linux-kernel - Rcceiving a generic netlink multicast - should be restricted to the root user?

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Date:	Fri, 6 Mar 2015 19:08:34 -0700
From:	Craig Davison <craig65535@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Rcceiving a generic netlink multicast - should be restricted to the
 root user?

I wrote a kernel module that sends generic Netlink multicasts, and
wrote a userland client using libmnl that receives them.

That all works fine, but my client works even when it's not the root user.

man 7 netlink says:

Only processes with an effective UID of 0 or the CAP_NET_ADMIN capability
may send or listen to a netlink multicast group.

The listen part of this is seemingly not true. I've tried this on
kernels 3.13 (Ubuntu 14.04), 2.6.32 (CentOS 6) and 2.6.18 (CentOS 5).

Is this a bug?

If not: I know that restricting receiving generic netlink commands
incoming to the kernel to being only from root is possible with
GENL_ADMIN_PERM flag, but is it possible to send multicasts from the
kernel that can only be received by root?

Thank you
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