netdev - multicast routing and multiple interfaces with same IP

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Message-ID: <1b9338490908111626j55b49177q71b8a373b6e6381b@mail.gmail.com>
Date:	Wed, 12 Aug 2009 02:26:21 +0300
From:	"Ilia K." <mail4ilia@...il.com>
To:	netdev@...r.kernel.org
Subject: multicast routing and multiple interfaces with same IP

Hi All,
When routing daemon wants to enable forwarding of multicast traffic it
performs something like:

	struct vifctl vc = {
		.vifc_vifi  = 1,
		.vifc_flags = 0,
		.vifc_threshold = 1,
		.vifc_rate_limit = 0,
		.vifc_lcl_addr = ip, /* <--- ip address of physical interface, e.g. eth0 */
		.vifc_rmt_addr.s_addr = htonl(INADDR_ANY),
	  };
	setsockopt(fd, IPPROTO_IP, MRT_ADD_VIF, &vc, sizeof(vc));

This leads (in the kernel) to call to vif_add() function call which
search the (physical) device using assigned IP address:
	dev = ip_dev_find(net, vifc->vifc_lcl_addr.s_addr);

It seems like API (struct vifctl) does not allow to specify an
interface other way than using it's IP, and if there are more than a
single interface with specified IP only the first one will be found
(for example it makes problems when tunnel is configured using the
same IP as underlying interface).

Am I correct in identifying the problem?
I can propose the attached patch against 2.6.30.4.

Regards,
Ilia.

View attachment "vif_add.patch" of type "text/x-diff" (1174 bytes)