Using vpptrace.sh for VPP Packet Tracing

VPP allows tracing of incoming packets using CLI commands trace add and show trace as explained [here](VPP_PACKET_TRACING_K8S.html), but it is a rather cumbersome process.

The buffer for captured packets is limited in size, and once it gets full the tracing stops. The user has to manually clear the buffer content, and then repeat the trace command to resume the packet capture, losing information about all packets received in the meantime.

Packet filtering exposed via the CLI command trace filter is also quite limited in what it can do. Currently there is just one available filter, which allows you to keep only packets that include a certain node in the trace or exclude a certain node in the trace. It is not possible to filter the traffic by its content (e.g., by the source/destination IP address, protocol, etc.).

Last but not least, it is not possible to trace packets on a selected interface like tcpdump, which allows tracing via the option -i. VPP is only able to capture packets on the RX side of selected devices (e.g., dpdk, virtio, af-packet). This means that interfaces based on the same device cannot be traced for incoming packets individually, but only all at the same time. In Contiv/VPP all pods are connected with VPP via the same kind of the TAP interface, meaning that it is not possible to capture packets incoming only from one selected pod.

Contiv/VPP ships with a simple bash script vpptrace.sh, which helps alleviate the aforementioned VPP limitations. The script automatically re-initializes buffers and traces whenever it is close to getting full, in order to avoid packet loss as much as possible. Next it allows you to filter packets by the content of the trace. There are two modes of filtering:

  • substring mode (default): packet trace must contain a given sub-string in order to be included in the output

  • regex mode: packet trace must match a given regex in order to be printed

The script is still limited, in that capture runs only on the RX side of all interfaces that are built on top of selected devices. Using filtering, however, it is possible to limit traffic by interface simply by using the interface name as a substring to match against.

Usage

Run the script with option -h to get the usage printed:

Usage: ./vpptrace.sh  [-i <VPP-IF-TYPE>]... [-a <VPP-ADDRESS>] [-r] [-f <REGEXP> / <SUBSTRING>]
   -i <VPP-IF-TYPE> : VPP interface *type* to run the packet capture on (e.g., dpdk-input, virtio-input, etc.)
                       - available aliases:
                         - af-packet-input: afpacket, af-packet, veth
                         - virtio-input: tap (version determined from the VPP runtime config), tap2, tapv2
                         - tapcli-rx: tap (version determined from the VPP config), tap1, tapv1
                         - dpdk-input: dpdk, gbe, phys*
                       - multiple interfaces can be watched at the same time - the option can be repeated with
                         different values
                       - default = dpdk + tap
   -a <VPP-ADDRESS> : IP address or hostname of the VPP to capture packets from
                      - not supported if VPP listens on a UNIX domain socket
                      - default = 127.0.0.1
   -r               : apply filter string (passed with -f) as a regexp expression
                      - by default the filter is NOT treated as regexp
   -f               : filter string that packet must contain (without -r) or match as regexp (with -r) to be printed
                      - default is no filtering

VPP-IF-TYPE is a repeated option used to select the set of devices (e.g., virtio, dpdk, etc.) to capture the incoming traffic. Script provides multiple aliases, which are much easier to remember than the device names. For dpdk-input one can enter just dpdk, or anything starting with phys, etc. For TAPs, the script is even smart enough to find out the TAP version used, which allows to enter just tap as the device name.

If VPP-IF-TYPE is not specified, then the default behaviour is to capture from both dpdk (traffic entering the node from outside) and tap (preferred interface type for pod-VPP and host-VPP interconnection, receiving node-initiated traffic).

vpptrace.sh can capture packets even from a VPP on a different host, provided that VPP-CLI listens on a port, and not on a UNIX domain socket (for security reasons IPC is the default communication link, see /etc/vpp/contiv-vswitch.conf). Enter the destination node IP address via the option -a(localhost is the default).

The capture can be filtered via the -f option. The output will include only packets whose trace matches contain the given expression/sub-string.

Option -r enables the regex mode for filtering.

Examples

  • Capture all packets entering VPP via tapcli-1 interface AND all packets leaving VPP via tapcli-1 that were sent from a pod, or the host on the same node (sent from tap, not Gbe):

$ vpptrace.sh -i tap -f "tapcli-1"



  • Capture all packets with source or destination IP address 10.1.1.3:

$ vpptrace.sh -i tap -i dpdk -f "10.1.1.3"

Or just:
$ vpptrace.sh "10.1.1.3"

  • Capture all SYN-ACKs received from outside:

$ vpptrace.sh -i dpdk -f "SYN-ACK"