Update

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

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:generate go run tailscale.com/cmd/viewer --type=Config --clonefunc
+
 // Package dns contains code to configure and manage DNS settings.
 package dns
 
@@ -8,8 +10,12 @@ import (
 	"bufio"
 	"fmt"
 	"net/netip"
+	"reflect"
+	"slices"
 	"sort"
 
+	"tailscale.com/control/controlknobs"
+	"tailscale.com/envknob"
 	"tailscale.com/net/dns/publicdns"
 	"tailscale.com/net/dns/resolver"
 	"tailscale.com/net/tsaddr"
@@ -47,11 +53,28 @@ type Config struct {
 	OnlyIPv6 bool
 }
 
-func (c *Config) serviceIP() netip.Addr {
+var magicDNSDualStack = envknob.RegisterBool("TS_DEBUG_MAGIC_DNS_DUAL_STACK")
+
+// serviceIPs returns the list of service IPs where MagicDNS is reachable.
+//
+// The provided knobs may be nil.
+func (c *Config) serviceIPs(knobs *controlknobs.Knobs) []netip.Addr {
 	if c.OnlyIPv6 {
-		return tsaddr.TailscaleServiceIPv6()
+		return []netip.Addr{tsaddr.TailscaleServiceIPv6()}
 	}
-	return tsaddr.TailscaleServiceIP()
+
+	// TODO(bradfitz,mikeodr,raggi): include IPv6 here too; tailscale/tailscale#15404
+	// And add a controlknobs knob to disable dual stack.
+	//
+	// For now, opt-in for testing.
+	if magicDNSDualStack() {
+		return []netip.Addr{
+			tsaddr.TailscaleServiceIP(),
+			tsaddr.TailscaleServiceIPv6(),
+		}
+	}
+
+	return []netip.Addr{tsaddr.TailscaleServiceIP()}
 }
 
 // WriteToBufioWriter write a debug version of c for logs to w, omitting
@@ -162,21 +185,16 @@ func sameResolverNames(a, b []*dnstype.Resolver) bool {
 		if a[i].Addr != b[i].Addr {
 			return false
 		}
-		if !sameIPs(a[i].BootstrapResolution, b[i].BootstrapResolution) {
+		if !slices.Equal(a[i].BootstrapResolution, b[i].BootstrapResolution) {
 			return false
 		}
 	}
 	return true
 }
 
-func sameIPs(a, b []netip.Addr) bool {
-	if len(a) != len(b) {
-		return false
+func (c *Config) Equal(o *Config) bool {
+	if c == nil || o == nil {
+		return c == o
 	}
-	for i := range a {
-		if a[i] != b[i] {
-			return false
-		}
-	}
-	return true
+	return reflect.DeepEqual(c, o)
 }