full-disclosure - [inbox] RE: RE: Example firewall script

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Date: Sun Aug 28 18:25:57 2005
From: exibar at thelair.com (Exibar)
Subject: [inbox] RE: RE: Example firewall script 

Exactly!  FireWall 101 if you will :-)

 Exibar

> -----Original Message-----
> From: Jan Nielsen [mailto:jan@...akasha.dk]
> Sent: Saturday, August 27, 2005 2:25 PM
> To: full-disclosure@...ts.grok.org.uk
> Subject: [inbox] RE: [Full-disclosure] RE: Example firewall script 
> 
> 
> I think the rules explained here are not intended to be actual rules in
> a firewall, but more of a way to explain what is secure and what is not,
> correct me if im wrong. Oh and btw, acl's ARE used in CBAC (cisco ios
> fw) they are just a tad more intelligently created than in a regular
> acl.
> 
> 
> Jan
> 
> -----Original Message-----
> From: ericscher@....com [mailto:ericscher@....com] 
> Sent: 27. august 2005 18:42
> To: full-disclosure@...ts.grok.org.uk
> Subject: [Full-disclosure] RE: Example firewall script 
> 
> 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 
> =================================
> ORIGINAL MESSAGE:
> -----------------
> 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
> =================================
> 
> REPLY:
> -------
> 
> 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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> 
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