Changing the NAT IP on demand?

[Leo Bicknell]

I'm considering options for a new project, and I think I've discovered
what I think is the best idea, but I don't think current software
supports the config.  I'd like to get some confirmation, and comments on
if it would be hard to implement.

Consider:


ISP #1-------\
              \
              FreeBSD Box----LAN
              /
ISP #2-------/

In this case the LAN would be 1918 space, the two ISP's would each
provide a public IP for the FreeBSD box.

Now, NAT would be required.  What I want to do is write an external
application to decide the performance of ISP #1 and ISP#2, and
somehow tell NAT which outside address to use.

That, by itself, is not hard.  Here's the trick.  I want the switch
to be seamless.  That is, if NAT is translating to ISP #1 and the
application says switch to #2 the existing translations to #1 (until
they go away naturally) should be kept, while new ones go to #2.

The only ways I know to change the outside address seem to tear down
all existing connections.

Is it possible to make this work today?  Would it be hard to fix if 
it doesn't work today?

-- 
       Leo Bicknell - bicknell at ufp.org - CCIE 3440
        PGP keys at http://www.ufp.org/~bicknell/
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