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 toand locate the traffic log for the two systems. Click theMonitorTrafficDetailed Log Viewicon 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.
For example, enter the source IP address192.168.2.10and the destination IP address10.43.14.55. To further filter the capture, setNon-IPtoexcludenon-IP traffic, such as broadcast traffic.
- Select.MonitorPacket Capture
- ClickClear All Settingsat the bottom of the window to clear any existing capture settings.
- ClickManage Filtersand clickAdd.
- SelectId1 and in theSourcefield enter the source IP address you are interested in and in theDestinationfield enter a destination IP address.
For example, in theSourcefield enter10.43.14.55and in theDestinationfield enter10.43.14.25. In theNon-IPdrop-down menu selectexclude.
- Addthe second filter and selectId2.
- 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 theHelpicon on the packet capture page.For example, to configure all packet capture stages and define a filename for each stage, perform the following procedure:
For example, selectreceiveas theStageand set theFilename totelnet-test-received.
- AddaStageto the packet capture configuration and define aFilename for the resulting packet capture.
- Continue toAddeachStageyou want to capture (receive,firewall,transmit, anddrop) and set a uniqueFilename for each stage.
- SetPacket CapturetoON.Note the warning that system performance can be degraded and then clickOK. If you define filters, the packet capture should have little impact on performance, but you should always turnOffpacket 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 captureOFFand 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 servertelnet 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. Note that 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.
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