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ASA Firewall single interface

Posted on 2013-11-07
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Last Modified: 2013-11-13
Hi
Please see my attached diagram.
My question is can I access server A from B and vices versa while using a single interface on asa to allow only certain/filter traffic.
Will this design work (I know it will work by using 2 interface in different subnets)
Please advice
Diagram.jpg
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Question by:ciscosupp
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Expert Comment

by:Ernie Beek
ID: 39631201
You should be able to when using VLANs and subinterfaces on the ASA. That way you can use the subinterfaces just like physical interfaces.
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Expert Comment

by:Ernie Beek
ID: 39631442
A small example:

interface GigabitEthernet0/4
 no nameif
 no security-level
 no ip address
!
interface GigabitEthernet0/4.100
 vlan 100
 nameif sub100
 security-level 80
 ip address 10.1.0.254 255.255.255.0
!
interface GigabitEthernet0/4.101
 vlan 101
 nameif sub101
 security-level 80
 ip address 10.2.0.254 255.255.255.0


That way you create the subinterfaces. On the switchport, create a trunk:

switchport trunk encapsulation dot1q
switchport trunk allowed vlan 100-101
switchport mode trunk
switchport nonegotiate
no cdp enable


This way you can have multiple subinterfaces on one physical interface.
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Author Comment

by:ciscosupp
ID: 39631512
It’s not what I want please refer to my diagram.

same-security –traffic permit intra interface
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by:Ernie Beek
ID: 39631588
Ah, wait. Let's see if I get it.

Server A is at another location which is connected through the router.
You want to have the connection from server B to server A to go through the firewall so you deny/allow certain traffic, right?
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Expert Comment

by:Henk van Achterberg
ID: 39631596
if you issue this command:

same-security-traffic permit intra-interface

you will be able to route traffic back to the same interface. The problem is the traffic from 10.0.0.0/24 to 192.168.1.0/24. This return traffic will not flow trough the ASA.

There are different solutions for this problem:

1. NAT traffic from 192.168.1.0/24 to 10.0.0.0/24 trough the inside interface on the ASA. This way the return traffic goes to the ASA first.
2. Disable TCP state in the inspection policy, this way the ASA will not drop connections with unknown state (which will happen when not all traffic hits the ASA).
3. Use the proxy arp to issue a second IP on the ASA 5505 inside interface to create a second subnet between the router and the ASA 5505: http://www.ducea.com/2008/05/31/adding-a-secondary-ip-address-on-a-cisco-asa-ethernet-interface/ (I have personally used this solution sometimes)
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by:Ernie Beek
ID: 39631621
@henkva: how did you know it's a 5505? Am I in need of glasses?
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Expert Comment

by:Ernie Beek
ID: 39631656
Further, using the ASA as a router does work (i agree) but it's not the most optimal solution. If ciscosupp wants to have all the traffic going through the ASA, thus separating the two networks, I still think using VLANs is a good option.
I know it will work by using 2 interface in different subnets
Using my formentioned setup would give the same result, but you would only need one interface on the ASA.

The question is, what do you (ciscosupp) have in mind?
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by:Henk van Achterberg
ID: 39631891
I mention the ASA 5505 but the TS does not specify the model. The commands however work for every ASA.

@erniebeek: Glasses can also be worn as fashion, who knows how pretty you are with glasses ;-)

I agree with Ernie that the VLAN setup is the best approach. But when you have a base license you do not have multiple VLANs on a single interface.
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by:Ernie Beek
ID: 39631926
@henkva: met of zonder bril, 't blijft even erg ;)

On topic: the base license issue only applies to the 5505. Even worse, you can't have subinterfaces on the 5505, only on 5510 and up. The ports on the 5505 are more like switchports than routerports (though an ASA is neither of those!) to put it simple.
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Author Comment

by:ciscosupp
ID: 39632690
Thanks All yes I want server a to b and vice versa to go through firewall so I can deny/allow certain traffic.

I also don’t like this setup but I’m working on a site with this kind of setup so try testing it in my lab.


So basically my question is traffic from server A to B and vice versa must go through firewall so I can deny allow traffic. I will try the things you mentioned in the post
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Assisted Solution

by:Henk van Achterberg
Henk van Achterberg earned 250 total points
ID: 39632807
I think the secondary "proxy arp" address on the inside interface is your only option to be sure that all traffic is flowing from and to the ASA.
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Accepted Solution

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Ernie Beek earned 250 total points
ID: 39632819
Do keep in mind that the current setup (as per your drawing) can't enforce the traffic flow through the ASA. As soon as someone changes the DG or adds a route,  the ASA is avoided.
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