Network Segmentation Approaches

From: Rich Kulawiec <>

The first rule in every firewall is of course
"deny all" and subsequent rulesets permit only
the traffic that is necessary.

I think you got this backward? That way all
traffic is blocked, so none is allowed through.

Nope, I said exactly what I intended (and what I do,
in practice). Doing so forces one to understand in
detail what traffic actually needs to pass in/out
and to craft specific rules for it. This in turn
helps avoid making mistake #1:

  The Six Dumbest Ideas in Computer Security
  The Six Dumbest Ideas in Computer Security it, this was sloppy phrasing on my part. I meant "first"
in the sense of "first rule that one should write". Depending on
the firewall type/implementation, that might be the rule that's
lexically first or last (or maybe somewhere else).

---rsk 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.".