net.ipv4.conf.all.mc_forwarding: why is my access denied?

I believe this thread describes your problem

In a nutshell, the proc entry containing that value is read-only, and cannot be made writable easily:

$ ls -ln /proc/sys/net/ipv4/conf/all/mc_forwarding 
-r--r--r-- 1 0 0 0 Jun 17 08:20 /proc/sys/net/ipv4/conf/all/mc_forwarding
$ sudo chmod u+x /proc/sys/net/ipv4/conf/all/mc_forwarding
chmod: changing permissions of `/proc/sys/net/ipv4/conf/all/mc_forwarding': Operation not permitted

But you can run your own router daemon to (e.g xorp) to get multicast forwarding.