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Date:      Thu, 13 Nov 2003 16:55:01 -0500
From:      Vincent Goupil <>
To:        "'Crist J. Clark'" <>, "''" <>, "''" <>, "''" <>
Subject:   RE: IPSec VPN & NATD (problem with alias_address vs redirect_addr ess)
Message-ID:  <F7D4BDA0E5A1D14B99D32C022AEB7366FE109F@alis-2k.alis.domain>

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But if I use this config file for natd:

redirect_address 208.x.y.120
redirect_address 208.x.y.121
redirect_address 208.x.y.122
redirect_address 208.x.y.123
alias_address 208.x.y.124

With this setup, I should be able to do 5 VPN IPSec connection at the same
Since, the ESP packet coming on 208.x.y.120 is mapped directly to and so on for the others using the redirect_address directive.
I also understand that I can use only one computer at a time for the others
using the alias_address (the rest of the network).

I'm currently using this setup.  I can do only IPSec with the witch is mapped by the alias_address.  The computer using
the IP from 208.x.y.120-123 can't use the VPN and I don't know why.


-----Original Message-----
From: Crist J. Clark []
Sent: 13 novembre, 2003 16:16
To: Vincent Goupil
Cc: ''; '';
Subject: Re: IPSec VPN & NATD (problem with alias_address vs

On Thu, Nov 13, 2003 at 12:46:24PM -0500, Vincent Goupil wrote:
> I setup a firewall with ipfw2 and natd on freebsd 4.9 release.
> I have mapped my subnet with alias_address
> I have mapped 4 private ip address with 4 public ip address
> Everything is working fine (web, email, ftp, etc..) for outgoing and
> incoming connexion for anyone on my network.
> With this configuration, 5 person at a time (on my network) could dial to
> the same VPN server.
> 4 with different IP and the one with the alias_address.  I supposed that
> only one person at a time can use the alias_address with the IPSec VPN (I
> think, tell me if I'm wrong)

Nope, that's right. You can have only one machine behind natd(8) using
ESP at a time (you could actually have one AH and one ESP at the same
time, but since NAT breaks AH, what's the point?). The reason within
natd(8) is that accept for a few protocols (TCP, UDP, ICMP, etc.), all
that it enters into its translation table is,

  IPproto: IPsrc_addr-IPdst_addr -> IPalias_addr-IPdst_addr

The obvious problem is that you can only have one mapping like
this. If you had more than one, when you receive a packet of IPproto
from IPdst_addr, to which internal machine do you send it?

Now, that's why natd(8) has problems. Why not add a feature to natd(8)
to get around it? Because there is no way to get around the
problem. ESP packets have this nice SPI field that one could
potentially use to map the traffic between multiple machines behind
NAT to a single VPN end point on the other side, but there is no
practical way for the NAT box to learn the SPI of incoming packets.
Crist J. Clark                     |
                                   |    |

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