topic Re: ASA- access-list issue in Network Security

ASA- access-list issue

mrafivan@cisco.com — Tue, 28 May 2019 10:07:36 GMT

Hi,

I have created a topology as shown in above diagram in vsphere client.Now i would like to ping from PC1 to PC2 and vice versa. But i could not able to ping.Below are the configurations that i have used for the setup.
PC1 configs:
IP : 192.168.1.2/24 ; default gateway : 192.168.1.1;
PC2 configs: 192.168.2.2/24; default gateway : 192.168.2.1;
ASA configs :
interface GigabitEthernet0/0
nameif inside
security-level 100
ip address 192.168.1.1 255.255.255.0
!
interface GigabitEthernet0/1
nameif outside
security-level 0
ip address 192.168.2.1 255.255.255.0

ciscoasa(config)# sh running-config access-list
access-list list1 extended permit icmp 192.168.1.0 255.255.255.0 192.168.2.0 255.255.255.0
ciscoasa(config)# sh running-config access-group
access-group list1 in interface inside
Please help me to sort out the issue.
Thanks in advance.

Re: ASA- access-list issue

Seb Rupik — Tue, 28 May 2019 10:29:08 GMT

Hi there,

Have you enabled ICMP inspection? Assuming you are using the default inspection policy:

!
policy-map global_policy
   class inspection_default
     inspect icmp
!

cheers,

Seb.

Re: ASA- access-list issue

Martin Carr — Tue, 28 May 2019 10:54:21 GMT

You do not require that ACL applied to the inside interface, the reason is it's a higher security level, so inherently it has access to the outside interface.

Conversely, the outside interface does not have any access to the inside, unless traffic is explicitly permitted.

Essentially, for PC2 to reach PC1, you would need to do the same.

In addition you would need inspect icmp, as stated.

Finally, you need to ensure the client firewalls (i.e. Windows Firewall) is configured to permit ICMP (which it does not, by default).

Martin

Re: ASA- access-list issue

mrafivan@cisco.com — Tue, 28 May 2019 11:40:12 GMT

Thanks for your response.

After enabling the icmp inspection now both sides able to ping.

but i couldn't understand what is the functionality of icmp inspection.

And i also i like to inform that inside interface is not able to access the outside interface without ACL.

Thanks.

Re: ASA- access-list issue

Martin Carr — Tue, 28 May 2019 12:35:31 GMT

You shouldn't need an ACL ( furthermore, it's not recommended)

As to the question, it's because ICMP is not stateful and does not contain sequence, port numbers etc.

'Inspect ICMP' creates a DACL, so that echo-reply's etc are permitted if the traffic originated from the inside interface.

Martin

Re: ASA- access-list issue

Seb Rupik — Tue, 28 May 2019 13:00:21 GMT

Hi there,

What @Martin Carr mentions about why the inspection is required is correct.

Regarding the need for the ACL...

Unless a permit statement is placed at the end of an ACL it has an implicit deny. In the case of your ACL, since it is INbound on the inside interface without that particular ACE the ICMP traffic will be denied. It is worth remembering that on an ASA in the absence of ACLs traffic is always permitted from a higher security level to a lower one, therefore this particular ACL is not required and the inspect statement will ensure that the return traffic is permitted without the need of further ACLs.

Hope that makes sense, and don't forget to mark correct/ helpful posts 😉

cheers,

Seb.