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Versions: 00                                                            
Internet Engineering Task Force                        R. G. Moskowitz
Internet Draft                                    Chrysler Corporation
Expires in six months                                 February 6, 1998





          Network Address Translation issues with IPsec
                 <draft-moskowitz-net66-vpn-00.txt>



Status of this Memo

   This document is an Internet-Draft.  Internet Drafts are
   working documents of the Internet Engineering Task Force
   (IETF), its areas, and its working Groups. Note that other
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   Distribution of this memo is unlimited.

Abstract

   This document looks at a number of issues surrounding the need
   for network address translation (NAT) when IPsec is used to
   create virtual private networks (VPN).  This document only
   looks at simple VPNs.  That is VPNs consisting of a single
   IPsec tunnel as compared to VPNs consisting of ‘chained’
   and/or ‘nested’ IPsec tunnels and/or transports.  It proposes
   a method to vastly reduce the extent that NAT is needed in a
   VPN.





R. Moskowitz                                                  [Page 1]


Internet Draft          NAT issues with IPsec        February 5, 1998


Table of Contents

   1. Introduction..............................................2
     1.1 Requirements...........................................2
   2. Network Description.......................................3
   3. Addressing for VPN Servers................................3
     3.1 Sample DNS entries for VPN Servers.....................4
     3.2 Internal addressing and routing in VPN Server networks.4
     3.3 Internal routing in VPN Client networks................4
     3.4 Sample Process flow....................................4
   4. Tunnel discovery the KX record............................5
     4.1 KX Process flow........................................5
     4.2 Alternatives to the KX record..........................6
   5. IANA functions............................................6
   6. Security Considerations...................................6
   7. References................................................6
   8. Acknowledgments...........................................6
   9. Author's Addresses........................................6


1. Introduction

   This document proposes a methodology to produce simple to
   manage Virtual Private Networks (VPNs). It assumes that the
   VPNs are constructed of tunnels (IP or link layer in IP)
   between IP networks across an internet.  If proposes a variant
   of ‘Address Allocation for Private Internets’ [RFC 1918] to
   minimize the need of Network Address Translation (NAT).

   The name NET66 has nothing to do with the addresses used in
   this document, network 7 is used as the example network that
   IANA would assign and manage for this VPN methodology.  Rather
   NET66 is an allusion to the per-interstate road across the US,
   Route 66.  Route 66 was a defined path across the western half
   of the US.  People planned their entrance and exit points from
   Route 7.  Likewise, NET66 means some planning for entrance and
   exit from the VPN, but the expectation of a smooth trip over
   it.



1.1 Requirements

   The Design objectives are:

          All servers in the VPN are globally addressed.

          The servers need not be globally routable:



R. Moskowitz                                                  [Page 2]


Internet Draft          NAT issues with IPsec        February 5, 1998


               Server addresses need to be routable in the
               server’s network.
               Server addresses need to be default or statically
               routed to the VPN gateway on the client’s network.

          The clients need a name they can resolve that yields an
          address that will route to the client’s gateway.

          The clients gateway needs to discover the server’s
          gateway’s address with only the server’s address as a
          clue.




2. Network Description

   The following diagram will be used in this discussion as a
   sample VPN of a community of interest.

          a.com                                          b.com

   Host(1)-----|                                 |-----Server(a)
               |                                 |
   Host(2)-----|---- GW(1)-------------GW(2) ----|-----Host(3)
               |                 |               |
   Server(b)---|                 |               |-----Server(c)
                                 |
                                 |
          c.com                  |                       d.com
                                 |
                                 |
   Host(4)-----|                 |               |-----Server(d)
               |                 |               |
   Host(5)-----|---- GW(3)-------------GW(4) ----|-----Server(e)
               |                                 |
   Server(f)---|                                 |-----Host(6)



3. Addressing for VPN Servers

   If Srva.b.com has a global address then Host1.a.com can
   resolve the name to the address.  A.com will have to have a
   route for this address to GW1.  If all the VPN accessible
   servers are addressed out of the same block, then a.com only
   needs one route to GW1 for all servers accessed by
   host1.a.com.  Since all of the traffic to the servers is
   within the VPN tunnels these addresses need not be routable


R. Moskowitz                                                  [Page 3]


Internet Draft          NAT issues with IPsec        February 5, 1998


   on the internet, they only need be registered in the
   internet’s DNS.



3.1 Sample DNS entries for VPN Servers

   Assume that the block of addresses for VPN Servers is
   7.0.0.0/8.  If the 4 companies in section 2 received an
   allocation of 2 addresses the DNS entries might look like:

   Srvb.a.com            IN   A    7.0.1.1
   Srva.b.com            IN   A    7.0.2.1
   Srvc.b.com            IN   A    7.0.2.2
   Srvf.c.com            IN   A    7.0.3.1
   Srvd.d.com            IN   A    7.0.4.1



3.2 Internal addressing and routing in VPN Server networks

   The addresses in 3.1 would be used both in the public DNS and
   as the actual address on the servers.  The router adjacent to
   the server would advertise a unique route for the server (e.g.
   7.0.1.1/32).  This way the packets will reach the server as
   they emerge from the gateway without any address translation
   required.


3.3 Internal routing in VPN Client networks

   The client networks (e.g. a.com for host1.a.com) advertise a
   route of 7.0.0.0/8 directed to their gateway.


3.4 Sample Process flow

   Host1.a.com needs to connect to a TCP application on
   Srva.b.com.  Host1 performs a DNS lookup and gets the address
   7.0.2.1.  Host1 (assume its address is 192.168.50.2) sends out
   a TCP SYN sourced from 192.168.50.2, destined for 7.0.2.1.

   A.com’s network routes this packet into GW1 since it has a
   route to 7.0.0.0/8.  GW1 (whose public address is
   199.175.30.1) has a policy (see sec 4) that a destination of
   7.0.2.1 is tunneled to 208.50.1.1 (GW2’s public address).  GW1
   places the packet it received into an IP packet sourced from
   199.175.30.1, destined for 208.50.1.1.

   The internet routes this packet to GW2.  GW2 removes the
   internal packet.  Since the source address is a private


R. Moskowitz                                                  [Page 4]


Internet Draft          NAT issues with IPsec        February 5, 1998


   address, GW2 MUST translate this address to an address usable
   within b.com (for example for the pool of 172.17.1.0/24).
   This requirement is based on the likelihood of GW2 having two
   or more tunnels from different hosts, all having the same
   private address.  If the source address had been a public
   address, GW2 COULD have left the address unchanged if b.com’s
   network is able to route that address (as a destination
   address) back to GW2 (this is a site option).  GW2 now
   delivers the packet into b.com’s network.

   B.com’s network routes this packet to Srva since there is a
   route to 7.0.2.1.  Srva reverses the source and destination
   addresses and creates an SYN/ACK.  B.com’s network routes this
   packet back to GW2.  GW2 reverses the address translation on
   the destination address (if it performed one) and places the
   packet into an IP packet sourced from 208.50.1.1, destined for
   199.175.30.1 (that is GW1).

   The internet delivers this packet to GW1.  It removes the
   inner packet and puts it on a.com’s network.  A.com’s network
   delivers the packet to Host1.




4. Tunnel discovery the KX record

   A key point in section 3.4 is GW1 ‘knowning’ that a packet
   destined for 7.0.2.1 was to be tunneled to 208.50.1.1.  This
   could be done via manual configuration; that is GW1’s
   administrator had set up a rule accordingly.  This method is
   easy to deploy on a small scale, but is not usable in a large
   context.  An alternative is for GW1 to be able to process DNS
   KX records [RFC 2230].  Consider the following DNS records:

   Srva.b.com.                IN   A    7.0.2.1
                              IN   KX   10, gw2.b.com.
   1.2.0.7.in-addr.arpa.      IN   A    Srva.b.com.
   gw2.b.com.                 IN   A    208.50.1.1


4.1 KX Process flow

   A configuration option on GW1 starts the KX processing if the
   destination address is 7.0.0.0/8.  GW1 does a reverse lookup
   on the destination address of 7.0.2.1 and gets Srva.b.com.  It
   next does a lookup for a KX record for Srva.b.com and gets
   gw2.b.com.  Finally it does a lookup up on gw2.b.com and gets
   208.50.1.1 as the tunnel end point.


R. Moskowitz                                                  [Page 5]


Internet Draft          NAT issues with IPsec        February 5, 1998



   Since KX processing’s purpose is to retrieve VPN configuration
   information, all of these queries MUST be secured with DNSSEC.
   This, plus the three DNS queries might be considered as a
   drawback to using the KX record.  Another drawback in the KX
   record is that it does not supply any tunnel policy
   information, like a hint at what type of tunneling technology
   or parameters to use.


4.2 Alternatives to the KX record

   There are many alternatives to the KX record.  SRVLOC, LDAP
   directory, and Attribute Certificates have all been proposed.
   Attribute certificates have the security needs stated above
   built into them.  A full analysis is needed to select a better
   alternative to the KX approach.



5. IANA functions

   IANA will reserve a large block of addresses, a /8 is
   recommended (and might not be enough until IPv6 is deployed
   eliminating the need of NET66).  IANA will develop the
   processes and procedures for an organization to license
   addresses from this block for their VPN accessible servers.


6. Security Considerations

   Network address translation, in conjunction with IPsec makes
   some large assumptions of trust.  Intermediate systems are
   changing IP addresses on behalf of other systems.

   The KX record relies on the deployment of DNSSEC, without this
   it cannot be trusted.  In fact the whole global server address
   block and its reverse lookup block needs to be secured with
   DNSSEC as well.


7. References

   [RFC 1918] Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de
   Groot & E. Lear  ‘Address Allocation for Private Internets’
   [RFC 2230] R. Atkinson  ‘Key Exchange Delegation Record for
   the DNS’





R. Moskowitz                                                  [Page 6]


Internet Draft          NAT issues with IPsec        February 5, 1998


8. Acknowledgments

   This document is based on discussions with Mark Johnson and
   Mike Papais of Chrysler Corporation, along with a host of
   others at the the Automotive Industry Action Group (AIAG) and
   within the IETF community.



9. Author's Addresses

     Robert G. Moskowitz
     rgm@icsa.net
     ICSA, Inc.





































R. Moskowitz                                                  [Page 7]