altnet parameter into pimd.conf file

Question

I need to route a multicast streams in my private network. I'm using pimd for multicast routing. The multicast routing did not work until I put altnet <multicast_source_ip> masklen 24 to the pimd.conf file. The problem that I do not always know what the <multicast_source_ip>.

I still not understand what the altnet directive is doing? According to the pimd.conf file:

# If you want to add "alternative (sub)net" to a physical interface,
# e.g., if you want to make incoming traffic with a non-local source address
# to appear as it is coming from a local subnet, then use the command:
#   phyint <local-addr | ifname> altnet <net-addr> masklen <len>
# XXX: if you use this command, make sure you know what you are doing!!

And how can I solve this problem without <multicast_source_ip> using?

Answer 1

The altnet directive to phyint specifies an alternative subnet for a given upstream interface.

Say you want to route from iface eth0 to eth1, the subnet on eth0 is 192.168.1.0/24 and eth1 has 10.0.0.0/24. However, even if the multicast source is located on the LAN eth0 is attached to, it operates on the 172.12.0.0/24 subnet. Adding altnet 172.12.0.0/24 to the eth0 phyint configuration makes pimd accept the traffic as locally originating.

I've added the following new paragraph to the man page for the project on GitHub:

altnet network/len: Alternative host(s)/network(s) to accept as locally attached multicast sources on a given interface. If a phyint is attached to multiple IP subnets, describe each additional subnet with the altnet keyword.

Usually you shouldn't need this option, so I'm a bit curious about your setup. Did you enable IP forwarding in sysctl.conf, or similar?