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Date:      Mon, 08 Apr 2002 14:20:12 -0700
From:      Lars Eggert <larse@ISI.EDU>
To:        "Rogier R. Mulhuijzen" <drwilco@drwilco.net>
Cc:        mgt@hytekblue.com, freebsd-net@FreeBSD.ORG
Subject:   Re: IPsec tunnel mode
Message-ID:  <3CB2098C.5080904@isi.edu>
References:  <5.1.0.14.0.20020408202757.01cac470@mail.drwilco.net>

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Rogier R. Mulhuijzen wrote:
 >> http://www.x-itec.de/projects/tuts/ipsec-howto.txt
 >
 > Unfortunately this howto, like any other mention of IPsec &
 > tunneling on the net uses the gif interface. Which is IPoverIP, and
 > this does not seem to match with  IPsec tunnel devices.

There are no IPsec tunnel devices in KAME. IPsec defines "security
associations" (SAs), which are not represented as devices in the routing
table in KAME. Thus, you can't use routes to direct traffic into these
tunnel mode SAs, you need to set up your security policies with the
correct selectors (think firewall-like matching).

*Many* tutorials on the net do not understand this disctinction, and
tell you to set up an IPIP tunnel (using a gif) and an IPsec tunnel
mode SA in parallel. This is a bad hack, since you (ab)use a side effect
of creating an IPIP tunnel device (it can be used for route entries) to
redirect traffic into your (separate) tunnel mode SA. Very roughly, you
set up the IPIP tunnel, then yank out the packets destined for it during 
outbound processing and force them over an IPsec tunnel mode SA.

Use EITHER IPsec tunnel mode alone OR IPIP tunnels and IP transport
mode (draft-touch-ipsec-vpn). Mixing both can work in some scenarios 
where the dependencies between side effects are just right, but in 
general, it's a broken approach.

Lars
-- 
Lars Eggert <larse@isi.edu>               Information Sciences Institute
http://www.isi.edu/larse/              University of Southern California

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