Related Documentation
Example: Configuring Packet Capture for Datapath Debugging
This example shows how to configure packet capture to monitor traffic that passes through the device. Packet Capture then dumps the packets into a PCAP file format that can be later examined by the tcpdump utility.
Requirements
Before you begin, see Debugging the Data Path (CLI Procedure).
Overview
A filter is defined to filter traffic; then an action profile is applied to the filtered traffic. The action profile specifies a variety of actions on the processing unit. One of the supported actions is packet dump, which sends the packet to the Routing Engine and stores it in proprietary form to be read using the show security datapath-debug capture command.
Configuration
CLI Quick Configuration
To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.
Step-by-Step Procedure
The following example requires you to navigate various
levels in the configuration hierarchy. For instructions on how to
do that, see Using the CLI Editor in Configuration Mode in the Junos OS CLI User Guide 
.
To configure packet capture:
- Edit the security datapath-debug option for the multiple
processing units along the packet-processing path:[edit]user@host# edit security datapath-debug
- Enable the capture file, the file format, the file size,
and the number of files. Size number limits the size of the capture
file. After the limit size is reached, if the file number is specified,
then the capture file will be rotated to filename x, where x is auto-incremented until it reaches
the specified index and then returns to zero. If no files index is
specified, the packets will be discarded after the size limit is reached.
The default size is 512 kilobytes.[edit security datapath-debug]user@host# set capture-file my-capture format pcap size 1m files 5[edit security datapath-debug]user@host# set maximum-capture-size 400
- Enable action profile and set the event. Set the action
profile as do-capture and the event type as np-ingress: [edit security datapath-debug]user@host# edit action-profile do-capture[edit security datapath-debug action-profile do-capture]user@host# edit event np-ingress
- Enable packet dump for the action profile:[edit security datapath-debug action-profile do-capture event np-ingress]user@host# set packet-dump
- Enable packet filter, action, and filter options. The
packet filter is set to my-filter, the action profile is set to do-capture,
and filter option is set to source-prefix 1.2.3.4/32.[edit security datapath-debug]user@host# set security datapath-debug packet-filter my-filter action-profile do-capture[edit security datapath-debug]user@host# set security datapath-debug packet-filter my-filter source-prefix 1.2.3.4/32
Results
From configuration mode, confirm your configuration by entering the show security datapath-debug command. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it. The following is output from the show security datapath-debug command:
If you are done configuring the device, enter commit from configuration mode.
Verification
To confirm that the configuration is working properly, perform these tasks:
Verifying Packet Capture
Purpose
Verify if the packet capture is working.
Action
From operational mode, enter the request security datapath-debug capture start command to start packet capture and enter the request security datapath-debug capture stop command to stop packet capture.
To view the results, from CLI operational mode, access the local UNIX shell and navigate to the directory /var/log/my-capture. The result can be read by using the tcpdump utility.
