netdev - Re: [PATCH] tcp: Generalized TTL Security Mechanism

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Date:	Thu, 14 Jan 2010 03:27:39 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	andi@...stfloor.org
Cc:	shemminger@...tta.com, netdev@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH] tcp: Generalized TTL Security Mechanism

From: Andi Kleen <andi@...stfloor.org>
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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