Tag: packet capture

Recently I had to troubleshoot some communication issues via a Cisco ASA device and the packet capture on the IOS comes in handy for this task.

When you have a lot of traffic over ASA and you’re interested in a particular IP address, the basic packet capture lesson says that you should configure an access-list to limit the captured packets for the interesting traffic only.

Let’s assume that I have a particular interest for the traffic to and from the IP address 10.0.0.10.

I created a standard ACL to match only the traffic related to 10.0.0.10:

access-list TS standard permit host 10.0.0.10

Afterward I attached the created ACL to a packet capture on a particular interface (let’s call it “lan”).

capture TSHOOT access-list TS interface lan

You can find the above lines in almost any how-to regarding packet capture on Cisco ASA.

Checking the capture I noticed that traffic is unidirectional captured:

FW# show capture TSHOOT

4 packets captured

   1: 20:15:32.757010       802.1Q vlan#10 P0 192.168.0.10 > 10.0.0.10: icmp: echo request
   2: 20:15:33.759283       802.1Q vlan#10 P0 192.168.0.10 > 10.0.0.10: icmp: echo request
   3: 20:15:34.761374       802.1Q vlan#10 P0 192.168.0.10 > 10.0.0.10: icmp: echo request
   4: 20:15:35.823748       802.1Q vlan#10 P0 192.168.0.10 > 10.0.0.10: icmp: echo request

This is not enough to troubleshoot complex communication scenarios.

Ok, maybe the standard ACL is not enough, so I tried to use extended one where 10.0.0.10 is source on one line and destination on another:

access-list TS extended permit ip host 10.0.0.10 any
access-list TS extended permit ip any host 10.0.0.10

This should do it…just that it doesn’t.

capture TSHOOT access-list TS interface lan

ERROR: Capture doesn't support access-list  containing mixed policies

Hmm, maybe it does not work with two lines in the ACL. I removed one, same error.

I was looking around to find a way to do it, but I couldn’t. This is why I wrote this article. Maybe my googling skills are not so good, as I’m sure it has to be an example somewhere out there.

However, here how I did it.

I gave up using the ACL. No, I’m was not going to capture the entire traffic :) Instead, I used inline restrictions for the IP address that I’m interested in.

capture TSHOOT interface lan match ip host 10.0.0.10 any

The result looks good now:

FW# show capture TSHOOT

8 packets captured

   1: 20:19:07.222553       802.1Q vlan#10 P0 192.168.0.10 > 10.0.0.10: icmp: echo request
   2: 20:19:07.223392       802.1Q vlan#10 P0 10.0.0.10 > 192.168.0.10: icmp: echo reply
   3: 20:19:08.229953       802.1Q vlan#10 P0 192.168.0.10 > 10.0.0.10: icmp: echo request
   4: 20:19:08.230670       802.1Q vlan#10 P0 10.0.0.10 > 192.168.0.10: icmp: echo reply
   5: 20:19:09.229327       802.1Q vlan#10 P0 192.168.0.10 > 10.0.0.10: icmp: echo request
   6: 20:19:09.230121       802.1Q vlan#10 P0 10.0.0.10 > 192.168.0.10: icmp: echo reply
   7: 20:19:10.252321       802.1Q vlan#10 P0 192.168.0.10 > 10.0.0.10: icmp: echo request
   8: 20:19:10.253130       802.1Q vlan#10 P0 10.0.0.10 > 192.168.0.10: icmp: echo reply

The packet capture shows now bidirectional traffic flow.

I hope you’ll find this useful during troubleshooting.