netdev - Re: A question about setting the net device for multicast forwarding in IPv4 and in IPv6

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Date:	Mon, 12 May 2008 11:57:05 +0300
From:	"Andy Johnson" <johnsonzjo@...il.com>
To:	"David Shwatrz" <dshwatrz@...il.com>
Cc:	netdev@...r.kernel.org,
	"YOSHIFUJI Hideaki / 吉藤英明" 
	<yoshfuji@...ux-ipv6.org>
Subject: Re: A question about setting the net device for multicast forwarding in IPv4 and in IPv6

Hello, David,

  - I wanted to note that there is one more difference I had noted
between IPv4 and IPv6
regarding multicast forwarding:

  - In IPv4, mc_forwarding is disabled by default (the value is 0). If
you will try to change it thus:
echo "1" >  /proc/sys/net/ipv4/conf/eth0/mc_forwarding
you will get:

/proc/sys/net/ipv4/conf/eth0/mc_forwarding: Permission denied.

ls -al /proc/sys/net/ipv4/conf/eth0/mc_forwarding
will show:
-r--r--r-- 1 root root 0 ...

But:
  chmod 666  /proc/sys/net/ipv4/conf/eth0/mc_forwarding
gives:

chmod 666  /proc/sys/net/ipv4/conf/eth0/mc_forwarding
chmod: changing permissions of
`/proc/sys/net/ipv4/conf/eth0/mc_forwarding': Operation not permitted

User space daemons (like xorp) will set
/proc/sys/net/ipv4/conf/all/mc_forwarding  to 1
(because they call setsockopt with MRT_INIT, which calls
IPV4_DEVCONF_ALL(sock_net(sk), MC_FORWARDING)++;
in ip_mroute_setsockopt(), as you mentioned.

 In IPV6, this does not happen.

In IPV6, Multicast Routing is also disabled by default.

**But **

echo "1" >  /proc/sys/net/ipv6/conf/all/mc_forwarding
**does** work and **does not** give an error (as in IPV4).

I wonder what is the reason for it ? Or maybe it should be changed to
have the same behaviour both in IPV6 and IPV4?

Regards,
Andy






On Tue, May 6, 2008 at 4:38 PM, David Shwatrz <dshwatrz@...il.com> wrote:
> Hi,
>  I had noticed that in IPV4 we set the net device for multicast
> forwarding in case
> it is not already configured so, when the user space app calls
> setsockopt() with MRT_INIT;
> this is done by calling IPV4_DEVCONF_ALL(sock_net(sk), MC_FORWARDING)++;
> in ip_mroute_setsockopt(); (net/ipv4/ipmr.c).
>
> On the other hand, the parallel method in IPv6 does **not** performs
> such action, as can be seen
> from ip6_mroute_setsockopt() in net/ipv6/ip6mr.c.
>
> My question is : is there any special for it to be so? (In case it
> should be also in IPv6, I will happy to send such a patch).
>
> Best Regards,
> DS
> --
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