cmd/derper: count bootstrap dns unique lookups.

Updates https://github.com/tailscale/corp/issues/13979

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

cmd/derper/bootstrap_dns.go

@@ -25,6 +25,7 @@ var (
 	dnsCache            syncs.AtomicValue[dnsEntryMap]
 	dnsCacheBytes       syncs.AtomicValue[[]byte] // of JSON
 	unpublishedDNSCache syncs.AtomicValue[dnsEntryMap]
+	bootstrapLookupMap  syncs.Map[string, bool]
 )
 
 var (
@@ -35,6 +36,12 @@ var (
 	unpublishedDNSMisses = expvar.NewInt("counter_bootstrap_dns_unpublished_misses")
 )
 
+func init() {
+	expvar.Publish("counter_bootstrap_dns_queried_domains", expvar.Func(func() any {
+		return bootstrapLookupMap.Len()
+	}))
+}
+
 func refreshBootstrapDNSLoop() {
 	if *bootstrapDNS == "" && *unpublishedDNS == "" {
 		return
@@ -107,6 +114,7 @@ func handleBootstrapDNS(w http.ResponseWriter, r *http.Request) {
 
 	// Try answering a query from our hidden map first
 	if q := r.URL.Query().Get("q"); q != "" {
+		bootstrapLookupMap.Store(q, true)
 		if ips, ok := unpublishedDNSCache.Load()[q]; ok && len(ips) > 0 {
 			unpublishedDNSHits.Add(1)
 

cmd/derper/bootstrap_dns_test.go

@@ -98,6 +98,7 @@ func resetMetrics() {
 	publishedDNSMisses.Set(0)
 	unpublishedDNSHits.Set(0)
 	unpublishedDNSMisses.Set(0)
+	bootstrapLookupMap.Clear()
 }
 
 // Verify that we don't count an empty list in the unpublishedDNSCache as a
@@ -148,4 +149,17 @@ func TestUnpublishedDNSEmptyList(t *testing.T) {
 			t.Errorf("got misses=%d; want 0", v)
 		}
 	})
+
+}
+
+func TestLookupMetric(t *testing.T) {
+	d := []string{"a.io", "b.io", "c.io", "d.io", "e.io", "e.io", "e.io", "a.io"}
+	resetMetrics()
+	for _, q := range d {
+		_ = getBootstrapDNS(t, q)
+	}
+	// {"a.io": true, "b.io": true, "c.io": true, "d.io": true, "e.io": true}
+	if bootstrapLookupMap.Len() != 5 {
+		t.Errorf("bootstrapLookupMap.Len() want=5, got %v", bootstrapLookupMap.Len())
+	}
 }

cmd/derper/depaware.txt

@@ -169,7 +169,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
         golang.org/x/crypto/nacl/secretbox                           from golang.org/x/crypto/nacl/box
         golang.org/x/crypto/salsa20/salsa                            from golang.org/x/crypto/nacl/box+
         golang.org/x/exp/constraints                                 from golang.org/x/exp/slices
-        golang.org/x/exp/maps                                        from tailscale.com/types/views
+        golang.org/x/exp/maps                                        from tailscale.com/types/views+
         golang.org/x/exp/slices                                      from tailscale.com/net/tsaddr+
    L    golang.org/x/net/bpf                                         from github.com/mdlayher/netlink+
         golang.org/x/net/dns/dnsmessage                              from net+