From: Andi Kleen <[email protected]>
Date: Thu, 14 Jan 2010 12:22:16 +0100

> It's just
> 
> +       if (iph->ttl < inet_sk(sk)->min_ttl)
> +               goto discard_and_relse;
> 
> I'm probably missing something, but naively I would expect all 
> packets with ttl > 0 to be discarded then when min_ttl is zero.

Andi, the feature works from top to bottom.

The idea is that the min_ttl is set very high, so that
you'll only accept packets from hosts that started with
a ttl of 255 and are within a hop or two from you.  (therefore
you'd set min_ttl to 254 or 253, something like that)

Since the ttl can never be less than zero, the test
will never hit when min_ttl is zero, and thus this is
that state where the socket option is not enabled.
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