After some investigation and trial and error I'm left with the sense that multicast is not entirely implemented on plan9. Here's what I tried.
I tried setting up a udp announce directly against /net/udp for port 564. Before listening I sent an addmulti message as described in the ip manual page. According to /net/iproute there is a new route for my multicast address but it is not bound to any of my interfaces despite specifying the ip of my adapter in the addmulti. There doesn't seem to be a way to specify the interface number in the iproute protocol. My process waits but when another process on another machine sends on that multicast address it is not received. I tried pinging 224.0.01 from another machine and my plan 9 box doesn't respond to the ping like other machines with multicast enabled. Finally, after looking at the code in /sys/src/9/ip I find that while a queue of multicast IPmulti structs are assembled nothing at the Ethernet/ARP layer seems to register the multicast with the adapter so that it can receive packets. Something else that's puzzling is that I can't seem to have multiple processes on the plan9 machine listening on the same udp port. On other OSes there are flags that can be set to allow reuse. I'm not seeing anything similar here. I'm hoping that there's something that I'm missing. Has anyone else had success setting multicast servers up in any of the plan 9 variants? Otherwise, it would be a project to write the code for it. Perhaps ARP will suit my need better for detecting hosts on the same Ethernet switch. Thanks, Chris
