netdev - Re: [RFC] Fixing up TCP/UDP checksum for UDP encap. ESP4 packets in transport mode

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Date:	Mon, 06 Jul 2009 19:02:35 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	christophe@...ut.de, netdev@...r.kernel.org
Subject: Re: [RFC] Fixing up TCP/UDP checksum for UDP encap. ESP4 packets
 in transport mode

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Tue, 7 Jul 2009 09:58:50 +0800

> On Mon, Jul 06, 2009 at 06:54:11PM -0700, David Miller wrote:
>> 
>> Hmmm, aren't we talking about packets which were protected by either a
>> hash, strong encryption, or both at some point?
> 
> Only if there is no inner NAT, i.e., only if this patch isn't
> needed.  Otherwise
> 
> Source  ---  GW1  ----  GW2  ---  Dest
> 
> the path between Source and GW1, will be unprotected if transport
> mode is used between GW1 and GW2.  The only bit protected by IPsec's
> hash is between GW1 and GW2.
> 
> When the traffic comes back, then the bit between Dest and GW2 will
> be unprotected.  This is why the only safe way to use this would be
> if your traffic is one-way and you only had inner NAT at the other
> end.

I see.  So people are using transport IPSEC + NAT as a kind of
specialized tunnel.

Indeed, there is no way to handle checksums sanely.  The whole
end-to-end protection of the checksum would be entirely subverted
if we fixed it up.
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