Subject: Re: MBone tunnel and FreeBSD

From: Bill Fenner <[email-masked]>
Date: Wed, 15 Apr 1998 11:38:35 PDT

>How about allowing an application to join on a loopback interface?

Sourcing packets on a loopback interface is problematic.  I can think
of 3 possibilities:

1. You get the loopback source address, and the packet can't be routed.
2. You get the loopback source address, and the kernel does some nasty
hack to rewrite the source address, and the packet can be routed.
3. You configure another {loopback|discard} interface with the same
global address as one of your real interfaces, and you get all the
trouble that happens when a UNIX box has multiple interfaces with the
same IP address.

3 is the cleanest architecturally, except that you stop being able to
refer to an interface uniquely by its IP address, which can get UNIX
pretty confused.

There's a 4th solution which we actually implemented in the SunOS kernel
but didn't like very much:

4. Add a flag to the interface which black-holes all output multicast
packets after the loopback to local processes but before calling
the interface's output routine.

It helps in this particular case, where you have a multicast-capable
interface that you don't want to actually output any packets on; it
doesn't help in the general case where you may not even have a
multicast-capable interface at all.

  Bill

---

From: [email-masked]
Date: Thu, 16 Apr 1998 09:29:26 -0400 (EDT)

> multicast-capable interface at all.

This sounds interesting. After all if you are going to do any multicasting
one would hope you have a proper interface. Also this could evolve into a
general solution for limiting multicast traffic to only the mrouted
machine. Any idea how feasible it would be to implement this on FreeBSD??

Regards 
nand