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From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] ipv4/ipv6: check hop limit field on input
Date: Tue, 02 Jun 2009 02:32:39 -0700 (PDT)
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From: John Dykstra <john.dykstra1@gmail.com>
Date: Tue, 02 Jun 2009 09:22:10 +0000

> On Mon, 2009-06-01 at 19:02 -0700, David Miller wrote:
>> And I even remember there are applications that use multicast and
>> a hop limit of zero explicitly to keep application traffic only on
>> the local subnet.  So any change like that proposed could break
>> things.
> 
> Are you thinking of multicast apps that explicitly set a hop limit, or Multicast
> Listener Discovery?  The hop limit specified for MLDv2 messages is one,
> not zero.

I am very much thinking of multicast apps that explicitly set a hop
limit.

The proof is in the pudding.  As mentioned in other parts of this
thread, we explicitly DO NOT allow a zero normal socket TTL option
but we DO allow a zero multicast TTL socket option to be set.