On Debian 12.04 LTS

I am trying to access the key net.ipv4.conf.all.mc_forwarding by doing :

sudo sysctl -w net.ipv4.conf.all.mc_forwarding=1

But every time I am doing this, I get the message

error: permission denied on key 'net.ipv4.conf.all.mc_forwarding'

Why do I get this message? How can I change this key?

  • 1
    Because /proc/sys/net/ipv4/conf/all/mc_forwarding is read-only, even for root. It seems like you have to run a multicast router instead of turning this option on directly.
    – scai
    Commented Jun 17, 2013 at 12:24

2 Answers 2


I believe this thread (EDIT: link to bit-coin miner site removed) 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.


you can also get error like this sysctl: setting key "net.ipv4.conf.all.mc_forwarding": No such file or directory

mc_forwarding - BOOLEAN
    Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
    and a multicast routing daemon is required.
    conf/all/mc_forwarding must also be set to TRUE to enable multicast
    routing for the interface

kernel should be compiled with option CONFIG_MROUTE, default kernel is not have this option set.

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