xdpcap

xdpcap is a tcpdump like tool for eXpress Data Path (XDP). It can capture packets and actions / return codes from XDP programs, using standard tcpdump / libpcap filter expressions.

Instrumentation

XDP programs need to expose at least one hook point:

struct bpf_map_def xdpcap_hook = {
	.type = BPF_MAP_TYPE_PROG_ARRAY,
	.key_size = sizeof(int),
	.value_size = sizeof(int),
	.max_entries = 4, // The max value of XDP_* constants
};

This map must be pinned inside a bpffs.

hook.h provides a convenience macro for declaring such maps:

#include "hook.h"

struct bpf_map_def xdpcap_hook = XDPCAP_HOOK();

return XDP_* statements should be modified to "feed" a hook:

#include "hook.h"

struct bpf_map_def xdpcap_hook = XDPCAP_HOOK();

int xdp_main(struct xdp_md *ctx) {
	return xdpcap_exit(ctx, &xdpcap_hook, XDP_PASS);
}

For a full example, see testdata/xdp_hook.c.

Depending on the granularity desired, a program can expose multiple hook points, or a hook can be reused across programs by using the same underlying map.

Package xdpcap provides a wrapper for creating and pinning the hook maps using the newtools/ebpf loader.

xdpcap supports attaching to XDP programs loaded with the BPF_F_XDP_HAS_FRAGS flag (annotated with xdp.frags). It will attempt to attach itself as usual to the XDP program and if that fails, it will retry with the BPF_F_XDP_HAS_FRAGS flag.

Installation

go get -u github.com/cloudflare/xdpcap/cmd/xdpcap

Usage

• Capture packets to a pcap: xdpcap /path/to/pinned/map dump.pcap "tcp and port 80"

• Display captured packets: sudo xdpcap /path/to/pinned/map - "tcp and port 80" | sudo tcpdump -r -

Limitations

• filters run after the instrumented XDP program. If the program modifies the packet, the filter should match the modified packet, not the original input packet.

• capturing multi-buffer packets xdpcap is currently unable to capture more than the first page of a packet. If the instrumented XDP program is loaded with BPF_F_XDP_HAS_FRAGS, then packets that span multiple physical pages won't be entirely captured.

Tests

• sudo -E $(which go) test