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.
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.
go get -u github.com/cloudflare/xdpcap/cmd/xdpcap
-
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 -
-
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
xdpcapis currently unable to capture more than the first page of a packet. If the instrumented XDP program is loaded withBPF_F_XDP_HAS_FRAGS, then packets that span multiple physical pages won't be entirely captured.
sudo -E $(which go) test
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