xdpdump(8)

NAME
xdpdump - a simple tcpdump like tool for capturing packets at the XDP layer

SYNOPSIS

xdpdump is a simple XDP packet capture tool that tries to behave similar to tcpdump, however, it has no packet filter or decode capabilities.

This can be used for debugging XDP programs that are already loaded on an interface. Packets can be dumped/inspected before on entry to XDP program, or after at exit from an XDP program. Furthermore, at exit the XDP action is also captured. This means that even packets that are dropped at the XDP layer can be captured via this tool.

xdpdump works by attaching a bpf trace program to the XDP entry and/or exit function which stores the raw packet in a perf trace buffer. If no XDP program is loaded this approach can not be used and the tool will use a libpcap live-capture to be backward compatible.

  Running xdpdump

The syntax for running xdpdump is:
    Usage: xdpdump [options]
     XDPDump tool to dump network traffic
    Options:
         --rx-capture <mode>    Capture point for the rx direction (valid values: entry,exit)
     -D, --list-interfaces      Print the list of available interfaces
     -i, --interface <ifname>   Name of interface to capture on
         --perf-wakeup <events>  Wake up xdpdump every <events> packets
     -p, --program-names <prog>  Specific program to attach to
     -s, --snapshot-length <snaplen>  Minimum bytes of packet to capture
         --use-pcap             Use legacy pcap format for XDP traces
     -w, --write <file>         Write raw packets to pcap file
     -x, --hex                  Print the full packet in hex
     -v, --verbose              Enable verbose logging (-vv: more verbose)
         --version              Display version information
     -h, --help                 Show this help

The options explained

The xdpdump tool tries to mimic the basic tcpdump options, but just in case below each of the available options is explained:

  --rx-capture <mode>

Specify where the ingress packet gets captured. Either at the entry of the XDP program and/or exit of the XDP program. Valid options are entry, exit, or both entry,exit. The packet at exit can be modified by the XDP program. If you are interested to see both the original and modified packet, use the entry,exit option. With this, each packet is captured twice. The default value for this is entry.

  -D, --list-interfaces

Display a list of available interfaces and any XDP program loaded

  --load-xdp-mode

Specifies which loader mode to use with the --load-xdp-program option. The valid values are ‘native’, which is the default in-driver XDP mode, ‘skb’, which causes the so-called skb mode (also known as generic XDP) to be used, ‘hw’ which causes the program to be offloaded to the hardware, or ‘unspecified’ which leaves it up to the kernel to pick a mode (which it will do by picking native mode if the driver supports it, or generic mode otherwise). Note that using ‘unspecified’ can make it difficult to predict what mode a program will end up being loaded in. For this reason, the default is ‘native’.

  --load-xdp-program

If no XDP program is loaded on the interface, by default, xdpdump will fallback to libpcap's live capture mode to capture the packets. Alternatively, with this option, you can ask xdpdump to load an XDP program to capture the packets directly.

  -i, --interface <ifname>

Listen on interface ifname. Note that if no XDP program is loaded on the interface it will use libpcap's live capture mode to capture the packets.

  --perf-wakeup <events>

Let the Kernel wake up xdpdump once for every <events> being posted in the perf ring buffer. The higher the number the less the impact is on the actual XDP program. The default value is 0, which automatically calculates the value based on the available CPUs/buffers. Use -v to see the actual used value.

  -p, --program-names [<prog>|all]

This option allows you to capture packets for a specific, set of, or all XDP programs loaded on the interface. You can either specify the actual program names or program IDs separated by commas. In the case where multiple programs are attached with the same name, you should use the program ID. Use the -D option to see the loaded programs and their IDs.

In addition, the Linux API does not provide the full name of the attached eBPF entry function if it's longer than 15 characters. xdpdump will try to guess the correct function name from the available BTF debug information. However, if multiple functions exist with the same leading name, it can not pick the correct one. It will dump the available functions, and you can choose the correct one, and supply it with this option. If you have programs with duplicate long names, you also need to specify the program ID with the full name. This can be done by adding the id to the name with the @<id> suffix.

  -P, --promiscuous-mode

This option puts the interface into promiscuous mode.

  -s, --snapshot-length <snaplen>

Capture snaplen bytes of a packet rather than the default 262144 bytes.

  --use-pcap

Use legacy pcap format for XDP traces. By default, it will use the PcapNG format so that it can store various metadata.

  -w, --write <file>

Write the raw packets to a pcap file rather than printing them out hexadecimal. Standard output is used if file is -.

  -x, --hex

When dumping packets on the console also print the full packet content in hex.

  -v, --verbose

Enable debug logging. Specify twice for even more verbosity.

  --version

Display xpdump version information and exit.

  -h, --help

Display a summary of the available options

Examples

The below will load the xdp-filter program on eth0, but it does not do any actual filtering:
    # xdp-filter load --mode skb eth0
    #
    # xdpdump -D
    Interface        Prio  Program name      Mode     ID   Tag               Chain actions
    --------------------------------------------------------------------------------------
    lo                     <No XDP program loaded!>
    eth0                   xdp_dispatcher    skb      10651 d51e469e988d81da
     =>              10     xdpfilt_alw_all           10669 0b394f43ab24501c  XDP_PASS

Now we can try xdpdump:
    # xdpdump -i eth0 -x
    listening on eth0, ingress XDP program ID 10651 func xdp_dispatcher, capture mode entry, capture size 262144 bytes
    1584373839.460733895: xdp_dispatcher()@entry: packet size 102 bytes, captured 102 bytes on if_index 2, rx queue 0, id 1
      0x0000:  52 54 00 db 44 b6 52 54 00 34 38 da 08 00 45 48  RT..D.RT.48...EH
      0x0010:  00 58 d7 dd 40 00 40 06 ec c3 c0 a8 7a 01 c0 a8  .X..@.@.....z...
      0x0020:  7a 64 9c de 00 16 0d d5 c6 bc 46 c9 bb 11 80 18  zd........F.....
      0x0030:  01 f5 7b b4 00 00 01 01 08 0a 77 0a 8c b8 40 12  ..{.......w...@.
      0x0040:  cc a6 00 00 00 10 54 ce 6e 20 c3 e7 da 6c 08 42  ......T.n ...l.B
      0x0050:  d6 d9 ee 42 42 f0 82 c9 4f 12 ed 7b 19 ab 22 0d  ...BB...O..{..".
      0x0060:  09 29 a9 ee df 89                                .)....
    1584373839.462340808: xdp_dispatcher()@entry: packet size 66 bytes, captured 66 bytes on if_index 2, rx queue 0, id 2
      0x0000:  52 54 00 db 44 b6 52 54 00 34 38 da 08 00 45 48  RT..D.RT.48...EH
      0x0010:  00 34 d7 de 40 00 40 06 ec e6 c0 a8 7a 01 c0 a8  .4..@.@.....z...
      0x0020:  7a 64 9c de 00 16 0d d5 c6 e0 46 c9 bc 85 80 10  zd........F.....
      0x0030:  01 f5 74 0c 00 00 01 01 08 0a 77 0a 8c ba 40 12  ..t.......w...@.
      0x0040:  d2 34                                            .4
    ^C
    2 packets captured
    0 packets dropped by perf ring

Below are two more examples redirecting the capture file to tcpdump or tshark:
    # xdpdump -i eth0 -w - | tcpdump -r - -n
    listening on eth0, ingress XDP program ID 10651 func xdp_dispatcher, capture mode entry, capture size 262144 bytes
    reading from file -, link-type EN10MB (Ethernet)
    15:55:09.075887 IP 192.168.122.1.40928 > 192.168.122.100.ssh: Flags [P.], seq 3857553815:3857553851, ack 3306438882, win 501, options [nop,nop,TS val 1997449167 ecr 1075234328], length 36
    15:55:09.077756 IP 192.168.122.1.40928 > 192.168.122.100.ssh: Flags [.], ack 37, win 501, options [nop,nop,TS val 1997449169 ecr 1075244363], length 0
    15:55:09.750230 IP 192.168.122.1.40928 > 192.168.122.100.ssh: Flags [P.], seq 36:72, ack 37, win 501, options [nop,nop,TS val 1997449842 ecr 1075244363], length 36

    # xdpdump -i eth0 -w - | tshark -r - -n
    listening on eth0, ingress XDP program ID 10651 func xdp_dispatcher, capture mode entry, capture size 262144 bytes
        1   0.000000 192.168.122.1 → 192.168.122.100 SSH 102 Client: Encrypted packet (len=36)
        2   0.000646 192.168.122.1 → 192.168.122.100 TCP 66 40158 → 22 [ACK] Seq=37 Ack=37 Win=1467 Len=0 TSval=1997621571 TSecr=1075416765
        3  12.218164 192.168.122.1 → 192.168.122.100 SSH 102 Client: Encrypted packet (len=36)

One final example capturing specific XDP programs loaded on the interface:
    # xdpdump -D
    Interface        Prio  Program name      Mode     ID   Tag               Chain actions
    --------------------------------------------------------------------------------------
    lo                     <No XDP program loaded!>
    eth0                   xdp_dispatcher    skb      10558 d51e469e988d81da
     =>              5      xdp_test_prog_w           10576 b5a46c6e9935298c  XDP_PASS
     =>              10     xdp_pass                  10582 3b185187f1855c4c  XDP_PASS
     =>              10     xdp_pass                  10587 3b185187f1855c4c  XDP_PASS

We would like to see the packets on the xdp_dispatcher() and the 2nd xdp_pass() program:
    # xdpdump -i eth0 --rx-capture=entry,exit -p xdp_dispatcher,xdp_pass@10587
      or
    # xdpdump -i eth0 --rx-capture=entry,exit -p 10558,10587
    listening on eth0, ingress XDP program ID 10558 func xdp_dispatcher, ID 10587 func xdp_pass, capture mode entry/exit, capture size 262144 bytes
    1607694215.501287259: xdp_dispatcher()@entry: packet size 102 bytes on if_index 2, rx queue 0, id 1
    1607694215.501371504: xdp_pass()@entry: packet size 102 bytes on if_index 2, rx queue 0, id 1
    1607694215.501383099: xdp_pass()@exit[PASS]: packet size 102 bytes on if_index 2, rx queue 0, id 1
    1607694215.501394709: xdp_dispatcher()@exit[PASS]: packet size 102 bytes on if_index 2, rx queue 0, id 1
    ^C
    4 packets captured
    0 packets dropped by perf ring

BUGS

Please report any bugs on Github: https://github.com/xdp-project/xdp-tools/issues

AUTHOR

xdpdump was written by Eelco Chaudron