RE: ACL design.

["David Gillett" <gillettdavid () fhda edu>]

  If I read you right, both sides of this router are on private
addresses and there should be no non-private addresses in the 
traffic.  You could enforce that in ACLs, just as a sanity measure.
(I sometimes see clients come onto our (guest) network with addresses 
from some other network; at one point, it was common to see them
show up with AOL addresses....)

  The other main use of ACLs in this case is to limit who can connect
to the router itself.  (The guest gateway's interface addresses are
not acceptable destinations for traffic originating within that
network.)

David Gillett

> -----Original Message-----
> From: listbounce () securityfocus com 
> [mailto:listbounce () securityfocus com] On Behalf Of WALI
> Sent: Saturday, May 12, 2007 11:00 AM
> To: Alex Nedelcu; security-basics () securityfocus com
> Subject: Re: ACL design.
>
> Off the subject a bit but I thought, I should ask this 
> question since it's been lingering on my mind for some time 
> now. Maybe guys around here can answer in detail.
>
> I have a remote site getting connected to my server farm. 
> It's our branch office. I have a router in the middle with no 
> fire wall and the addresses on both sides of the interface 
> are private, say 10.10.10.0/24 on my side and 10.20.20.0/24 
> on the other.
>
> The only thing the branch users access on this side of the 
> router is AD authentication, Exchange (SMTP) and some file shares.
> What should be my minimal extended ACL? Currently, it' all 
> through and through and I feel that's highly insecure.
>
> Any advise??
>
> At 08:58 AM 5/9/2007 +0300, Alex Nedelcu wrote:
> > It's also important where you place your ACLS.
> >
> > If you have an advanced ACL that takes into consideration 
> the source, 
> > destination, ports, TOS etc you should place it as close to 
> the source 
> > of traffic as possible.
> >
> > If the ACL is based solely on source addresses they should 
> be placed as 
> > close as possible to the destination.
> >
> > Another thing that you should take into consideration is to 
> never apply 
> > ACLs in the core area of your network, in a hierarchical 
> model network 
> > the traffic policies should be applied at the distribution 
> layer. You 
> > should analyze carefully the design of your network and find 
> the ideal 
> > places where you should implement filtering, if you choose badly you 
> > may get decreased perfomance.