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Cisco ASA 5200 ACL question

Posted on 2006-12-01
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Last Modified: 2008-01-16
Folks,

   We have an ASA 5200 which we may be using to provide firewall services for more than one client.  With that in mind I went to great lengths and some pain to configure it using the minimum number of interfaces necessary for the existing client using subinterfaces of a single interface.  

   Behind the subinterfaces for this client we plan to have exchange, web, citrix and sundry other applications which will need access to the internet.  I've fiddled with the static (int, int) command and have it functioning for the sub interfaces.

   I then began to fiddle with configuring the ACL.  Ideally I'd like to permit pretty much everything in through the outside interface then allow or block traffic at the subinterface level.  I created two separate ACL's, one called Global_ACL the other called AWF_ACL.  The Global_ACL had a permit ip any any statement in it and the AWF_ACL had the more granular permit statements.

   I then applied the Global_ACL to the outside interface and the AWF_ACL to the subinterface which I needed traffic to pass on.  I tested my connection using RDP remapped to a different port and was able to connect to the server behind the ASA.

   I looked at the access-lists and saw hits on the Global_ACL but none on the AWF_ACL.  Curious to see if the AWF_ACL was being parsed I then removed it from the subinterface and found that I could still connect via RDP.  

   My question is this: how can I configure the ASA so that I am allowing everything into the outside interface but only what port traffic I want into each sub interface?

   I can post the relevant portion of the config if needed.

Thanks
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Question by:winningtech
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Expert Comment

by:Freya28
ID: 18056319
i do not think that you can do that being the security device's primary role is to block everything incoming, unless requested from the inside.

the only thing i can thik of is to add an access list that opens everything up and bind it to the outside interface, then add your neccessary access-lists to block what you want on the sub interfaces

access-list inbound permit ip any any
access-group inbound in interface outside

this will open up the device with basically no security.  then add you acl's for the subs
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Author Comment

by:winningtech
ID: 18056471
Thanks for the reply.  

This is essentially what I have done:  The Global_ACL has a permit ip any any statement and the AWF_ACL has more granular statements.

The ACL on the subinterface seems to have no effect.  It's as though the ASA assumes the traffic is alright for any interface once it's matched the ACL on the outside interface.  

I do appreciate the thoughts though!



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Accepted Solution

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lrmoore earned 2000 total points
ID: 18057394
You can now apply acls "out" on each interface.
Allow all/most "in" on the outside interface.
Restrict "out" on the vlan interfaces

Example:
 access-list inbound permit ip any any
 access-group inbound in interface outside

 access-list to_vlan1 permit tcp any eq www any
 access-list to_vlan1 permit udp any eq domain any
 access-list to_vlan1 permit tcp any any eq https any
 access-group to_vlan1 out interface vlan1

 access-list to_vlan2 permit tcp any host a.b.c.d eq www
 access-list to_vlan2 permit tcp any host a.b.c.d eq https
 access-list to_vlan2 permit tcp any host alb.c.e eq smtp
 access-group to_vlan2 out interface vlan2

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Author Comment

by:winningtech
ID: 18057494
I'll test this but looking at it wouldn't this restrict traffic outbound from inside the interface?  

I'm trying to wrap my brain around how this would work...
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Author Comment

by:winningtech
ID: 18057510
oh... wait... I think I see what you're saying.

So, If I have the following interfaces:

gig0/1
gig0/1.10
gig0/1.20

and I create an ACL which permits all IP traffic and apply it on gi0/1 inbound

then I create an ACL which restricts traffic and apply it to gi0/1.10 outbound

the result should be that all traffic flows into the primary interface but is restricted flowing through it to the sub-interface?
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LVL 79

Expert Comment

by:lrmoore
ID: 18057537
No. Think of the ASA as a big box with multiple doors.
From the outside of the box, all doors say "IN"
From the inside of the box, all doors say "OUT"
packets can come in the outside interface, but can't go out the vlan interface
In the example above, "inside" and vlan3 can use FTP, RDP, etc, while users on vlan1 and 2 are restricted to http, https, dns, smtp, etc
 
These are just examples. Be careful what you wish for and plan it very carefully and don't forget about udp/53 for dns
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LVL 79

Expert Comment

by:lrmoore
ID: 18057736
No/Yes.
No to your question "wouldn't this restrict traffic outbound from inside the interface? "
Yes to your last post. We must have cross-posted at the same time.

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Author Comment

by:winningtech
ID: 18058320
lrmoore:

Thanks for the replies.  So I understand correctly I described applying the ACL's correctly to achieve what I wish to accomplish, correct?

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LVL 79

Expert Comment

by:lrmoore
ID: 18058523
correct
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Author Comment

by:winningtech
ID: 18058832
Thanks, I'll give it a try tomorrow and award points if it works!
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LVL 79

Expert Comment

by:lrmoore
ID: 18107831
Are you still working on this? Can you close out this question before the cleanup crew gets around to it?
Thanks!
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Author Comment

by:winningtech
ID: 18183904
I was yanked off of this project for a week or so but finally had a chance to apply the suggested solution.  The solution did work!

Thanks, awarding points and closing now.
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