Update

vendor/tailscale.com/net/dns/flush_windows.go

@@ -1,34 +1,80 @@
-// Copyright (c) Tailscale Inc & AUTHORS
+// Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
 package dns
 
 import (
-	"fmt"
+	"log"
 	"os/exec"
 	"syscall"
+	"time"
 
 	"golang.org/x/sys/windows"
+	"tailscale.com/util/coalescedop"
 )
 
-func flushCaches() error {
+var (
+	flushDNSOp    = coalescedop.New(doFlushDNS)
+	registerDNSOp = coalescedop.New(doRegisterDNS)
+)
+
+func doFlushDNS() {
+	t0 := time.Now()
 	cmd := exec.Command("ipconfig", "/flushdns")
 	cmd.SysProcAttr = &syscall.SysProcAttr{
 		CreationFlags: windows.DETACHED_PROCESS,
 	}
-	out, err := cmd.CombinedOutput()
+	err := cmd.Run()
+	d := time.Since(t0).Round(time.Millisecond)
 	if err != nil {
-		return fmt.Errorf("%v (output: %s)", err, out)
+		log.Printf("error running ipconfig /flushdns after %v: %v", d, err)
+	} else {
+		log.Printf("ran ipconfig /flushdns in %v", d)
 	}
+}
+
+func doRegisterDNS() {
+	t0 := time.Now()
+	cmd := exec.Command("ipconfig", "/registerdns")
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		CreationFlags: windows.DETACHED_PROCESS,
+	}
+	err := cmd.Run()
+	d := time.Since(t0).Round(time.Millisecond)
+	if err != nil {
+		log.Printf("error running ipconfig /registerdns after %v: %v", d, err)
+	} else {
+		log.Printf("ran ipconfig /registerdns in %v", d)
+	}
+}
+
+func flushCaches() error {
+	flushDNSOp.Do()
 	return nil
 }
 
 // Flush clears the local resolver cache.
 //
-// Only Windows has a public dns.Flush, needed in router_windows.go. Other
-// platforms like Linux need a different flush implementation depending on
-// the DNS manager. There is a FlushCaches method on the manager which
-// can be used on all platforms.
-func Flush() error {
-	return flushCaches()
+// Only Windows has a public dns.Flush, needed in tailscaled_windows.go.
+// Other platforms like Linux need a different flush implementation
+// depending on the DNS manager. There is a FlushCaches method on the manager
+// which can be used on all platforms.
+//
+// Flush is non-blocking; it coalesces concurrent and rapid successive
+// calls so that at most one ipconfig /flushdns is in flight at a time,
+// with at most one additional execution queued.
+func Flush() {
+	flushDNSOp.Do()
+}
+
+// registerDNS forces DNS re-registration in Active Directory. This
+// invokes an undocumented hidden function that forces Windows to
+// notice that adapter settings have changed, which makes the DNS
+// settings actually take effect.
+//
+// registerDNS is non-blocking; it coalesces concurrent and rapid
+// successive calls so that at most one ipconfig /registerdns is in
+// flight at a time, with at most one additional execution queued.
+func registerDNS() {
+	registerDNSOp.Do()
 }