On Wed, 22 Nov 2006, David Young wrote:
>     -I ifaddr
>             Send multicast datagrams on the network interface specified by
>             the interface's hostname or IP address.

How about:

  -I ipaddr
 	Send datagrams on the network interface specified by the
 	interface's hostname or IP address, and use that as the
 	source address of outgoing packets. On hosts with more than one IP
 	address, this option can be used to force the source address to be
  	something other than the IP address of the interface the probe
 	packet is sent on. If the IP address is not one of this machine's
 	interface addresses, an error is returned and nothing is sent.

(taken mostly from FreeBSD; someone please confirm if the error handling 
is actually true!)


  - Hubert