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ORIGINAL MESSAGE:
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Date: Sat, 27 Aug 2005
From: "Exibar"
Subject: Example firewall script
The absolute worse Firewal rule
you can have:
Allow ANY ANY
The best:
Deny ANY ANY
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REPLY:
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Actually, that's not true.
I would agree that as a general rule of thumb
you should have a deny statement at the end
of every ACL. In fact, Cisco places an implicit
DENY ANY ANY at the end of their ACL's
automatically.
However, Access Control Lists are not firewalls.
Yes, we use them as firewalls, but that's not what
they are.
ACL's ARE TRAFFIC SHAPING DEVICES.
As traffic shaping devices, they can be used for
security, but they are also used for management
purposes. For instance; many Autonomous Systems
are multi-homed. There are decisions to be made
about how traffic will flow in and out of the AS.
You also have to decide if you wish to be a
transit AS or not.
ACLs are the tool that you use to control your
traffic.
While an ACL being used as a security device
should have a deny statement at the end, proper
construction of the ACL is more about following
the proper construction rules.
This is actually a huge subject, far too big
for an individual e-mail to a list.
But there are some basic rules to keep in mind:
ACL's analyze traffic from top to bottom, so
keep your most specific entries at the top,
with more general entries near the bottom;
and do your "permits" before your "denys".
That means you deal with hosts first, then
subnets, then networks, and at each level
you have your permit statements before your
deny statements. The reason for this is because
once a packet matches a line, it's dealt with
right then and there. You don't want to have
a packet thrown away just before a line that
would have permitted it.
There are also issues of what KIND of ACL to
use and where to place them; Inbound or Outbound.
In terms of the original question, the only
difference between a "good" line item or a
"bad" line item is whether or not the syntax
is correct.
The only difference between a "good" ACL
and a "bad" ACL is whether or not it's
structure is properly designed and whether
or not it's placed in the proper location.
This subject REALLY calls for a book, not
an e-mail response. I've said very little
in this post and look at all the room
it took up.
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