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Date:      Fri, 11 Oct 2002 17:44:11 -0400 (EDT)
From:      Jason Hunt <leth@primus.ca>
To:        MrWebby <mrwebby@bigfoot.com>
Cc:        freebsd-questions@freebsd.org
Subject:   Re: IPsec Tunneling (VPN) from WIN2K (client) to FreeBSD (Server)
Message-ID:  <20021011172140.T59992-100000@lethargic.dyndns.org>
In-Reply-To: <3DA73754.8080103@bigfoot.com>

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I have CC'd this message to the questions mailing list, for "archival"
purposes, and because someone else might have more insight.  My apologies
if your message was intended to be private.

Anyhow ...


On Fri, 11 Oct 2002, MrWebby wrote:

> This is really good stuff. Both of your assumptions were correct. I
> failed to include details. But I will include them here just in case you
> want to reply (again). For testing purposes, I have my laptop (client)
> behind the same NAT/Gateway/Firewall as the server is in. That can
> really create a problem. I was ignoring that all this time. Now, your
> comments suggest that my laptop will be behind a NAT which makes sense.
> I failed to realize that it will alter the packets. BUT since I'm trying
> to setup IPsec in tunnel mode, wouldn't it matter if the client is
> behind a firewall since tunnel mode will encrypt the whole packet and
> not the IP header?
>

I am honestly not 100% sure, but my understanding of this is as follows:

IPSec consists of two parts, Authentication Header (AH) and Encapsulation
Security Payload (ESP).  AH is for authenticating the identify of who the
packet came from.  ESP is for encrypting the packets that you are
tunelling.

The AH function is what causes IPSec servers/clients to not work properly
behind NAT, since the packets will fail to authenticate.  If you only use
ESP (as opposed both ESP+AH), then IPSec should work behind NAT.  I think
I might be wrong however.  Has anyone else ever tried this?

I'll have look at this a bit more later, since I am on my way out.  You
might want to look around google for "IPSec NAT", and/or around
www.kame.net


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