Packet Capture
The Packet Capture tool is a valuable asset for use in debugging connectivity and configuration issues and for understanding traffic flows through your Firepower 4100/9300 chassis. You can use the Packet Capture tool to log traffic that is going through specific interfaces on your Firepower 4100/9300 chassis.
You can create multiple packet capture sessions, and each session can capture traffic on multiple interfaces. For each interface included in a packet capture session, a separate packet capture (PCAP) file will be created.
Backplane Port Mappings
The backplane or uplink interface is an internal interface that connects the security module (SM) to the internal switch. In case of 2 backplane interfaces per module, the internal switch and the applications on the modules perform traffic load-balancing over the 2 blackplane interfaces.The Firepower 4100/9300 chassis uses the following mappings for internal backplane ports:
Platform |
Number of supported security modules |
Backplane/uplink interfaces |
Mapped application interfaces |
---|---|---|---|
Firepower 4100 (except Firepower 4110/4112) |
1 |
SM1: Ethernet1/9 Ethernet1/10 |
Internal-Data0/0 Internal-Data0/1 |
Firepower 4110/4112 |
1 |
Ethernet1/9 |
Internal-Data0/0 |
Firepower 9300 |
3 |
SM1: Ethernet1/9 Ethernet1/10 |
Internal-Data0/0 Internal-Data0/1 |
SM2: Ethernet1/11 Ethernet1/12 |
Internal-Data0/0 Internal-Data0/1 |
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SM3: Ethernet1/13 Ethernet1/14 |
Internal-Data0/0 Internal-Data0/1 |
Guidelines and Limitations for Packet Capture
The Packet Capture tool has the following limitations:
-
Can capture only up to 100 Mbps.
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Packet capture sessions can be created even when there is not enough storage space available to run the packet capture session. You should verify that you have enough storage space available before you start a packet capture session.
-
For packet capture sessions on a single-wide 4x100Gbps or 2x100Gbps network module (part numbers FPR-NM-4X100G and FPR-NM-2X100G respectively), if the module
adminstate
is set tooff
, the capture session is automatically disabled with an “Oper State Reason: Unknown Error.” You will have to restart the capture session after the moduleadminstate
is set toon
again.With all other network modules, packet capture sessions continue across module
adminstate
changes. -
Does not support multiple active packet capturing sessions.
-
Captures only at the ingress stage of the internal switch.
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Filters are not effective on packets that cannot be understood by the internal switch (for example Security Group Tag and Network Service Header packets).
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You can only capture packets for one subinterface per session, even if you have multiple subinterfaces on one or more parents.
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You cannot capture packets for an EtherChannel as a whole or for subinterfaces of an EtherChannel. However, for an EtherChannel allocated to a logical device, you can capture packets on each member interface of the EtherChannel. If you allocate a subinterface, but not the parent interface, then you cannot capture packets on member interfaces.
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You cannot copy or export a PCAP file while the capture session is still active.
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When you delete a packet capture session, all packet capture files associated with that session are also deleted.
Creating or Editing a Packet Capture Session
Procedure
Step 1 |
Choose .The Capture Session tab displays a list of currently configured packet capture sessions. If no packet capture sessions are currently configured, a message stating so is displayed instead. |
Step 2 |
Do one of the following:
The left side of the window lets you select a specific application instance and then shows a representation of that instance. This representation is used to select the interfaces on which you would like to capture packets. The right side of the window contains fields for defining the packet capture session. |
Step 3 |
Select an Instance from the drop-down menu. |
Step 4 |
Click the interfaces on which you want to capture traffic. Selected interfaces show a check mark. |
Step 5 |
For subinterfaces, click the icon to the left of the parent interface to view subinterfaces in the Subinterface selection column. Click one subinterface in the column; you can only capture packets for one subinterface per capture session, even if you have multiple subinterfaces on one or more parents. In the case of multiple subinterfaces, the icon will be labled Subinterfaces(n); for a single subinterface, it will be labeled with the subinterface ID. If the parent interface is also allocated to the instance, you can either choose the parent interface or a subinterface; you cannot choose both. If the parent is not allocated, it will be grayed out. Subinterfaces for EtherChannels are not supported. |
Step 6 |
To capture traffic from the logical device going out over the backplane ports: |
Step 7 |
Enter a name for the packet capture session in the Session Name field. |
Step 8 |
Specify the buffer size to use for this packet capture session by selecting one of the pre-defined values from the Buffer Size list, or by selecting Custom in MB and then entering the desired buffer size. The specified buffer size must be between 1 and 2048 MB. |
Step 9 |
Specify the length of the packet that you want to capture in the Snap Length field. Valid values are from 64 to 9006 bytes. The default snap length is 1518 bytes. |
Step 10 |
Specify whether you want to overwrite existing PCAP files or append data to the PCAP files when this packet capture session is executed. |
Step 11 |
To capture traffic between the application instance and a specific interface: |
Step 12 |
To filter the traffic being captured: |
Step 13 |
Do one of the following:
You see the Capture Session tab with your session listed along with any other sessions that have been created. If you selected Save and Run, your packet capture session will be capturing packets. You will need to stop capturing before you can download the PCAP files from your session. |
Configuring Filters for Packet Capture
You can create filters to limit the traffic that is included in a packet capture session. You can select which interfaces should use a specific filter while creating a packet capture session.
![]() Note |
If you modify or delete a filter that is applied to a packet capture session that is currently running, the changes will not take affect until you disable that session and then reenable it. |
Procedure
Step 1 |
Choose .The Capture Session tab displays a list of currently configured packet capture sessions. If no packet capture sessions are currently configured, a message stating so is displayed instead. |
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Step 2 |
Do one of the following:
You see the Create or Edit Packet Filter dialog box. |
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Step 3 |
Enter a name for the packet capture filter in the Filter Name field. |
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Step 4 |
To filter on a specific protocol, select it from the Protocol list, or selectCustom and then enter the desired protocol. The custom protocol must be an IANA defined protocol in decimal format (0-255). |
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Step 5 |
To filter on a specific EtherType, select it from the EtherType list, or selectCustom and then enter the desired EtherType. The custom EhterType must be an IANA defined EtherType in decimal format (for example, IPv4 = 2048, IPv6 = 34525, ARP = 2054, and SGT = 35081). |
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Step 6 |
To filter traffic based on an Inner VLAN (VLAN ID while ingressing the port) or Outer VLAN (VLAN ID added by the Firepower 4100/9300 chassis), enter the VLAN ID in the specified field. |
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Step 7 |
To filter traffic from a specific source or destination, enter the IP address and port or enter the MAC address in the specified source or destination fields.
|
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Step 8 |
Click Save to save the filter, You see the Filter List tab with your filter listed along with any other filters that have been created. |
Starting and Stopping a Packet Capture Session
Procedure
Step 1 |
Choose .The Capture Session tab displays a list of currently configured packet capture sessions. If no packet capture sessions are currently configured, a message stating so is displayed instead. |
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Step 2 |
To start a packet capture session, click the Enable Session button for that session and then click Yes to confirm.
The PCAP files for the interfaces included in the session will start collecting traffic. If the session is configured to overwrite session data, the existing PCAP data will be erased. If not, data will be appended to the existing file (if any). While the packet capture session is running, the file size for the individual PCAP files will increase as traffic is captured. Once the Buffer Size limit is reached, the system will start dropping packets and you will see the Drop Count field increase. |
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Step 3 |
To stop a packet capture session, click the Disable Session button for that session and then click Yes to confirm. After the session has been disabled, you can then download the PCAP files (see Downloading a Packet Capture File). |
Downloading a Packet Capture File
You can download the Packet Capture (PCAP) files from a session to your local computer so that they can be analyzed using a network packet analyzer.
Procedure
Step 1 |
Choose .The Capture Session tab displays a list of currently configured packet capture sessions. If no packet capture sessions are currently configured, a message stating so is displayed instead. |
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Step 2 |
To download the PCAP file for a specific interface from a packet capture session, click the Download button that corresponds to that interface.
Depending on your browser, the specified PCAP file is either automatically downloaded to your default download location or you are prompted to save the file. |
Deleting Packet Capture Sessions
You can delete an individual packet capture session if it is not currently running or you can delete all inactive packet capture sessions.
Procedure
Step 1 |
Choose .The Capture Session tab displays a list of currently configured packet capture sessions. If no packet capture sessions are currently configured, a message stating so is displayed instead. |
Step 2 |
To delete a specific packet capture session, click the Delete button that corresponds to that session. |
Step 3 |
To delete all inactive packet capture sessions, click the Delete All Sessions button above the list of packet capture sessions. |