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Re: Private port numbers?

  • From: David G. Andersen
  • Date: Wed Aug 13 23:05:23 2003

On Wed, Aug 13, 2003 at 10:40:30PM +0000, Christopher L. Morrow quacked:
> what about ports that start as 'private' and are eventually ubiquitously
> used on a public network? (Sean Donelan noted that 137->139 were
> originally intended to be used in private networks... and they became
> 'public' over time)

 You run it on a different port. I actually really like this idea,
because it makes shipping a more secure default configuration
easier for vendors without having to coordinate between firewall
vendors and implementors.

The "gotcha" is that it makes life pretty weird for you if you
then want to make your service work in the wide-area... but
that's pretty easy to do with intelligent defaults:

Ports 1-1024:  Well-known-ports
Ports 60001-61024:  Private well-known-port analogues

Applications would try:

 if (!connect(..., public port #))
   connect(..., private port #))

In fact, this (impractically) generalizes to a nice way of
signifying whether or not you want external people to be
able to talk to your service:

   port bit[0] == 0:  Public service, please do not filter
   port bit[0] == 1:  Private service, please filter at
                      organizational boundary

I suddenly wish the port space was 32 bits. :)

People _could_, of course, implement all of this with
tcpwrappers and host-local firewalls.  But experience has
shown that they don't.  It might be easier for them if they
could just click "private" when they configured the service,
though experience has shown that services migrate to the less
restrictive mode as debugging and time goes on...


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      MIT Laboratory for Computer Science 
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