Subject: Re: [Bugme-new] [Bug 13856] New: sk_bound_dev_if should be set for a socket when using IP_MULITCAST_IF or IP_ADD_MEMBERSHIO

From: Andrew Morton <[email-masked]>
Date: Wed, 29 Jul 2009 12:52:03 -0700

(switched to email.  Please respond via emailed reply-to-all, not via the
bugzilla web interface).

On Tue, 28 Jul 2009 03:42:38 GMT
[email-masked] wrote:

> http://bugzilla.kernel.org/show_bug.cgi?id=13856
> 
>            Summary: sk_bound_dev_if should be set for a socket when using
>                     IP_MULITCAST_IF or IP_ADD_MEMBERSHIO
>            Product: Networking
>            Version: 2.5
>     Kernel Version: 2.6.27.7
>           Platform: All
>         OS/Version: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: normal
>           Priority: P1
>          Component: IPV4
>         AssignedTo: [email-masked]
>         ReportedBy: [email-masked]
>         Regression: No
> 
> 
> Currently there is no way to tell which interface a multicast packet arrived on
> except by using a raw socket and issuing the SO_BINDTODEVICE option which
> forces the sk_bound_dev_if for the socket to be set.  
> 
> Consider a multi-homed system where a service needs to listen for multicast
> SSDP messages on a number of interfaces.  The system needs to know which
> interface a search message came in on in order to respond with the correct URL. 
> 
> The service needs to join the multicast group on all interfaces.  It must bind
> on INADDR_ANY and then issue the IP_ADD_MEMBERSHIP call.  
> 
> 1. If it uses one socket and joins on all interfaces it will receive mulitcast
> from all interfaces but cannot tell which interface the message came in on
> 
> 2. It it uses a socket per interface each bound on INADDR_ANY and issues
> IP_MULTICAST_IF.  In this case any mulitcast socket, no matter what interface,
> will be forwarded to each socket because sk_bound_dev_if was not set (i.e. 0) -
> see net/ipv4/udp.c [ udp_v4_mcast_next ]
> 
> Suggested solution is to set sk_bound_dev_if when IP_MULTICAST_IF is issued.
>