cmd/derpprobe,prober: add ability to continuously probe bandwidth and track queuing delay

Setting the bw-interval to a negative value enables continuous bandwidth probing.
When probing bandwidth continuously, the prober also tracks packet transit times,
which can be used as a measure of queuing delay.

Updates tailscale/corp#24522

Signed-off-by: Percy Wegmann <percy@tailscale.com>

cmd/derpprobe/derpprobe.go

@@ -27,7 +27,7 @@ var (
 	meshInterval = flag.Duration("mesh-interval", 15*time.Second, "mesh probe interval")
 	stunInterval = flag.Duration("stun-interval", 15*time.Second, "STUN probe interval")
 	tlsInterval  = flag.Duration("tls-interval", 15*time.Second, "TLS probe interval")
-	bwInterval   = flag.Duration("bw-interval", 0, "bandwidth probe interval (0 = no bandwidth probing)")
+	bwInterval   = flag.Duration("bw-interval", 0, "bandwidth probe interval (0 = no bandwidth probing, > 0 = periodic bandwidth probing, < 0 = continuous bandwidth probing)")
 	bwSize       = flag.Int64("bw-probe-size-bytes", 1_000_000, "bandwidth probe size")
 	regionCode   = flag.String("region-code", "", "probe only this region (e.g. 'lax'); if left blank, all regions will be probed")
 )
@@ -45,7 +45,7 @@ func main() {
 		prober.WithSTUNProbing(*stunInterval),
 		prober.WithTLSProbing(*tlsInterval),
 	}
-	if *bwInterval > 0 {
+	if *bwInterval != 0 {
 		opts = append(opts, prober.WithBandwidthProbing(*bwInterval, *bwSize))
 	}
 	if *regionCode != "" {
@@ -106,7 +106,7 @@ func getOverallStatus(p *prober.Prober) (o overallStatus) {
 			// Do not show probes that have not finished yet.
 			continue
 		}
-		if i.Result {
+		if i.Status == prober.ProbeStatusSucceeded {
 			o.addGoodf("%s: %s", p, i.Latency)
 		} else {
 			o.addBadf("%s: %s", p, i.Error)

derp/derphttp/derphttp_client.go

@@ -748,7 +748,7 @@ func (c *Client) dialNode(ctx context.Context, n *tailcfg.DERPNode) (net.Conn, e
 				select {
 				case <-ctx.Done():
 					// Either user canceled original context,
-					// it timed out, or the v6 dial succeeded.
+					// if timed out, or the v6 dial succeeded.
 					t.Stop()
 					return
 				case <-tChannel:
@@ -757,6 +757,9 @@ func (c *Client) dialNode(ctx context.Context, n *tailcfg.DERPNode) (net.Conn, e
 			}
 			dst := cmp.Or(dstPrimary, n.HostName)
 			port := "443"
+			if !c.useHTTPS() {
+				port = "3340"
+			}
 			if n.DERPPort != 0 {
 				port = fmt.Sprint(n.DERPPort)
 			}

prober/derp.go

@@ -8,6 +8,7 @@ import (
 	"cmp"
 	"context"
 	crand "crypto/rand"
+	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"expvar"
@@ -15,10 +16,12 @@ import (
 	"log"
 	"net"
 	"net/http"
+	"reflect"
 	"strconv"
 	"strings"
 	"sync"
 	"time"
+	"unsafe"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"tailscale.com/client/tailscale"
@@ -55,7 +58,7 @@ type derpProber struct {
 	tlsProbeFn  func(string) ProbeClass
 	udpProbeFn  func(string, int) ProbeClass
 	meshProbeFn func(string, string) ProbeClass
-	bwProbeFn   func(string, string, int64) ProbeClass
+	bwProbeFn   func(string, string, int64, bool) ProbeClass
 
 	sync.Mutex
 	lastDERPMap   *tailcfg.DERPMap
@@ -69,6 +72,11 @@ type DERPOpt func(*derpProber)
 // WithBandwidthProbing enables bandwidth probing. When enabled, a payload of
 // `size` bytes will be regularly transferred through each DERP server, and each
 // pair of DERP servers in every region.
+//
+// If `size` is less than 0, this will trigger the probe to continuously send
+// data as fast as possible. Continuous mode can consume huge amounts of
+// network bandwidth, so it is best used when probing DERP servers on the same
+// LAN/datacenter.
 func WithBandwidthProbing(interval time.Duration, size int64) DERPOpt {
 	return func(d *derpProber) {
 		d.bwInterval = interval
@@ -94,7 +102,7 @@ func WithSTUNProbing(interval time.Duration) DERPOpt {
 
 // WithTLSProbing enables TLS probing that will check TLS certificate on port
 // 443 of each DERP server every `interval`.
-func WithTLSProbing(interval time.Duration) DERPOpt {
+func WithTLSProbing(interval time.Duration, regions ...string) DERPOpt {
 	return func(d *derpProber) {
 		d.tlsInterval = interval
 	}
@@ -196,12 +204,12 @@ func (d *derpProber) probeMapFn(ctx context.Context) error {
 					}
 				}
 
-				if d.bwInterval > 0 && d.bwProbeSize > 0 {
+				if d.bwInterval != 0 && d.bwProbeSize > 0 {
 					n := fmt.Sprintf("derp/%s/%s/%s/bw", region.RegionCode, server.Name, to.Name)
 					wantProbes[n] = true
 					if d.probes[n] == nil {
 						log.Printf("adding DERP bandwidth probe for %s->%s (%s) %v bytes every %v", server.Name, to.Name, region.RegionName, d.bwProbeSize, d.bwInterval)
-						d.probes[n] = d.p.Run(n, d.bwInterval, labels, d.bwProbeFn(server.Name, to.Name, d.bwProbeSize))
+						d.probes[n] = d.p.Run(n, d.bwInterval, labels, d.bwProbeFn(server.Name, to.Name, d.bwProbeSize, d.bwInterval < 0))
 					}
 				}
 			}
@@ -246,11 +254,19 @@ func (d *derpProber) probeMesh(from, to string) ProbeClass {
 // through a pair of DERP servers (or just one server, if 'from' and 'to' are
 // the same). 'from' and 'to' are expected to be names (DERPNode.Name) of two
 // DERP servers in the same region.
-func (d *derpProber) probeBandwidth(from, to string, size int64) ProbeClass {
+func (d *derpProber) probeBandwidth(from, to string, size int64, continuous bool) ProbeClass {
 	derpPath := "mesh"
 	if from == to {
 		derpPath = "single"
 	}
+	if continuous {
+		return d.probeBandwidthContinuous(from, to, derpPath, size)
+	} else {
+		return d.probeBandwidthOnce(from, to, derpPath, size)
+	}
+}
+
+func (d *derpProber) probeBandwidthOnce(from, to, derpPath string, size int64) ProbeClass {
 	var transferTime expvar.Float
 	return ProbeClass{
 		Probe: func(ctx context.Context) error {
@@ -258,7 +274,7 @@ func (d *derpProber) probeBandwidth(from, to string, size int64) ProbeClass {
 			if err != nil {
 				return err
 			}
-			return derpProbeBandwidth(ctx, d.lastDERPMap, fromN, toN, size, &transferTime)
+			return derpProbeBandwidthOnce(ctx, d.lastDERPMap, fromN, toN, size, &transferTime)
 		},
 		Class:  "derp_bw",
 		Labels: Labels{"derp_path": derpPath},
@@ -271,6 +287,76 @@ func (d *derpProber) probeBandwidth(from, to string, size int64) ProbeClass {
 	}
 }
 
+func (d *derpProber) probeBandwidthContinuous(from, to, derpPath string, size int64) ProbeClass {
+	var bytesTransferred expvar.Float
+	var transferTime expvar.Float
+	var qdh queuingDelayHistogram
+	qdh.reset()
+	return ProbeClass{
+		Probe: func(ctx context.Context) error {
+			fromN, toN, err := d.getNodePair(from, to)
+			if err != nil {
+				return err
+			}
+			return derpProbeBandwidthContinuous(ctx, d.lastDERPMap, fromN, toN, size, &bytesTransferred, &transferTime, &qdh)
+		},
+		Class:  "derp_bw",
+		Labels: Labels{"derp_path": derpPath},
+		Metrics: func(l prometheus.Labels) []prometheus.Metric {
+			qdh.mx.Lock()
+			result := []prometheus.Metric{
+				prometheus.MustNewConstMetric(prometheus.NewDesc("derp_bw_probe_continuous_size_bytes", "Payload size of the bandwidth prober", nil, l), prometheus.GaugeValue, float64(size)),
+				prometheus.MustNewConstMetric(prometheus.NewDesc("derp_bw_probe_continuous_bytes_total", "Total data transferred", nil, l), prometheus.CounterValue, float64(bytesTransferred.Value())),
+				prometheus.MustNewConstMetric(prometheus.NewDesc("derp_bw_probe_continuous_transfer_time_seconds_total", "Time it took to transfer data", nil, l), prometheus.CounterValue, transferTime.Value()),
+				prometheus.MustNewConstHistogram(prometheus.NewDesc("derp_bw_probe_continuous_queuing_delays_seconds", "Distribution of queuing delays", nil, l), qdh.count, qdh.sum, qdh.buckets),
+			}
+			qdh.resetLocked()
+			qdh.mx.Unlock()
+			return result
+		},
+	}
+}
+
+// queuingDelayHistogram allows tracking a histogram of queuing delays
+type queuingDelayHistogram struct {
+	count   uint64
+	sum     float64
+	buckets map[float64]uint64
+	mx      sync.Mutex
+}
+
+// qdhBuckets defines the buckets (in seconds) for the queuingDelayHistogram.
+var qdhBuckets = []float64{0.0001, 0.0002, 0.0005}
+
+func (qdh *queuingDelayHistogram) reset() {
+	qdh.mx.Lock()
+	defer qdh.mx.Unlock()
+	qdh.resetLocked()
+}
+
+func (qdh *queuingDelayHistogram) resetLocked() {
+	qdh.count = 0
+	qdh.sum = 0
+	qdh.buckets = make(map[float64]uint64, len(qdhBuckets))
+}
+
+func (qdh *queuingDelayHistogram) add(d time.Duration) {
+	qdh.mx.Lock()
+	defer qdh.mx.Unlock()
+
+	seconds := float64(d.Seconds())
+	qdh.count++
+	qdh.sum += seconds
+
+	for _, b := range qdhBuckets {
+		if seconds > b {
+			continue
+		}
+		qdh.buckets[b] += 1
+		break
+	}
+}
+
 // getNodePair returns DERPNode objects for two DERP servers based on their
 // short names.
 func (d *derpProber) getNodePair(n1, n2 string) (ret1, ret2 *tailcfg.DERPNode, _ error) {
@@ -411,9 +497,9 @@ func derpProbeUDP(ctx context.Context, ipStr string, port int) error {
 	return nil
 }
 
-// derpProbeBandwidth sends a payload of a given size between two local
+// derpProbeBandwidthOnce sends a payload of a given size between two local
 // DERP clients connected to two DERP servers.
-func derpProbeBandwidth(ctx context.Context, dm *tailcfg.DERPMap, from, to *tailcfg.DERPNode, size int64, transferTime *expvar.Float) (err error) {
+func derpProbeBandwidthOnce(ctx context.Context, dm *tailcfg.DERPMap, from, to *tailcfg.DERPNode, size int64, transferTime *expvar.Float) (err error) {
 	// This probe uses clients with isProber=false to avoid spamming the derper logs with every packet
 	// sent by the bandwidth probe.
 	fromc, err := newConn(ctx, dm, from, false)
@@ -437,7 +523,39 @@ func derpProbeBandwidth(ctx context.Context, dm *tailcfg.DERPMap, from, to *tail
 	start := time.Now()
 	defer func() { transferTime.Add(time.Since(start).Seconds()) }()
 
-	if err := runDerpProbeNodePair(ctx, from, to, fromc, toc, size); err != nil {
+	if err := runDerpProbeNodePairOnce(ctx, from, to, fromc, toc, size); err != nil {
+		// Record pubkeys on failed probes to aid investigation.
+		return fmt.Errorf("%s -> %s: %w",
+			fromc.SelfPublicKey().ShortString(),
+			toc.SelfPublicKey().ShortString(), err)
+	}
+	return nil
+}
+
+// derpProbeBandwidthContinuous continuously sends data between two local
+// DERP clients connected to two DERP servers.
+func derpProbeBandwidthContinuous(ctx context.Context, dm *tailcfg.DERPMap, from, to *tailcfg.DERPNode, size int64, bytesTransferred, transferTime *expvar.Float, qdh *queuingDelayHistogram) (err error) {
+	// This probe uses clients with isProber=false to avoid spamming the derper logs with every packet
+	// sent by the bandwidth probe.
+	fromc, err := newConn(ctx, dm, from, false)
+	if err != nil {
+		return err
+	}
+	defer fromc.Close()
+	toc, err := newConn(ctx, dm, to, false)
+	if err != nil {
+		return err
+	}
+	defer toc.Close()
+
+	// Wait a bit for from's node to hear about to existing on the
+	// other node in the region, in the case where the two nodes
+	// are different.
+	if from.Name != to.Name {
+		time.Sleep(100 * time.Millisecond) // pretty arbitrary
+	}
+
+	if err := runDerpProbeNodePairContinuously(ctx, from, to, fromc, toc, size, bytesTransferred, transferTime, qdh); err != nil {
 		// Record pubkeys on failed probes to aid investigation.
 		return fmt.Errorf("%s -> %s: %w",
 			fromc.SelfPublicKey().ShortString(),
@@ -468,7 +586,7 @@ func derpProbeNodePair(ctx context.Context, dm *tailcfg.DERPMap, from, to *tailc
 	}
 
 	const meshProbePacketSize = 8
-	if err := runDerpProbeNodePair(ctx, from, to, fromc, toc, meshProbePacketSize); err != nil {
+	if err := runDerpProbeNodePairOnce(ctx, from, to, fromc, toc, meshProbePacketSize); err != nil {
 		// Record pubkeys on failed probes to aid investigation.
 		return fmt.Errorf("%s -> %s: %w",
 			fromc.SelfPublicKey().ShortString(),
@@ -504,10 +622,10 @@ func packetsForSize(size int64) [][]byte {
 	return pkts
 }
 
-// runDerpProbeNodePair takes two DERP clients (fromc and toc) connected to two
+// runDerpProbeNodePairOnce takes two DERP clients (fromc and toc) connected to two
 // DERP servers (from and to) and sends a test payload of a given size from one
 // to another.
-func runDerpProbeNodePair(ctx context.Context, from, to *tailcfg.DERPNode, fromc, toc *derphttp.Client, size int64) error {
+func runDerpProbeNodePairOnce(ctx context.Context, from, to *tailcfg.DERPNode, fromc, toc *derphttp.Client, size int64) error {
 	// To avoid derper dropping enqueued packets, limit the number of packets in flight.
 	// The value here is slightly smaller than perClientSendQueueDepth in derp_server.go
 	inFlight := syncs.NewSemaphore(30)
@@ -544,6 +662,8 @@ func runDerpProbeNodePair(ctx context.Context, from, to *tailcfg.DERPNode, fromc
 					recvc <- fmt.Errorf("got data packet %d from unexpected source, %v", idx, v.Source)
 					return
 				}
+				// This assumes that the packets are received reliably and in order.
+				// The DERP protocol does not guarantee this, but this probe assumes it.
 				if got, want := v.Data, pkts[idx]; !bytes.Equal(got, want) {
 					recvc <- fmt.Errorf("unexpected data packet %d (out of %d)", idx, len(pkts))
 					return
@@ -577,6 +697,91 @@ func runDerpProbeNodePair(ctx context.Context, from, to *tailcfg.DERPNode, fromc
 	return nil
 }
 
+func runDerpProbeNodePairContinuously(ctx context.Context, from, to *tailcfg.DERPNode, fromc, toc *derphttp.Client, pktSize int64, bytesTransferred, transferTime *expvar.Float, qdh *queuingDelayHistogram) error {
+	// To avoid derper dropping enqueued packets, limit the number of packets in flight.
+	// The value here is slightly smaller than perClientSendQueueDepth in derp_server.go
+	inFlight := syncs.NewSemaphore(30)
+
+	// Send the packets.
+	sendc := make(chan error, 1)
+	pkt := make([]byte, pktSize) // sized slightly smaller than MTU to avoid fragmentation
+	crand.Read(pkt)
+	go func() {
+		for {
+			inFlight.AcquireContext(ctx)
+			now := time.Now()
+			// Write the monotonic time into the first 16 bytes of the packet
+			wall, ext := wallAndExt(now)
+			binary.BigEndian.PutUint64(pkt, wall)
+			binary.BigEndian.PutUint64(pkt[8:], uint64(ext))
+			if err := fromc.Send(toc.SelfPublicKey(), pkt); err != nil {
+				sendc <- fmt.Errorf("sending packet %w", err)
+				return
+			}
+		}
+	}()
+
+	// Increment transfer time every 1 second.
+	ticker := time.NewTicker(1 * time.Second)
+	go func() {
+		for {
+			select {
+			case <-ticker.C:
+				transferTime.Add(1)
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+
+	// Receive the packets.
+	recvc := make(chan error, 1)
+	go func() {
+		defer close(recvc) // to break out of 'select' below.
+		for {
+			m, err := toc.Recv()
+			if err != nil {
+				recvc <- err
+				return
+			}
+			switch v := m.(type) {
+			case derp.ReceivedPacket:
+				now := time.Now()
+				inFlight.Release()
+				if v.Source != fromc.SelfPublicKey() {
+					recvc <- fmt.Errorf("got data packet from unexpected source, %v", v.Source)
+					return
+				}
+				wall := binary.BigEndian.Uint64(v.Data)
+				ext := int64(binary.BigEndian.Uint64(v.Data[8:]))
+				sent := fromWallAndExt(wall, ext)
+				qdh.add(now.Sub(sent))
+				bytesTransferred.Add(float64(pktSize))
+
+			case derp.KeepAliveMessage:
+				// Silently ignore.
+			default:
+				log.Printf("%v: ignoring Recv frame type %T", to.Name, v)
+				// Loop.
+			}
+		}
+	}()
+
+	select {
+	case <-ctx.Done():
+		return fmt.Errorf("timeout: %w", ctx.Err())
+	case err := <-sendc:
+		if err != nil {
+			return fmt.Errorf("error sending via %q: %w", from.Name, err)
+		}
+	case err := <-recvc:
+		if err != nil {
+			return fmt.Errorf("error receiving from %q: %w", to.Name, err)
+		}
+	}
+	return nil
+}
+
 func newConn(ctx context.Context, dm *tailcfg.DERPMap, n *tailcfg.DERPNode, isProber bool) (*derphttp.Client, error) {
 	// To avoid spamming the log with regular connection messages.
 	l := logger.Filtered(log.Printf, func(s string) bool {
@@ -597,6 +802,9 @@ func newConn(ctx context.Context, dm *tailcfg.DERPMap, n *tailcfg.DERPNode, isPr
 	if err != nil {
 		return nil, err
 	}
+
+	// Only verify TLS state if this is a prober.
+	if isProber {
 		cs, ok := dc.TLSConnectionState()
 		if !ok {
 			dc.Close()
@@ -610,6 +818,7 @@ func newConn(ctx context.Context, dm *tailcfg.DERPMap, n *tailcfg.DERPNode, isPr
 			dc.Close()
 			return nil, fmt.Errorf("TLS server name %q != derp hostname %q", cs.ServerName, n.HostName)
 		}
+	}
 
 	errc := make(chan error, 1)
 	go func() {
@@ -645,3 +854,27 @@ func httpOrFileTransport() http.RoundTripper {
 	tr.RegisterProtocol("file", http.NewFileTransport(http.Dir("/")))
 	return tr
 }
+
+// wallAndExt extracts the wall and ext fields from a time.Time,
+// allowing us to marshal a monotonic clock reading.
+func wallAndExt(t time.Time) (uint64, int64) {
+	v := reflect.ValueOf(&t).Elem()
+	return exposeField(v.Field(0)).Uint(), exposeField(v.Field(1)).Int()
+}
+
+// fromWallAndExt constructs a time.Time from wall and ext fields,
+// allowing us to unmarshal a monotonic clock reading.
+func fromWallAndExt(wall uint64, ext int64) time.Time {
+	var t time.Time
+	v := reflect.ValueOf(&t).Elem()
+	exposeField(v.Field(0)).SetUint(wall)
+	exposeField(v.Field(1)).SetInt(ext)
+	return t
+}
+
+func exposeField(v reflect.Value) reflect.Value {
+	if v.CanSet() {
+		return v
+	}
+	return reflect.NewAt(v.Type(), unsafe.Pointer(v.UnsafeAddr())).Elem()
+}

prober/derp_test.go

@@ -192,7 +192,7 @@ func TestRunDerpProbeNodePair(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
 	defer cancel()
 
-	err = runDerpProbeNodePair(ctx, &tailcfg.DERPNode{Name: "c1"}, &tailcfg.DERPNode{Name: "c2"}, c1, c2, 100_000_000)
+	err = runDerpProbeNodePairOnce(ctx, &tailcfg.DERPNode{Name: "c1"}, &tailcfg.DERPNode{Name: "c2"}, c1, c2, 100_000_000)
 	if err != nil {
 		t.Error(err)
 	}
@@ -237,3 +237,12 @@ func Test_packetsForSize(t *testing.T) {
 		})
 	}
 }
+
+func TestWallAndExt(t *testing.T) {
+	now := time.Now()
+	now2 := fromWallAndExt(wallAndExt(now))
+	diff := now2.Sub(now)
+	if now2.Sub(now) != 0 {
+		t.Fatalf("fromWallAndExt(wallAndExt(now)) = %v; want 0", diff)
+	}
+}

prober/prober.go

@@ -256,6 +256,11 @@ type Probe struct {
 	latencyHist *ring.Ring
 }
 
+// IsContinuous indicates that this is a continuous probe.
+func (p *Probe) IsContinuous() bool {
+	return p.interval < 0
+}
+
 // Close shuts down the Probe and unregisters it from its Prober.
 // It is safe to Run a new probe of the same name after Close returns.
 func (p *Probe) Close() error {
@@ -288,6 +293,22 @@ func (p *Probe) loop() {
 		return
 	}
 
+	if p.IsContinuous() {
+		// Probe function is going to run continuously.
+		for {
+			p.run()
+			// Wait and then retry if probe fails. We use the inverse of the
+			// configured negative interval as our sleep period.
+			// TODO(percy):implement exponential backoff.
+			select {
+			case <-time.After(-1 * p.interval):
+				p.run()
+			case <-p.ctx.Done():
+				return
+			}
+		}
+	}
+
 	p.tick = p.prober.newTicker(p.interval)
 	defer p.tick.Stop()
 	for {
@@ -323,9 +344,17 @@ func (p *Probe) run() (pi ProbeInfo, err error) {
 			p.recordEnd(err)
 		}
 	}()
+	ctx := p.ctx
+	if p.IsContinuous() {
+		p.mu.Lock()
+		p.lastErr = nil
+		p.mu.Unlock()
+	} else {
 		timeout := time.Duration(float64(p.interval) * 0.8)
-	ctx, cancel := context.WithTimeout(p.ctx, timeout)
+		var cancel func()
+		ctx, cancel = context.WithTimeout(ctx, timeout)
 		defer cancel()
+	}
 
 	err = p.probeClass.Probe(ctx)
 	p.recordEnd(err)
@@ -365,6 +394,16 @@ func (p *Probe) recordEnd(err error) {
 	p.successHist = p.successHist.Next()
 }
 
+// ProbeStatus indicates the status of a probe.
+type ProbeStatus string
+
+const (
+	ProbeStatusUnknown   = "unknown"
+	ProbeStatusRunning   = "running"
+	ProbeStatusFailed    = "failed"
+	ProbeStatusSucceeded = "succeeded"
+)
+
 // ProbeInfo is a snapshot of the configuration and state of a Probe.
 type ProbeInfo struct {
 	Name            string
@@ -374,7 +413,7 @@ type ProbeInfo struct {
 	Start           time.Time
 	End             time.Time
 	Latency         time.Duration
-	Result          bool
+	Status          ProbeStatus
 	Error           string
 	RecentResults   []bool
 	RecentLatencies []time.Duration
@@ -402,6 +441,10 @@ func (pb ProbeInfo) RecentMedianLatency() time.Duration {
 	return pb.RecentLatencies[len(pb.RecentLatencies)/2]
 }
 
+func (pb ProbeInfo) Continuous() bool {
+	return pb.Interval < 0
+}
+
 // ProbeInfo returns the state of all probes.
 func (p *Prober) ProbeInfo() map[string]ProbeInfo {
 	out := map[string]ProbeInfo{}
@@ -429,9 +472,14 @@ func (probe *Probe) probeInfoLocked() ProbeInfo {
 		Labels:   probe.metricLabels,
 		Start:    probe.start,
 		End:      probe.end,
-		Result:   probe.succeeded,
 	}
-	if probe.lastErr != nil {
+	inf.Status = ProbeStatusUnknown
+	if probe.end.Before(probe.start) {
+		inf.Status = ProbeStatusRunning
+	} else if probe.succeeded {
+		inf.Status = ProbeStatusSucceeded
+	} else if probe.lastErr != nil {
+		inf.Status = ProbeStatusFailed
 		inf.Error = probe.lastErr.Error()
 	}
 	if probe.latency > 0 {
@@ -467,7 +515,7 @@ func (p *Prober) RunHandler(w http.ResponseWriter, r *http.Request) error {
 	p.mu.Lock()
 	probe, ok := p.probes[name]
 	p.mu.Unlock()
-	if !ok {
+	if !ok || probe.IsContinuous() {
 		return tsweb.Error(http.StatusNotFound, fmt.Sprintf("unknown probe %q", name), nil)
 	}
 
@@ -531,7 +579,8 @@ func (p *Probe) Collect(ch chan<- prometheus.Metric) {
 	if !p.start.IsZero() {
 		ch <- prometheus.MustNewConstMetric(p.mStartTime, prometheus.GaugeValue, float64(p.start.Unix()))
 	}
-	if p.end.IsZero() {
+	// For periodic probes that haven't ended, don't collect probe metrics yet.
+	if p.end.IsZero() && !p.IsContinuous() {
 		return
 	}
 	ch <- prometheus.MustNewConstMetric(p.mEndTime, prometheus.GaugeValue, float64(p.end.Unix()))

prober/prober_test.go

@@ -316,7 +316,7 @@ func TestProberProbeInfo(t *testing.T) {
 			Interval:        probeInterval,
 			Labels:          map[string]string{"class": "", "name": "probe1"},
 			Latency:         500 * time.Millisecond,
-			Result:          true,
+			Status:          ProbeStatusSucceeded,
 			RecentResults:   []bool{true},
 			RecentLatencies: []time.Duration{500 * time.Millisecond},
 		},
@@ -324,6 +324,7 @@ func TestProberProbeInfo(t *testing.T) {
 			Name:            "probe2",
 			Interval:        probeInterval,
 			Labels:          map[string]string{"class": "", "name": "probe2"},
+			Status:          ProbeStatusFailed,
 			Error:           "error2",
 			RecentResults:   []bool{false},
 			RecentLatencies: nil, // no latency for failed probes
@@ -349,7 +350,7 @@ func TestProbeInfoRecent(t *testing.T) {
 	}{
 		{
 			name:                    "no_runs",
-			wantProbeInfo:           ProbeInfo{},
+			wantProbeInfo:           ProbeInfo{Status: ProbeStatusUnknown},
 			wantRecentSuccessRatio:  0,
 			wantRecentMedianLatency: 0,
 		},
@@ -358,7 +359,7 @@ func TestProbeInfoRecent(t *testing.T) {
 			results: []probeResult{{latency: 100 * time.Millisecond, err: nil}},
 			wantProbeInfo: ProbeInfo{
 				Latency:         100 * time.Millisecond,
-				Result:          true,
+				Status:          ProbeStatusSucceeded,
 				RecentResults:   []bool{true},
 				RecentLatencies: []time.Duration{100 * time.Millisecond},
 			},
@@ -369,7 +370,7 @@ func TestProbeInfoRecent(t *testing.T) {
 			name:    "single_failure",
 			results: []probeResult{{latency: 100 * time.Millisecond, err: errors.New("error123")}},
 			wantProbeInfo: ProbeInfo{
-				Result:          false,
+				Status:          ProbeStatusFailed,
 				RecentResults:   []bool{false},
 				RecentLatencies: nil,
 				Error:           "error123",
@@ -390,7 +391,7 @@ func TestProbeInfoRecent(t *testing.T) {
 				{latency: 80 * time.Millisecond, err: nil},
 			},
 			wantProbeInfo: ProbeInfo{
-				Result:        true,
+				Status:        ProbeStatusSucceeded,
 				Latency:       80 * time.Millisecond,
 				RecentResults: []bool{false, true, true, false, true, true, false, true},
 				RecentLatencies: []time.Duration{
@@ -420,7 +421,7 @@ func TestProbeInfoRecent(t *testing.T) {
 				{latency: 110 * time.Millisecond, err: nil},
 			},
 			wantProbeInfo: ProbeInfo{
-				Result:        true,
+				Status:        ProbeStatusSucceeded,
 				Latency:       110 * time.Millisecond,
 				RecentResults: []bool{true, true, true, true, true, true, true, true, true, true},
 				RecentLatencies: []time.Duration{
@@ -483,7 +484,7 @@ func TestProberRunHandler(t *testing.T) {
 				ProbeInfo: ProbeInfo{
 					Name:          "success",
 					Interval:      probeInterval,
-					Result:        true,
+					Status:        ProbeStatusSucceeded,
 					RecentResults: []bool{true, true},
 				},
 				PreviousSuccessRatio: 1,
@@ -498,7 +499,7 @@ func TestProberRunHandler(t *testing.T) {
 				ProbeInfo: ProbeInfo{
 					Name:          "failure",
 					Interval:      probeInterval,
-					Result:        false,
+					Status:        ProbeStatusFailed,
 					Error:         "error123",
 					RecentResults: []bool{false, false},
 				},

prober/status.go

@@ -62,7 +62,8 @@ func (p *Prober) StatusHandler(opts ...statusHandlerOpt) tsweb.ReturnHandlerFunc
 	return func(w http.ResponseWriter, r *http.Request) error {
 		type probeStatus struct {
 			ProbeInfo
-			TimeSinceLast time.Duration
+			TimeSinceLastStart time.Duration
+			TimeSinceLastEnd   time.Duration
 			Links              map[string]template.URL
 		}
 		vars := struct {
@@ -81,12 +82,15 @@ func (p *Prober) StatusHandler(opts ...statusHandlerOpt) tsweb.ReturnHandlerFunc
 
 		for name, info := range p.ProbeInfo() {
 			vars.TotalProbes++
-			if !info.Result {
+			if info.Error != "" {
 				vars.UnhealthyProbes++
 			}
 			s := probeStatus{ProbeInfo: info}
+			if !info.Start.IsZero() {
+				s.TimeSinceLastStart = time.Since(info.Start).Truncate(time.Second)
+			}
 			if !info.End.IsZero() {
-				s.TimeSinceLast = time.Since(info.End).Truncate(time.Second)
+				s.TimeSinceLastEnd = time.Since(info.End).Truncate(time.Second)
 			}
 			for textTpl, urlTpl := range params.probeLinks {
 				text, err := renderTemplate(textTpl, info)

prober/status.html

@@ -73,8 +73,9 @@
             <th>Name</th>
             <th>Probe Class & Labels</th>
             <th>Interval</th>
-            <th>Last Attempt</th>
+            <th>Last Finished</th>
-            <th>Success</th>
+            <th>Last Started</th>
+            <th>Status</th>
             <th>Latency</th>
             <th>Last Error</th>
         </tr></thead>
@@ -85,10 +86,12 @@
                 {{$name}}
                 {{range $text, $url := $probeInfo.Links}}
                 <br/>
+                {{if not $probeInfo.Continuous}}
                     <button onclick="location.href='{{$url}}';" type="button">
                         {{$text}}
                     </button>
                 {{end}}
+                {{end}}
             </td>
             <td>{{$probeInfo.Class}}<br/>
                 <div class="small">
@@ -97,28 +100,48 @@
                 {{end}}
                 </div>
             </td>
-            <td>{{$probeInfo.Interval}}</td>
+            <td>
-            <td data-sort="{{$probeInfo.TimeSinceLast.Milliseconds}}">
+                {{if $probeInfo.Continuous}}
-                {{if $probeInfo.TimeSinceLast}}
+                    Continuous
-                    {{$probeInfo.TimeSinceLast.String}} ago<br/>
+                {{else}}
+                    {{$probeInfo.Interval}}
+                {{end}}
+            </td>
+            <td data-sort="{{$probeInfo.TimeSinceLastEnd.Milliseconds}}">
+                {{if $probeInfo.TimeSinceLastEnd}}
+                    {{$probeInfo.TimeSinceLastEnd.String}} ago<br/>
                     <span class="small">{{$probeInfo.End.Format "2006-01-02T15:04:05Z07:00"}}</span>
                 {{else}}
                     Never
                 {{end}}
             </td>
-            <td>
+            <td data-sort="{{$probeInfo.TimeSinceLastStart.Milliseconds}}">
-                {{if $probeInfo.Result}}
+                {{if $probeInfo.TimeSinceLastStart}}
-                    {{$probeInfo.Result}}
+                    {{$probeInfo.TimeSinceLastStart.String}} ago<br/>
+                    <span class="small">{{$probeInfo.Start.Format "2006-01-02T15:04:05Z07:00"}}</span>
                 {{else}}
-                    <span class="error">{{$probeInfo.Result}}</span>
+                    Never
+                {{end}}
+            </td>
+            <td>
+                {{if $probeInfo.Error}}
+                    <span class="error">{{$probeInfo.Status}}</span>
+                {{else}}
+                    {{$probeInfo.Status}}
                 {{end}}<br/>
+                {{if not $probeInfo.Continuous}}
                     <div class="small">Recent: {{$probeInfo.RecentResults}}</div>
                     <div class="small">Mean: {{$probeInfo.RecentSuccessRatio}}</div>
+                {{end}}
             </td>
             <td data-sort="{{$probeInfo.Latency.Milliseconds}}">
+                {{if $probeInfo.Continuous}}
+                    n/a
+                {{else}}
                     {{$probeInfo.Latency.String}}
                     <div class="small">Recent: {{$probeInfo.RecentLatencies}}</div>
                     <div class="small">Median: {{$probeInfo.RecentMedianLatency}}</div>
+                {{end}}
             </td>
             <td class="small">{{$probeInfo.Error}}</td>
         </tr>