[PATCH] don't put multicasts with mc_ttl=0 on the wire

[PATCH] don't put multicasts with mc_ttl=0 on the wire

[akepner]

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@sgi.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

Re: [PATCH] don't put multicasts with mc_ttl=0 on the wire

[David Miller]

From: akepner@sgi.com
Date: Tue, 15 May 2007 12:56:02 -0700

> 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@sgi.com>

This is actually used by some things if I remember correctly.
See this command and code in net/ipv4/route.c:

			/* Special hack: user can direct multicasts
			   and limited broadcast via necessary interface
			   without fiddling with IP_MULTICAST_IF or IP_PKTINFO.
			   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.
			   From the viewpoint of routing cache they are broken,
			   because we are not allowed to build multicast path
			   with loopback source addr (look, routing cache
			   cannot know, that ttl is zero, so that packet
			   will not leave this host and route is valid).
			   Luckily, this hack is good workaround.
			 */

			fl.oif = dev_out->ifindex;
			goto make_route;

Re: [PATCH] don't put multicasts with mc_ttl=0 on the wire

[David Stevens]

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@vger.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@sgi.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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