Network Segmentation Approaches

Jimmy Hess mysidia at
Thu May 7 23:39:09 UTC 2015

On Thu, May 7, 2015 at 7:12 AM, Rich Kulawiec <rsk at> wrote:
> it, this was sloppy phrasing on my part.  I meant "first"
> in the sense of "first rule that one should write".  Depending on

Security best practice to always have an active "cleanup" rule  for
every traffic direction
applicable to every pair of zones   (or interfaces)  with a default DROP,
to  catch traffic  matching no accept rule.

In practice... however....  in the real world,  many firewalls get
configured with
this only in the INBOUND direction (Default deny Write packet to
Higher integrity level
zone  from lower level security zone),  and Default Accept   for
packet from more secure
zone to less secure zone,     Since this has superior usability and is
 lower maintenance.

And for client devices, in a low security environment: with just a
simple Layer4 stateful
inspection firewall, this is probably the right solution.

"Permit only traffic that is necessary"

Only works out if you are able to rigidly define what exactly that
traffic is in advance.

Which is feasible to do for servers and other single-purpose devices,
but   very expensive to do for clients,   at least without a firewall aware of
the communications at the application layer   that can look at those
UDP connections
and say  "OKAY,  This is skype...  allow it",

Or...   "This connection going out on port 80..  it's not a valid HTTP request,
Drop the connection now and cache a rule to Deny further connections
to that IP:Port number pair.".

> the firewall type/implementation, that might be the rule that's
> lexically first or last (or maybe somewhere else).
> ---rsk

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