tun_disabled: reply to ICMP Echo Request (#342)

This change allows a server running with `tun.disabled: true` (usually
a lighthouse) to still reply to ICMP EchoRequest packets. This allows
you to "ping" the lighthouse Nebula IP as a quick check to make sure the
tunnel is up, even when running with tun.disabled.

This is still gated by allowing `icmp` packets in the inbound firewall
rules.

main.go

@@ -123,7 +123,7 @@ func Main(config *Config, configTest bool, buildVersion string, logger *logrus.L
 
 		switch {
 		case config.GetBool("tun.disabled", false):
-			tun = newDisabledTun(tunCidr, l)
+			tun = newDisabledTun(tunCidr, config.GetInt("tun.tx_queue", 500), config.GetBool("stats.message_metrics", false), l)
 		case tunFd != nil:
 			tun, err = newTunFromFd(
 				*tunFd,

tun_disabled.go

@@ -1,26 +1,43 @@
 package nebula
 
 import (
+	"encoding/binary"
 	"fmt"
 	"io"
 	"net"
 	"strings"
 
+	"github.com/rcrowley/go-metrics"
+	"github.com/sirupsen/logrus"
 	log "github.com/sirupsen/logrus"
 )
 
 type disabledTun struct {
-	block  chan struct{}
+	read   chan []byte
 	cidr   *net.IPNet
 	logger *log.Logger
+
+	// Track these metrics since we don't have the tun device to do it for us
+	tx metrics.Counter
+	rx metrics.Counter
 }
 
-func newDisabledTun(cidr *net.IPNet, l *log.Logger) *disabledTun {
-	return &disabledTun{
+func newDisabledTun(cidr *net.IPNet, queueLen int, metricsEnabled bool, l *log.Logger) *disabledTun {
+	tun := &disabledTun{
 		cidr:   cidr,
-		block:  make(chan struct{}),
+		read:   make(chan []byte, queueLen),
 		logger: l,
 	}
+
+	if metricsEnabled {
+		tun.tx = metrics.GetOrRegisterCounter("messages.tx.message", nil)
+		tun.rx = metrics.GetOrRegisterCounter("messages.rx.message", nil)
+	} else {
+		tun.tx = &metrics.NilCounter{}
+		tun.rx = &metrics.NilCounter{}
+	}
+
+	return tun
 }
 
 func (*disabledTun) Activate() error {
@@ -36,12 +53,73 @@ func (*disabledTun) DeviceName() string {
 }
 
 func (t *disabledTun) Read(b []byte) (int, error) {
-	<-t.block
-	return 0, io.EOF
+	r, ok := <-t.read
+	if !ok {
+		return 0, io.EOF
+	}
+
+	if len(r) > len(b) {
+		return 0, fmt.Errorf("packet larger than mtu: %d > %d bytes", len(r), len(b))
+	}
+
+	t.tx.Inc(1)
+	if l.Level >= logrus.DebugLevel {
+		t.logger.WithField("raw", prettyPacket(r)).Debugf("Write payload")
+	}
+
+	return copy(b, r), nil
+}
+
+func (t *disabledTun) handleICMPEchoRequest(b []byte) bool {
+	// Return early if this is not a simple ICMP Echo Request
+	if !(len(b) >= 28 && len(b) <= mtu && b[0] == 0x45 && b[9] == 0x01 && b[20] == 0x08) {
+		return false
+	}
+
+	// We don't support fragmented packets
+	if b[7] != 0 || (b[6]&0x2F != 0) {
+		return false
+	}
+
+	buf := make([]byte, len(b))
+	copy(buf, b)
+
+	// Swap dest / src IPs and recalculate checksum
+	ipv4 := buf[0:20]
+	copy(ipv4[12:16], b[16:20])
+	copy(ipv4[16:20], b[12:16])
+	ipv4[10] = 0
+	ipv4[11] = 0
+	binary.BigEndian.PutUint16(ipv4[10:], ipChecksum(ipv4))
+
+	// Change type to ICMP Echo Reply and recalculate checksum
+	icmp := buf[20:]
+	icmp[0] = 0
+	icmp[2] = 0
+	icmp[3] = 0
+	binary.BigEndian.PutUint16(icmp[2:], ipChecksum(icmp))
+
+	// attempt to write it, but don't block
+	select {
+	case t.read <- buf:
+	default:
+		t.logger.Debugf("tun_disabled: dropped ICMP Echo Reply response")
+	}
+
+	return true
 }
 
 func (t *disabledTun) Write(b []byte) (int, error) {
-	t.logger.WithField("raw", prettyPacket(b)).Debugf("Disabled tun received unexpected payload")
+	t.rx.Inc(1)
+
+	// Check for ICMP Echo Request before spending time doing the full parsing
+	if t.handleICMPEchoRequest(b) {
+		if l.Level >= logrus.DebugLevel {
+			t.logger.WithField("raw", prettyPacket(b)).Debugf("Disabled tun responded to ICMP Echo Request")
+		}
+	} else if l.Level >= logrus.DebugLevel {
+		t.logger.WithField("raw", prettyPacket(b)).Debugf("Disabled tun received unexpected payload")
+	}
 	return len(b), nil
 }
 
@@ -55,9 +133,9 @@ func (t *disabledTun) NewMultiQueueReader() (io.ReadWriteCloser, error) {
 }
 
 func (t *disabledTun) Close() error {
-	if t.block != nil {
-		close(t.block)
-		t.block = nil
+	if t.read != nil {
+		close(t.read)
+		t.read = nil
 	}
 	return nil
 }
@@ -76,3 +154,22 @@ func (p prettyPacket) String() string {
 
 	return s.String()
 }
+
+func ipChecksum(b []byte) uint16 {
+	var c uint32
+	sz := len(b) - 1
+
+	for i := 0; i < sz; i += 2 {
+		c += uint32(b[i]) << 8
+		c += uint32(b[i+1])
+	}
+	if sz%2 == 0 {
+		c += uint32(b[sz]) << 8
+	}
+
+	for (c >> 16) > 0 {
+		c = (c & 0xffff) + (c >> 16)
+	}
+
+	return ^uint16(c)
+}