netdev - Re: [PATCH] don't put multicasts with mc

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Date:	Tue, 15 May 2007 14:40:55 -0700
From:	David Stevens <dlstevens@...ibm.com>
To:	akepner@....com
Cc:	arenaud@....com, netdev@...r.kernel.org,
	netdev-owner@...r.kernel.org
Subject: Re: [PATCH] don't put multicasts with mc_ttl=0 on the wire

Arthur,
        I assume you're making use of the hack mentioned in route.c:

 "... This hack is not just for fun, it allows vic, vat and friends to 
work.
They bind socket to loopback, set ttl to zero and expect that it will 
work."
        I don't know the details of the intent for this hack, but did you
test that it won't break them?
        A multicast application that relies on the (unicast) routing table
at all is broken IMHO, as is an app that sets the TTL to 0, but shouldn't
all ttl 0 packets about to be sent to the wire be dropped? Wouldn't that
be a better way to handle this case?

                                                +-DLS

netdev-owner@...r.kernel.org wrote on 05/15/2007 12:56:02 PM:

> A colleague of mine found that multicasts with a ttl of 0
> can be sent on the wire. This happens if the sender doesn't
> belong to the destination multicast group.
> 
> With the following the multicast ttl is respected whether
> or not the sender belongs to the destination multicast group.
> 
> Signed-off-by: Arthur Kepner <akepner@....com>
> 
> ---
> 
>  net/ipv4/route.c |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index cb76e3c..bf25cf5 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -2249,8 +2249,7 @@ static inline int __mkroute_output(struct rtable 
**result,
>     }
>     if (flags & (RTCF_BROADCAST | RTCF_MULTICAST)) {
>        rth->rt_spec_dst = fl->fl4_src;
> -      if (flags & RTCF_LOCAL &&
> -          !(dev_out->flags & IFF_LOOPBACK)) {
> +      if (!(dev_out->flags & IFF_LOOPBACK)) {
>           rth->u.dst.output = ip_mc_output;
>           RT_CACHE_STAT_INC(out_slow_mc);
>        }
> 
> -- 
> Arthur
> 
> -
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