Re: Internet accessible screened subnet - use public orprivateIPs?

[David Lang <david.lang () digitalinsight com>]

On Fri, 22 Jul 2005, Victor Williams wrote:

> Everyone has missed the point.
>
> The whole issue of using NAT or not has nothing to do with work associated 
> with either.  The whole reason NAT was implemented was because of a very 
> finite (and quickly running out supply, dependending on who you ask) number 
> of publicly routable IP addresses.  Instead of assigning every machine that 
> wanted internet access a public IP address, it was just more cost-effective 
> (IP addresses cost money) to use NAT or masquerading...whatever your lingo 
> is...to address those hosts that only needed outgoing access--who weren't 
> serving content.

however, for a DMZ (the question that was asked) you are typicaly 
providing service to the Internet, and for that you run into a bunch of 
very interesting issues if you try to use NAT to reduce the number of IP 
addresses you use.

David Lang

> Whether you address your publicly accessible hosts directly with public ip 
> addresses or you use static NAT translations is up to the preference of the 
> administrator.  If you have enough public IP addresses and $ isn't an object, 
> then your preference for assigning them all public IP addresses really 
> doesn't make a difference.  If you don't have enough public IP addresses and 
> you have a limited budget and have to allow many services on the internet 
> with less public IP addresses, then it sounds like you'll be using NAT or 
> PAT.
>
> There is no clear-cut *better* way universally.  Several different ways work 
> if you have your head screwed on straight.
>
> My personal preference is to use private ip addresses everywhere inside my 
> firewall...even in my DMZ.  That way I control my public IP addresses at one 
> point only, and that's my firewall.  If for some reason I change ISP's or my 
> ISP wants to change my IP address range (which hasn't happened in over 9 
> years), I make my IP address changes in two spots: my firewall(s), and my DNS 
> servers.  Nothing else changes.  To me, it's simpler.  Others like to be 
> complicated...so YMMV.
>
>
> David Lang wrote:
> > On Fri, 22 Jul 2005, Dave Piscitello wrote:
> >
> > > Isn't this a question of whether you want to route or NAT?
> > >
> > > A server that is Internet-facing has to have (or be reachable via) a
> > > public IP. If your ISP changes your block of public IP addresses, you
> > > have to change:
> > >
> > > 1) the mapping between your private IP addresses and the new public
> > > IP addresses (the static or 1:1 NAT case) or
> > > 2) the IP addresses of all the servers, the IPs of the trusted and
> > > external interfaces on the firewall, and the routing table (or
> > > routing protocol configuration)
> > >
> > > (2) seems like a whole lot more work to me.
> >
> >
> > first off, how frequently does your ISP reallocate your address range?
> >
> > secondly you are ignoring all the other work that you need to do when this 
> > change takes place. with all that in mind the difference in the amount of 
> > work seems a lot less.
> >
> > and as I said below, the trade off for simplifying this rare occurance of 
> > changeing your IP range comes with day-to-day costs in running NAT.
> >
> > David Lang
> >
> > > On 21 Jul 2005 at 18:28, David Lang wrote:
> > >
> > > > On Thu, 21 Jul 2005, Paul D. Robertson wrote:
> > > >
> > > > > On Fri, 15 Jul 2005, Matt Bazan wrote:
> > > > >
> > > > > > Is there a preferred method of setting up a Internet facing
> > > > > > screened subnet and the use of public or private IP addresses?
> > > > > > Looking at redesinging our DMZ to only include public resources
> > > > > > (www, smtp, imap, ftp).  Presently we use a private IP address
> > > > > > range for this that is NAT'ed at our firewall.  Any reasons to
> > > > > > change this policy to using public IPs in the DMZ?  Thanks,
> > > > >
> > > > >
> > > > > If you're NATing to your internal network, then a rework is
> > > > > necessary- public stuff should be on its own (preferably) physical
> > > > > subnet.
> > > > >
> > > > > IP addressing doesn't matter much, since you'll be letting stuff
> > > > > through the most likely exploit vectors anyway.
> > > >
> > > >
> > > > The thing I've been eharing for years about why NAT is better is that
> > > > you may change ISP's and end up with a new set of IP addresses which
> > > > are easier to change if you NAT.
> > > >
> > > > this may be true (I've actually never seen anyone acutally DO this),
> > > > but you are trading one-time headaches (which I personally believe are
> > > > no more severe then all the other changes that you need to make when
> > > > changing things, firewalls, DNS, NAT tables, etc) for ongoing overhead
> > > > (performance on your NAT device, troubleshooting, bugs in the NAT
> > > > implementation, overloading of the NAT tables, etc)
> > > >
> > > > I would definantly have things that server the Internet use public
> > > > addresses, once you get behind that layer and have devices that only
> > > > talk to internal stuff, then make it all private addresses.
> > > >
> > > > David Lang
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > --
> > > > There are two ways of constructing a software design. One way is to
> > > > make it so simple that there are obviously no deficiencies. And the
> > > > other way is to make it so complicated that there are no obvious
> > > > deficiencies.
> > > >   -- C.A.R. Hoare
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> > >
> > >
> > >
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--
There are two ways of constructing a software design. One way is to make it so simple that there are obviously no 
deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies.
 -- C.A.R. Hoare
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