captured: implement a packet ring buffer

So that when you connect with Wireshark, you’ll see the most recent
packets (takes up to 7 MB of RAM).

cmd/captured/captured.go

@@ -1,8 +1,19 @@
 package main
 
 import (
+	"container/ring"
+	"context"
 	"flag"
 	"log"
+	"sync"
+
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
+	"github.com/google/gopacket/pcapgo"
+
+	"golang.org/x/sync/errgroup"
+
+	_ "net/http/pprof"
 )
 
 var (
@@ -11,8 +22,78 @@ var (
 		"path to a PEM-encoded RSA, DSA or ECDSA private key (create using e.g. ssh-keygen -f /perm/breakglass.host_key -N '' -t rsa)")
 )
 
+func capturePackets(ctx context.Context) (chan gopacket.Packet, error) {
+	packets := make(chan gopacket.Packet)
+	for _, ifname := range []string{"uplink0", "lan0"} {
+		handle, err := pcapgo.OpenEthernet(ifname)
+		if err != nil {
+			return nil, err
+		}
+
+		if err := handle.SetBPF(instructions); err != nil {
+			return nil, err
+		}
+
+		pkgsrc := gopacket.NewPacketSource(handle, layers.LayerTypeEthernet)
+		go func() {
+			defer handle.Close()
+			for packet := range pkgsrc.Packets() {
+				select {
+				case packets <- packet:
+				case <-ctx.Done():
+					return
+				}
+			}
+		}()
+	}
+	return packets, nil
+}
+
+type packetRingBuffer struct {
+	sync.Mutex
+	r *ring.Ring
+}
+
+func newPacketRingBuffer(size int) *packetRingBuffer {
+	return &packetRingBuffer{
+		r: ring.New(size),
+	}
+}
+
+func (prb *packetRingBuffer) writePacket(p gopacket.Packet) {
+	prb.Lock()
+	defer prb.Unlock()
+	prb.r.Value = p
+	prb.r = prb.r.Next()
+}
+
+func (prb *packetRingBuffer) packetsLocked() []gopacket.Packet {
+	packets := make([]gopacket.Packet, 0, prb.r.Len())
+	prb.r.Do(func(x interface{}) {
+		if x != nil {
+			packets = append(packets, x.(gopacket.Packet))
+		}
+	})
+	return packets
+}
+
 func logic() error {
-	return listenAndServe()
+	prb := newPacketRingBuffer(5000)
+
+	var eg errgroup.Group
+	eg.Go(func() error { return listenAndServe(prb) })
+	eg.Go(func() error {
+		packets, err := capturePackets(context.Background())
+		if err != nil {
+			return err
+		}
+		for packet := range packets {
+			prb.writePacket(packet)
+		}
+		return nil
+	})
+
+	return eg.Wait()
 }
 
 func main() {

cmd/captured/ssh.go

@@ -8,13 +8,12 @@ import (
 	"net"
 
 	"github.com/gokrazy/gokrazy"
-	"github.com/google/gopacket"
 	"github.com/google/gopacket/layers"
 	"github.com/google/gopacket/pcapgo"
 	"golang.org/x/crypto/ssh"
 )
 
-func handleChannel(newChannel ssh.NewChannel) {
+func handleChannel(newChannel ssh.NewChannel, prb *packetRingBuffer) {
 	if t := newChannel.ChannelType(); t != "session" {
 		newChannel.Reject(ssh.UnknownChannelType, fmt.Sprintf("unknown channel type: %q", t))
 		return
@@ -30,7 +29,7 @@ func handleChannel(newChannel ssh.NewChannel) {
 	go func(channel ssh.Channel, requests <-chan *ssh.Request) {
 		s := session{channel: channel}
 		for req := range requests {
-			if err := s.request(req); err != nil {
+			if err := s.request(req, prb); err != nil {
 				errmsg := []byte(err.Error())
 				// Append a trailing newline; the error message is
 				// displayed as-is by ssh(1).
@@ -49,7 +48,7 @@ type session struct {
 	channel ssh.Channel
 }
 
-func (s *session) request(req *ssh.Request) error {
+func (s *session) request(req *ssh.Request, prb *packetRingBuffer) error {
 	switch req.Type {
 	case "exec":
 		if got, want := len(req.Payload), 4; got < want {
@@ -65,34 +64,22 @@ func (s *session) request(req *ssh.Request) error {
 			return err
 		}
 
-		packets := make(chan gopacket.Packet)
-		for _, ifname := range []string{"uplink0", "lan0"} {
-			handle, err := pcapgo.OpenEthernet(ifname)
-			//handle, err := pcap.OpenLive("uplink0", 1600, false /* promisc */, pcap.BlockForever)
-			if err != nil {
-				return err
-			}
-
-			if err := handle.SetBPF(instructions); err != nil {
-				//if err := handle.SetBPFFilter("icmp6 or (udp and (port 67 or port 68 or port 546 or port 547))"); err != nil {
-				return err
-			}
-
-			pkgsrc := gopacket.NewPacketSource(handle, layers.LayerTypeEthernet)
-			go func() {
-				defer handle.Close()
-				for packet := range pkgsrc.Packets() {
-					select {
-					case packets <- packet:
-					case <-ctx.Done():
-						return
-					}
-				}
-			}()
+		prb.Lock()
+		packets, err := capturePackets(ctx)
+		buffered := prb.packetsLocked()
+		prb.Unlock()
+		if err != nil {
+			return err
 		}
 
 		req.Reply(true, nil)
 
+		for _, packet := range buffered {
+			if err := pcapw.WritePacket(packet.Metadata().CaptureInfo, packet.Data()); err != nil {
+				return fmt.Errorf("pcap.WritePacket(): %v", err)
+			}
+		}
+
 		for packet := range packets {
 			if err := pcapw.WritePacket(packet.Metadata().CaptureInfo, packet.Data()); err != nil {
 				return fmt.Errorf("pcap.WritePacket(): %v", err)
@@ -117,7 +104,7 @@ func loadHostKey(path string) (ssh.Signer, error) {
 	return ssh.ParsePrivateKey(b)
 }
 
-func listenAndServe() error {
+func listenAndServe(prb *packetRingBuffer) error {
 	config := &ssh.ServerConfig{
 		PublicKeyCallback: func(conn ssh.ConnMetadata, pubKey ssh.PublicKey) (*ssh.Permissions, error) {
 			return nil, nil // authorize all users
@@ -149,7 +136,7 @@ func listenAndServe() error {
 				go ssh.DiscardRequests(reqs)
 
 				for newChannel := range chans {
-					handleChannel(newChannel)
+					handleChannel(newChannel, prb)
 				}
 			}(conn)
 		}