9. DPDK packet capture libraries and tools

This document describes how the Data Plane Development Kit (DPDK) Packet Capture Framework is used for capturing packets on DPDK ports. It is intended for users of DPDK who want to know more about the Packet Capture feature and for those who want to monitor traffic on DPDK-controlled devices.

The DPDK packet capture framework was introduced in DPDK v16.07 and enhanced in 21.11. The DPDK packet capture framework consists of the libraries for collecting packets librte_pdump and writing packets to a file librte_pcapng. There are two sample applications: dpdk-dumpcap and older dpdk-pdump.

9.1. Introduction

The librte_pdump library provides the API required to allow users to initialize the packet capture framework and to enable or disable packet capture. The library works on a multi-process communication model and its usage is recommended for debugging purposes.

The librte_pcapng library provides the API to format packets and write them to a file in Pcapng format.

The dpdk-dumpcap is a tool that captures packets in like Wireshark dumpcap does for Linux. It runs as a DPDK secondary process and captures packets from one or more interfaces and writes them to a file in Pcapng format. The dpdk-dumpcap tool is designed to take most of the same options as the Wireshark dumpcap command.

Without any options it will use the packet capture framework to capture traffic from the first available DPDK port.

The dpdk-testpmd application can be used to initialize the packet capture framework and acts as a server, and the dpdk-dumpcap tool acts as a client. To view Rx or Tx packets of dpdk-testpmd, the application should be launched first, and then the dpdk-dumpcap tool. Packets from dpdk-testpmd will be sent to the tool, and then to the Pcapng file.

Some things to note:

  • All tools using librte_pdump can only be used in conjunction with a primary application which has the packet capture framework initialized already. In dpdk, only testpmd is modified to initialize packet capture framework, other applications remain untouched. So, if the dpdk-dumpcap tool has to be used with any application other than the testpmd, the user needs to explicitly modify that application to call the packet capture framework initialization code. Refer to the app/test-pmd/testpmd.c code and look for pdump keyword to see how this is done.

  • The dpdk-pdump tool is an older tool created as demonstration of librte_pdump library. The dpdk-pdump tool provides more limited functionality and depends on the Pcap PMD. It is retained only for compatibility reasons; users should use dpdk-dumpcap instead.

9.2. Test Environment

The overview of using the Packet Capture Framework and the dpdk-dumpcap utility for packet capturing on the DPDK port in Fig. 9.1.

../_images/packet_capture_framework.svg

Fig. 9.1 Packet capturing on a DPDK port using the dpdk-dumpcap utility.

9.3. Running the Application

The following steps demonstrate how to run the dpdk-dumpcap tool to capture Rx side packets on dpdk_port0 in Fig. 9.1 and inspect them using tcpdump.

  1. Launch testpmd as the primary application:

    sudo <build_dir>/app/dpdk-testpmd -c 0xf0 -n 4 -- -i --port-topology=chained

  2. Launch the dpdk-dumpcap as follows:

    sudo <build_dir>/app/dpdk-dumpcap -w /tmp/capture.pcapng

  3. Send traffic to dpdk_port0 from traffic generator. Inspect packets captured in the file capture.pcapng using a tool such as tcpdump or tshark that can interpret Pcapng files:

    $ tcpdump -nr /tmp/capture.pcapng
reading from file /tmp/capture.pcap, link-type EN10MB (Ethernet)
11:11:36.891404 IP 4.4.4.4.whois++ > 3.3.3.3.whois++: UDP, length 18
11:11:36.891442 IP 4.4.4.4.whois++ > 3.3.3.3.whois++: UDP, length 18
11:11:36.891445 IP 4.4.4.4.whois++ > 3.3.3.3.whois++: UDP, length 18