Subject: RE: [NAT] a basic question

From: "Shahrier, Sharif M." <[email-masked]>
Date: Thu, 10 May 2001 13:14:08 -0400

Bala,

	Let's consider we're just using Basic-NAT, i.e. without any TCP/UDP
port numbers.
	Suppose that NAT-A and NAT-B are assigned 3 global addresses each.
	Of course, the hosts on each network will be aware of this.

	Suppose that there are 100 hosts in each network with private
addresses. 
	So the 3 global addresses cannot map to all these addresses.
	In this case, when the datagram arrives at B, how can the NAT decide
which host to
	send the packet to based on the global address? There is no mapping
entry in its table.

 	Thus, the table in NAT-B had to have been set up for translation
from public to
	a specific internal host address, before the first packet ever came
in from network A.
	So, is there a separate procedure that does this when theTCP/UDP
connection is first set up?

	Now, going back the other way from B --> A is straightforward
because NAT-A already has the
	mapping of public to private host entry already in its table. This
was done when the packet 
	was first sent out.

Sharif.

Sharif,

Nat B will have the mapping already..else how will the
host associated with NAT A know the destination public address.

If there is a single global address available with NAT B then the
contention will be resolved through mapping of ports..

Bala

> I have a basis question on NAT, and would be grateful of a response.
> 
> Suppose a host on private network A wants to send to a host on private
> network B. 
> For the outgoing datagram, the NAT replaces local s-addr with its
> public p-addr. It makes an entry in its table.
> The packet is sent to NAT-B to its public address.
> When the packet reaches NAT-B, I assumed that NAT-B would have to do a
> look-up to replace the public dest-addr
> in the packet header, with the private address of the local host to
> which the datagram was destinted. 
> There is no way to know what the mapping is, unless a mapping entry was
> established by some other means beforehand.
> In that case, several hosts may have been pre-mapped to the same public
> address.
> Thus, the NAT-B will not know unambiguously to which host to forward
> the datagram to.
> Security, problems......
> 
> I am sure that thi issue has been solved, and thus would be grateful if
> someone can send me a response.
> 
> Thanks.
> 
> -Sharif.
> 
> _______________________________________________
> nat mailing list
> [email-masked]
> http://www.ietf.org/mailman/listinfo/nat

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---

From: Pyda Srisuresh <[email-masked]>
Date: Thu, 10 May 2001 10:32:58 -0700 (PDT)

RFC 2694 (DNS_ALG) is probably what you should take a look at. 
Thanks.

cheers,
suresh

> > someone can send me a response.

=====

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