cmd/derpprobe: support 'local' derpmap to get derp map via LocalAPI

To make it easier for people to monitor their custom DERP fleet.

Updates tailscale/corp#20654

Change-Id: Id8af22936a6d893cc7b6186d298ab794a2672524
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/derpprobe/derpprobe.go

@@ -20,7 +20,7 @@ import (
 )
 
 var (
-	derpMapURL   = flag.String("derp-map", "https://login.tailscale.com/derpmap/default", "URL to DERP map (https:// or file://)")
+	derpMapURL   = flag.String("derp-map", "https://login.tailscale.com/derpmap/default", "URL to DERP map (https:// or file://) or 'local' to use the local tailscaled's DERP map")
 	versionFlag  = flag.Bool("version", false, "print version and exit")
 	listen       = flag.String("listen", ":8030", "HTTP listen address")
 	probeOnce    = flag.Bool("once", false, "probe once and print results, then exit; ignores the listen flag")

prober/derp.go

@@ -21,6 +21,7 @@ import (
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
+	"tailscale.com/client/tailscale"
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
 	"tailscale.com/net/netmon"
@@ -35,7 +36,7 @@ import (
 // based on the current DERPMap.
 type derpProber struct {
 	p            *Prober
-	derpMapURL   string
+	derpMapURL   string // or "local"
 	udpInterval  time.Duration
 	meshInterval time.Duration
 	tlsInterval  time.Duration
@@ -97,6 +98,9 @@ func WithTLSProbing(interval time.Duration) DERPOpt {
 }
 
 // DERP creates a new derpProber.
+//
+// If derpMapURL is "local", the DERPMap is fetched via
+// the local machine's tailscaled.
 func DERP(p *Prober, derpMapURL string, opts ...DERPOpt) (*derpProber, error) {
 	d := &derpProber{
 		p:          p,
@@ -268,8 +272,18 @@ func (d *derpProber) getNodePair(n1, n2 string) (ret1, ret2 *tailcfg.DERPNode, _
 	return ret1, ret2, nil
 }
 
+var tsLocalClient tailscale.LocalClient
+
 // updateMap refreshes the locally-cached DERP map.
 func (d *derpProber) updateMap(ctx context.Context) error {
+	var dm *tailcfg.DERPMap
+	if d.derpMapURL == "local" {
+		var err error
+		dm, err = tsLocalClient.CurrentDERPMap(ctx)
+		if err != nil {
+			return err
+		}
+	} else {
 		req, err := http.NewRequestWithContext(ctx, "GET", d.derpMapURL, nil)
 		if err != nil {
 			return nil
@@ -290,10 +304,11 @@ func (d *derpProber) updateMap(ctx context.Context) error {
 		if res.StatusCode != 200 {
 			return fmt.Errorf("fetching %s: %s", d.derpMapURL, res.Status)
 		}
-	dm := new(tailcfg.DERPMap)
+		dm = new(tailcfg.DERPMap)
 		if err := json.NewDecoder(res.Body).Decode(dm); err != nil {
 			return fmt.Errorf("decoding %s JSON: %v", d.derpMapURL, err)
 		}
+	}
 
 	d.Lock()
 	defer d.Unlock()