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main.go
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main.go
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// Program xdpcap produces tcpdump compatible PCAPs from a BPF map.
//
// You can use it to introspect traffic dropped or redirected by XDP. It
// is also useful to examine any transformations done to a packet.
//
// xdpcap requires you to instrument your XDP code in a specific way,
// check the documentation at https://github.com/cloudflare/xdpcap
// for details.
//
// Once you have done so, you can capture into a file, or pipe
// straight into tcpdump:
//
// xdpcap /path/to/pinned/map file.pcap
// xdpcap /path/to/pinned/map - | tcpdump -r -
//
// xdpcap supports tcpdump / libpcap-style filter expressions.
package main
import (
"bytes"
"flag"
"fmt"
"io"
"math"
"os"
"os/signal"
"syscall"
"time"
"github.com/google/gopacket"
"github.com/google/gopacket/layers"
"github.com/google/gopacket/pcapgo"
"github.com/pkg/errors"
)
func main() {
flags, err := parseFlags(os.Args[0], os.Args[1:])
switch {
case err == flag.ErrHelp:
fmt.Fprintln(os.Stderr, flags.Usage())
os.Exit(0)
case err != nil:
fmt.Fprintf(os.Stderr, "Error: %v\n\nUsage: %s", err, flags.Usage())
os.Exit(1)
}
defer flags.pcapFile.Close()
err = capture(flags)
if err != nil {
fmt.Fprintln(os.Stderr, "Error:", err)
os.Exit(2)
}
}
func capture(flags flags) error {
// BPF progs, maps and the perf buffer are stored in locked memory
err := unlimitLockedMemory()
if err != nil {
fmt.Fprintln(os.Stderr, "Error setting locked memory limit:", err)
}
// Exit gracefully
sigs := make(chan os.Signal, 1)
signal.Notify(sigs, os.Interrupt, syscall.SIGTERM)
filter, err := newFilter(flags.mapPath, flags.filterOpts)
if err != nil {
return errors.Wrap(err, "creating filter")
}
// Need to close the filter after the pcap writer
pcapWriter, interfaces, err := newPcapWriter(flags.pcapFile, flags.filterExpr, filter.actions)
if err != nil {
filter.close()
return errors.Wrap(err, "writing pcap header")
}
defer pcapWriter.Flush()
defer filter.close()
if !flags.quiet {
// Print metrics every 1 second
go func() {
ticker := time.NewTicker(time.Second)
defer ticker.Stop()
for range ticker.C {
str := bytes.Buffer{}
metrics, err := filter.metrics()
if err != nil {
fmt.Fprintln(os.Stderr, "Error getting metrics:", err)
continue
}
for _, action := range filter.actions {
fmt.Fprintf(&str, "%v: %d/%d\t", action, metrics[action].receivedPackets, metrics[action].matchedPackets)
}
str.WriteString("(received/matched packets)\n")
os.Stderr.WriteString(str.String())
}
}()
}
// Write out a pcap file from aggregated packets
go func() {
for {
pkt, err := filter.read()
switch {
case err == errFilterClosed:
return
case err != nil:
fmt.Fprintln(os.Stderr, "Error:", err)
continue
}
info := gopacket.CaptureInfo{
Timestamp: time.Now(),
CaptureLength: len(pkt.data),
Length: len(pkt.data),
InterfaceIndex: interfaces[pkt.action],
}
err = pcapWriter.WritePacket(info, pkt.data)
if err != nil {
fmt.Fprintln(os.Stderr, "Error writing packet:", err)
}
if flags.flush {
err = pcapWriter.Flush()
if err != nil {
fmt.Fprintln(os.Stderr, "Error flushing data:", err)
}
}
}
}()
<-sigs
return nil
}
// newPcapWriter creates a pcapWriter with an pcap interface (metadata) per xdp action
func newPcapWriter(w io.Writer, filterExpr string, actions []xdpAction) (*pcapgo.NgWriter, map[xdpAction]int, error) {
if len(actions) == 0 {
return nil, nil, errors.New("can't create pcap with no actions")
}
var interfaces []pcapgo.NgInterface
actionIfcs := make(map[xdpAction]int)
for id, action := range actions {
interfaces = append(interfaces, pcapgo.NgInterface{
Name: fmt.Sprintf("XDP%s", action.String()),
Comment: "XDP action",
Filter: filterExpr,
LinkType: layers.LinkTypeEthernet,
SnapLength: uint32(math.MaxUint16),
})
actionIfcs[action] = id
}
pcapWriter, err := pcapgo.NewNgWriterInterface(w, interfaces[0], pcapgo.NgWriterOptions{})
if err != nil {
return nil, nil, err
}
for _, ifc := range interfaces[1:] {
_, err := pcapWriter.AddInterface(ifc)
if err != nil {
return nil, nil, err
}
}
// Flush the header out in case we're writing to stdout, this lets tcpdump print a reassuring message
err = pcapWriter.Flush()
if err != nil {
return nil, nil, errors.Wrap(err, "writing pcap header")
}
return pcapWriter, actionIfcs, nil
}