgot to turn on IP tunneling in your Linux router.
It's a kind of stupid error when you know it but, believe me, its
quite time-consuming when you don't, and there is no apparent reason
that explains what is going wrong. A sniffer proves to be quite useful
in these situations!
Once you have compiled and installed your new kernel, you should
provide a default route for multicast traffic. The goal is to add a
route to the network 224.0.0.0.
The problem most people seem to face in this stage of the
configuration is with the value of the mask to supply. If you have
read Terry Dawson's excellent NET-3-HOWTO, it should not be difficult
to guess the correct value, though. As explained there, the netmask is
a 32 bit number filled with all-1s in the network part of your IP
address, and with all-0s in the host part. Recall from section 2.1
that a class D multicast address has no netwok/host sections. Instead
it has a 28-bit group identifier and a 4-bit class D identifier. Well,
this 4 bits are the network part and the remaining 28 the host part.
So the netmask needed is 11110000000000000000000000000000 or, easier
to read: 240.0.0.0. Then, the full command should be:
route add 224.0.0.0 netmask 240.0.0.0 dev eth0
Depending on how old your route program is, you might need to add the
-net flag after the add.
Here we supposed that eth0 was multicast-capable and that, when not
otherwise specified, we wanted multicast traffic to be output there.
If this is not your case, change the dev parameter as appropriate.
The /proc filesystem proves here to be useful once again: you can
check /proc/net/igmp to see the groups your host is currently subscribed to.
The current release version of mrouted is 3.8. Version 3.9 is in beta.
You can find both versions at:
ftp.parc.xerox.com/pub/net-research/ipmulti/
Unfortunately, there are no precompiled binaries for Linux at this site, but
you should be able to build a working version from source. Julian Highfield
has published some helpful
notes on building the latest release of mrouted. There is also a
Linux-specific mrouted (version 3.81) is available at:
www.video.ja.net/mice/mrouted/Linux/
This version seems to be maintained by the
Belgian Multimedia Integrated
Conferencing for Europe National Support Center.
I know nothing about this group or this version of mrouted.
The project seems to have ended in December 1995, but the tar file
has a last modified date of 13-Mar-97. Your mileage may vary.
Here's another ftp site with Linux-specific versions (3.81 and 3.9 beta)
of mrouted. Again, I know nothing about this release:
ftp.dc.ufscar.br/pub/mbone/
Yet another place to find mrouted 3.81, this time in Italian:
http://caristudenti.cs.unibo.it/~tonon/mrouted.htm
Please, no more requests for help on finding or building mrouted.
If someone would like to write up a short HOW-TO on building mrouted, I'd be
happy to publish it here, or provide a link. I'll try to provide some more
information soon....
