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Re: IPSEC and PAT
From: "Tony Rall" <trall () almaden ibm com>
Date: Thu, 13 Sep 2001 20:23:47 -0700


On Thursday, 2001/09/13 at 21:43 AST, "Steven M. Bellovin" 
<smb () research att com> wrote:
I repeat -- it doesn't do PAT.  Some "routers" -- they're really no
such thing, of course; they're NAT boxes and/or bridges -- allow one
host behind them to speak IPsec.  If a host emits a packet using ESP,
it's tagged as *the* IPsec user; return IPsec packets are routed to
that host.  (Some of these boxes may use manual configuration instead
or in addition.)  You can't have two IPsec hosts, because there's no
way to know which should receive incoming packets -- there's no
relationship between inbound and outbound SPIs.

Actually you can have multiple IPSEC sessions hidden behind a NAT box with 
a single public IP address - we've found several vendors' "routers" that 
can work in this environment.  I believe the key is that each tunnel must 
be to distinct remote IP addresses.  All the NAT box has available to 
separate the traffic for the different tunnels (which use IP protocol 50) 
is the address of the other end of the tunnel, but that is all it needs.

Of course, many users would like to have multiple tunnels to the same 
partner.  I don't know how that is possible with current IPSEC technology.

Tony Rall


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