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PIX 6.3: ACLs and Security Levels

Posted on 2006-06-21
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I'm reworking my PIX configuration to get rid of conduits in favor of ACLs.

My question is regarding how ACLs and the security levels of the various interfaces interact.

Since every access-list ends with an implicit deny all statement, does this override the fact that higher security interfaces can usually get to lower security interfaces?

For example, if I apply an ACL to my DMZ interface (seclevel 50) permitting access to a server behind my inside interface, will the implicit deny at the end of the ACL prevent things in the DMZ from accessing the Internet (i.e., networks on the outside, seclevel 0 interface?)
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Question by:titan6400
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by:Cyclops3590
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yes.  dmz clients will no longer reach anywhere to the internet after you apply an interface.

if you want that functionality you need to make explicit rules to handle what you want
lets say you have 192.168.1.x as your inside network and 192.168.2.x as your dmz
you want 192.168.2.10 to talk to 192.168.1.5 web purposes for some reason, you'd need to add

access-list dmz-in permit tcp host 192.168.2.10 host 192.168.1.5 eq 80
and apply it to the interface. of course that essentially add this line to the end of the acl
access-list dmz-in deny ip any any
so what you want to do is after that first acl add these lines to keep the functionality of the security levels
access-list dmz-in deny ip 192.168.2.0 255.255.255.0 192.168.1.0 255.255.255.0
access-list dmz-in permit ip any any

of course if there are any other networks on a higher security level interface, you'd need to add those rules in as well and make sure they are before that last permit any any line
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lrmoore earned 75 total points
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Yes...
All traffic from higher security to lower security (inside-->outside, inside-->DMZ, DMZ-->out) is permitted by default. Use acls only to restrict outbound traffic.
All traffic from lower to higher (outside-->DMZ, outside-->inside, DMZ-->inside) is blocked by default. Use access-lists for exceptions.
Yes, if you do something like this:
 access-list DMZ permit ip host <dmz host> host <inside host>
 access-group DMZ in interface DMZ

It will now block all other traffic into that interface. You have to be careful:
 access-list DMZ permit ip host <dmz host> host <inside host>
 access-list DMZ deny ip any <inside subnet>
 access-list DMZ permit ip any any

Of course, you are also constained by the static NAT between inside and DMZ, so you may want to do a same/same nat for the inside host to dmz:
 static (inside,dmz) <inside host ip> <inside host ip> netmask 255.255.255.255

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by:Cyclops3590
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forgot about the translation entries
btw, you'll also need
static (dmz,inside) <dmz host ip> <dmz host ip> netmask 255.255.255.255

of course I prefer
static (inside, dmz) <inside subnet> <inside subnet> netmask <inside subnet mask>
static (dmz, inside) <dmz subnet> <dmz subnet> netmask <dmz subnet mask>

but that is mostly because i'm lazy and it covers the entire subnet right away instead of doing a lot of static entries to cover individual hosts
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