Take a Custom Packet Capture

Before you start a packet capture, identify the attributes of the traffic that you want to capture.

For example, to determine the source IP address, source NAT IP address, and the destination IP address for traffic between two systems, perform a ping from the source system to the to the destination system. After the ping is complete, go to

Monitor

Traffic

and locate the traffic log for the two systems. Click the

Detailed Log View

icon located in the first column of the log and note the source address, source NAT IP, and the destination address.

The following example shows how to use a packet capture to troubleshoot a Telnet connectivity issue from a user in the Trust zone to a server in the DMZ zone.

Set packet capture filters, so the firewall only captures traffic you are interested in.

Using filters makes it easier for you to locate the information you need in the packet capture and will reduce the processing power required by the firewall to take the packet capture. To capture all traffic, do not define filters and leave the filter option off.

For example, if you configured NAT on the firewall, you will need to apply two filters. The first one filters on the pre-NAT source IP address to the destination IP address and the second one filters traffic from the destination server to the source NAT IP address.

Select
Monitor
Packet Capture
.
Click
Clear All Settings
at the bottom of the window to clear any existing capture settings.
Click
Manage Filters
and click
Add
.
Select
Id
1 and in the
Source
field enter the source IP address you are interested in and in the
Destination
field enter a destination IP address.
For example, enter the source IP address
192.168.2.10
and the destination IP address
10.43.14.55
. To further filter the capture, set
Non-IP
to
exclude
non-IP traffic, such as broadcast traffic.
Add
the second filter and select
Id
2.
For example, in the
Source
field enter
10.43.14.55
and in the
Destination
field enter
10.43.14.25
. In the
Non-IP
drop-down menu select
exclude
.
Click
OK
.

Set

Filtering

to

On

.

Specify the traffic stage(s) that trigger the packet capture and the filename(s) to use to store the captured content. For a definition of each stage, click the

Help

icon on the packet capture page.

For example, to configure all packet capture stages and define a filename for each stage, perform the following procedure:

Add

a

Stage

to the packet capture configuration and define a

File

name for the resulting packet capture.

For example, select

receive

as the

Stage

and set the

File

name to telnet-test-received.

Continue to

Add

each

Stage

you want to capture (

receive, firewall

,

transmit

, and

drop

) and set a unique

File

name for each stage.

Set

Packet Capture

to

ON

.

The firewall or appliance warns you that system performance can be degraded; acknowledge the warning by clicking

OK

. If you define filters, the packet capture should have little impact on performance, but you should always turn

Off

packet capture after the firewall captures the data that you want to analyze.

Generate traffic that matches the filters that you defined.

For this example, generate traffic from the source system to the Telnet-enabled server by running the following command from the source system (192.168.2.10):

telnet 10.43.14.55

Turn packet capture

OFF

and then click the refresh icon to see the packet capture files.

Notice that in this case, there were no dropped packets, so the firewall did not create a file for the drop stage.

Download the packet captures by clicking the filename in the File Name column.

View the packet capture files using a network packet analyzer.

In this example, the received.pcap packet capture shows a failed Telnet session from the source system at 192.168.2.10 to the Telnet-enabled server at 10.43.14.55. The source system sent the Telnet request to the server, but the server did not respond. In this example, the server may not have Telnet enabled, so check the server.

Enable the Telnet service on the destination server (10.43.14.55) and turn on packet capture to take a new packet capture.

Generate traffic that will trigger the packet capture.

Run the Telnet session again from the source system to the Telnet-enabled server

telnet 10.43.14.55

Download and open the received.pcap file and view it using a network packet analyzer.

The following packet capture now shows a successful Telnet session from the host user at 192.168.2.10 to the Telnet-enabled server at 10.43.14.55.

You also see the NAT address 10.43.14.25. When the server responds, it does so to the NAT address. You can see the session is successful as indicated by the three-way handshake between the host and the server and then you see Telnet data.