core/dnsserver: support explicit registration for embedded hosts (#8525)

* core/dnsserver: test host-plugin embedding with forward

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

* core/dnsserver: add a directive setter for embedded hosts

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

* core/dnsserver: support explicit server registration for embedded hosts

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

---------

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

.github/workflows/go.test.yml

@@ -32,6 +32,12 @@ jobs:
           ( cd core; go test -race  ./... )
           ( cd coremain; go test -race ./... )
 
+      - name: Build manual-registration mode
+        run: go build -tags=coredns_manual_registration ./...
+
+      - name: Test manual-registration mode
+        run: go test -race -tags=coredns_manual_registration ./core/dnsserver ./coremain
+
   test-windows:
     name: Test Windows
     runs-on: windows-latest
@@ -57,6 +63,12 @@ jobs:
           ( cd core; go test -race  ./... )
           ( cd coremain; go test -race ./... )
 
+      - name: Build manual-registration mode
+        run: go build -tags=coredns_manual_registration ./...
+
+      - name: Test manual-registration mode
+        run: go test -race -tags=coredns_manual_registration ./core/dnsserver ./coremain
+
   test-plugins:
     name: Test Plugins
     strategy:

core/dnsserver/directives.go

@@ -0,0 +1,33 @@
+package dnsserver
+
+import (
+	"fmt"
+	"slices"
+)
+
+// SetDirectives replaces [Directives] with a copy of directives, in execution
+// order. Names must be nonempty and unique. On error, Directives is unchanged.
+// An empty or nil list disables all directives.
+//
+// This selects directives but does not import or register plugins. A host may
+// register its plugins after this call, before starting Caddy. Unregistered
+// directives used in a Corefile are rejected by Caddy during startup.
+//
+// The list is process-wide, not per instance. Call SetDirectives before starting
+// any servers, not while servers are running. The caller must serialize it with
+// all other access to Directives, including startup and reload. It is not a
+// runtime reconfiguration API.
+func SetDirectives(directives []string) error {
+	seen := make(map[string]struct{}, len(directives))
+	for i, name := range directives {
+		if name == "" {
+			return fmt.Errorf("empty directive name at index %d", i)
+		}
+		if _, ok := seen[name]; ok {
+			return fmt.Errorf("duplicate directive %q", name)
+		}
+		seen[name] = struct{}{}
+	}
+	Directives = slices.Clone(directives)
+	return nil
+}

core/dnsserver/directives_test.go

@@ -0,0 +1,73 @@
+package dnsserver
+
+import (
+	"slices"
+	"strings"
+	"testing"
+
+	"github.com/coredns/caddy"
+)
+
+func TestSetDirectives(t *testing.T) {
+	original := Directives
+	t.Cleanup(func() { Directives = original })
+
+	for _, tc := range []struct {
+		name       string
+		directives []string
+		wantError  string
+	}{
+		{name: "ordered", directives: []string{"bind", "test_host", "forward"}},
+		{name: "replacement", directives: []string{"whoami", "bind"}},
+		{name: "nil", directives: nil},
+		{name: "empty", directives: []string{}},
+		{name: "empty name", directives: []string{"bind", ""}, wantError: "empty directive name"},
+		{name: "duplicate", directives: []string{"bind", "forward", "bind"}, wantError: `duplicate directive "bind"`},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			Directives = []string{"test_previous"}
+			previous := Directives
+			input := slices.Clone(tc.directives)
+			err := SetDirectives(input)
+			if tc.wantError != "" {
+				if err == nil || !strings.Contains(err.Error(), tc.wantError) {
+					t.Fatalf("SetDirectives(%v) = %v, want %q", input, err, tc.wantError)
+				}
+				if !slices.Equal(Directives, previous) || &Directives[0] != &previous[0] {
+					t.Fatalf("invalid list changed Directives: got %v, want %v", Directives, previous)
+				}
+			} else {
+				if err != nil {
+					t.Fatal(err)
+				}
+				if got := caddy.ValidDirectives("dns"); !slices.Equal(got, tc.directives) {
+					t.Fatalf("registered server directives = %v, want %v", got, tc.directives)
+				}
+				if len(input) > 0 {
+					input[0] = "test_mutated"
+					if !slices.Equal(Directives, tc.directives) {
+						t.Fatalf("caller mutation changed Directives: %v", Directives)
+					}
+					input[0] = tc.directives[0]
+				}
+			}
+			if !slices.Equal(input, tc.directives) {
+				t.Fatalf("SetDirectives changed the input: got %v, want %v", input, tc.directives)
+			}
+		})
+	}
+}
+
+func TestSetDirectivesCopiesCurrentList(t *testing.T) {
+	original := Directives
+	t.Cleanup(func() { Directives = original })
+	Directives = []string{"bind", "test_host", "forward"}
+	previous := Directives
+	if err := SetDirectives(Directives[1:]); err != nil {
+		t.Fatal(err)
+	}
+	previous[1] = "test_mutated"
+	if want := []string{"test_host", "forward"}; !slices.Equal(Directives, want) {
+		t.Fatalf("old list mutation changed Directives: got %v, want %v", Directives, want)
+	}
+}

core/dnsserver/embedding_test.go

@@ -0,0 +1,340 @@
+package dnsserver_test
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"net"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/coredns/caddy"
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+	_ "github.com/coredns/coredns/plugin/bind"
+	_ "github.com/coredns/coredns/plugin/forward"
+	_ "github.com/coredns/coredns/plugin/whoami"
+
+	"github.com/miekg/dns"
+)
+
+// Register once, but outside init, as an embedding host would do before Start.
+var registerEmbeddingObserver sync.Once
+
+func TestEmbeddingForward(t *testing.T) {
+	configureEmbedding(t)
+	upstream := startEmbeddingUpstream(t)
+	input := caddy.CaddyfileInput{
+		Filepath: "Corefile",
+		Contents: fmt.Appendf(nil, `.:0 {
+	bind 127.0.0.1
+	forward . %s
+	test_observe
+}
+`, upstream),
+		ServerTypeName: "dns",
+	}
+
+	// Two live instances exercise host-owned plugin state and independent shutdown.
+	first, firstObserver, stopFirst := startEmbeddedForwarder(t, input)
+	second, secondObserver, _ := startEmbeddedForwarder(t, input)
+	for _, instance := range []*caddy.Instance{first, second} {
+		instance.StorageMu.RLock()
+		observer := instance.Storage[embeddingObserverKey{}].(*embeddingObserver)
+		instance.StorageMu.RUnlock()
+		for _, network := range []string{"udp", "tcp"} {
+			for _, qtype := range []uint16{dns.TypeA, dns.TypeAAAA} {
+				checkEmbeddedQuery(t, instance, observer, network, qtype)
+			}
+		}
+	}
+	stopFirst()
+	if firstObserver.stops != 1 || secondObserver.stops != 0 {
+		t.Fatalf("shutdown callbacks ran for the wrong instances: first=%d second=%d", firstObserver.stops, secondObserver.stops)
+	}
+	listener, err := net.Listen("tcp", first.Servers()[0].Addr().String())
+	if err != nil {
+		t.Fatalf("embedded TCP listener was not released: %v", err)
+	}
+	listener.Close()
+	packet, err := net.ListenPacket("udp", first.Servers()[0].LocalAddr().String())
+	if err != nil {
+		t.Fatalf("embedded UDP listener was not released: %v", err)
+	}
+	packet.Close()
+	checkEmbeddedQuery(t, second, secondObserver, "udp", dns.TypeA)
+	checkEmbeddedQuery(t, second, secondObserver, "tcp", dns.TypeAAAA)
+}
+
+func TestEmbeddingInvalidPlugin(t *testing.T) {
+	configureEmbedding(t)
+	for _, tc := range []struct {
+		name       string
+		directives []string
+		config     string
+		wantError  string
+	}{
+		{
+			name:       "imported but not enabled",
+			directives: []string{"bind", "test_observe", "forward"},
+			config:     ".:0 {\nbind 127.0.0.1\nwhoami\n}\n",
+			wantError:  "Unknown directive 'whoami'",
+		},
+		{
+			name:       "enabled but not registered",
+			directives: []string{"bind", "test_missing"},
+			config:     ".:0 {\nbind 127.0.0.1\ntest_missing\n}\n",
+			wantError:  "no action found for directive 'test_missing'",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if err := dnsserver.SetDirectives(tc.directives); err != nil {
+				t.Fatal(err)
+			}
+			instance, err := caddy.Start(caddy.CaddyfileInput{
+				Filepath:       "Corefile",
+				Contents:       []byte(tc.config),
+				ServerTypeName: "dns",
+			})
+			if err == nil {
+				stopEmbeddedInstance(t, instance)
+				t.Fatal("expected a configuration error")
+			}
+			if !strings.Contains(err.Error(), tc.wantError) {
+				t.Fatalf("expected %q, got %v", tc.wantError, err)
+			}
+			if len(instance.Servers()) != 0 {
+				t.Fatal("invalid configuration opened listeners")
+			}
+		})
+	}
+}
+
+func TestEmbeddingDoesNotRegisterCLIFlags(t *testing.T) {
+	// Check the original process FlagSet, not an empty replacement which would
+	// hide flags registered by imports. The host must not import coremain.
+	for _, name := range []string{"conf", "dns.port", "p", "pidfile", "plugins", "version", "quiet"} {
+		if flag.Lookup(name) != nil {
+			t.Errorf("embedding imports registered the CLI flag %q", name)
+		}
+	}
+}
+
+func configureEmbedding(t *testing.T) {
+	t.Helper()
+	oldDirectives, oldCaddyQuiet, oldDNSQuiet := dnsserver.Directives, caddy.Quiet, dnsserver.Quiet
+	t.Cleanup(func() {
+		dnsserver.Directives, caddy.Quiet, dnsserver.Quiet = oldDirectives, oldCaddyQuiet, oldDNSQuiet
+	})
+	directives := []string{"bind", "test_observe", "forward"}
+	if err := dnsserver.SetDirectives(directives); err != nil {
+		t.Fatal(err)
+	}
+	// A host can reuse its input and register a selected plugin after setting
+	// the execution order, as long as both happen before startup.
+	directives[1] = "test_missing"
+	registerEmbeddingObserver.Do(func() {
+		plugin.Register("test_observe", setupEmbeddingObserver)
+	})
+	caddy.Quiet, dnsserver.Quiet = true, true
+}
+
+type embeddingObserverKey struct{}
+
+type embeddingObservation struct {
+	question dns.Question
+	response *dns.Msg
+}
+
+type embeddingObserver struct {
+	next         plugin.Handler
+	config       *dnsserver.Config
+	observations chan embeddingObservation
+	starts       int
+	stops        int
+}
+
+func setupEmbeddingObserver(c *caddy.Controller) error {
+	for c.Next() {
+		if len(c.RemainingArgs()) != 0 {
+			return c.ArgErr()
+		}
+	}
+	observer := &embeddingObserver{
+		config:       dnsserver.GetConfig(c),
+		observations: make(chan embeddingObservation, 1),
+	}
+	observer.config.AddPlugin(func(next plugin.Handler) plugin.Handler {
+		observer.next = next
+		return observer
+	})
+	c.OnStartup(func() error { observer.starts++; return nil })
+	c.OnShutdown(func() error { observer.stops++; return nil })
+	c.Set(embeddingObserverKey{}, observer)
+	return nil
+}
+
+func (o *embeddingObserver) Name() string { return "test_observe" }
+
+func (o *embeddingObserver) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	writer := &embeddingResponseWriter{ResponseWriter: w}
+	rcode, err := plugin.NextOrFailure(o.Name(), o.next, ctx, writer, r)
+	o.observations <- embeddingObservation{question: r.Question[0], response: writer.response}
+	return rcode, err
+}
+
+type embeddingResponseWriter struct {
+	dns.ResponseWriter
+	response *dns.Msg
+}
+
+func (w *embeddingResponseWriter) WriteMsg(m *dns.Msg) error {
+	w.response = m.Copy()
+	return w.ResponseWriter.WriteMsg(m)
+}
+
+func startEmbeddedForwarder(t *testing.T, input caddy.CaddyfileInput) (*caddy.Instance, *embeddingObserver, func()) {
+	t.Helper()
+	instance, err := caddy.Start(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	stop := sync.OnceFunc(func() { stopEmbeddedInstance(t, instance) })
+	t.Cleanup(stop)
+	instance.StorageMu.RLock()
+	observer, ok := instance.Storage[embeddingObserverKey{}].(*embeddingObserver)
+	instance.StorageMu.RUnlock()
+	if !ok {
+		t.Fatal("custom plugin was not set up")
+	}
+	if observer.starts != 1 {
+		t.Fatalf("startup callbacks: got %d, want 1", observer.starts)
+	}
+	handlers := observer.config.Handlers()
+	names := make([]string, 0, len(handlers))
+	for _, handler := range handlers {
+		names = append(names, handler.Name())
+	}
+	if !slices.Equal(names, []string{"test_observe", "forward"}) {
+		t.Fatalf("handler order: got %v, want [test_observe forward]", names)
+	}
+	return instance, observer, stop
+}
+
+func stopEmbeddedInstance(t *testing.T, instance *caddy.Instance) {
+	t.Helper()
+	shutdownErr := errors.Join(instance.ShutdownCallbacks()...)
+	stopErr := instance.Stop()
+	instance.Wait()
+	if err := errors.Join(shutdownErr, stopErr); err != nil {
+		t.Errorf("stop embedded instance: %v", err)
+	}
+}
+
+func checkEmbeddedQuery(t *testing.T, instance *caddy.Instance, observer *embeddingObserver, network string, qtype uint16) {
+	t.Helper()
+	server := instance.Servers()[0]
+	addr := server.LocalAddr()
+	if network == "tcp" {
+		addr = server.Addr()
+	}
+	query := new(dns.Msg)
+	query.SetQuestion("embedded.example.", qtype)
+	client := &dns.Client{Net: network, Timeout: 2 * time.Second}
+	response, _, err := client.Exchange(query, addr.String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	wantAnswer := "embedded.example.\t60\tIN\tA\t192.0.2.53"
+	if qtype == dns.TypeAAAA {
+		wantAnswer = "embedded.example.\t60\tIN\tAAAA\t2001:db8::53"
+	}
+	if response.Rcode != dns.RcodeSuccess || len(response.Answer) != 1 || response.Answer[0].String() != wantAnswer {
+		t.Fatalf("unexpected forwarded response: %v", response)
+	}
+	select {
+	case observed := <-observer.observations:
+		if observed.question != query.Question[0] || observed.response == nil ||
+			observed.response.Rcode != response.Rcode || len(observed.response.Answer) != 1 ||
+			observed.response.Answer[0].String() != wantAnswer {
+			t.Fatalf("custom plugin did not observe the question and answer: %+v", observed)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("custom plugin did not observe the forwarded query")
+	}
+}
+
+func startEmbeddingUpstream(t *testing.T) string {
+	t.Helper()
+	var listener net.Listener
+	var packet net.PacketConn
+	var err error
+	// A free TCP port may already be in use by UDP. Reserve both before serving.
+	for range 5 {
+		listener, err = net.Listen("tcp", "127.0.0.1:0")
+		if err != nil {
+			t.Fatal(err)
+		}
+		packet, err = net.ListenPacket("udp", listener.Addr().String())
+		if err == nil {
+			break
+		}
+		listener.Close()
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		listener.Close()
+		packet.Close()
+	})
+	handler := dns.HandlerFunc(func(w dns.ResponseWriter, r *dns.Msg) {
+		response := new(dns.Msg)
+		response.SetReply(r)
+		question := r.Question[0]
+		header := dns.RR_Header{Name: question.Name, Rrtype: question.Qtype, Class: dns.ClassINET, Ttl: 60}
+		switch question.Qtype {
+		case dns.TypeA:
+			response.Answer = []dns.RR{&dns.A{Hdr: header, A: net.ParseIP("192.0.2.53")}}
+		case dns.TypeAAAA:
+			response.Answer = []dns.RR{&dns.AAAA{Hdr: header, AAAA: net.ParseIP("2001:db8::53")}}
+		}
+		if err := w.WriteMsg(response); err != nil {
+			t.Errorf("upstream response: %v", err)
+		}
+	})
+	for _, server := range []*dns.Server{
+		{Net: "tcp", Listener: listener, Handler: handler},
+		{Net: "udp", PacketConn: packet, Handler: handler},
+	} {
+		ready := make(chan struct{})
+		done := make(chan error, 1)
+		server.NotifyStartedFunc = func() { close(ready) }
+		go func() { done <- server.ActivateAndServe() }()
+		t.Cleanup(func() {
+			ctx, cancel := context.WithTimeout(context.WithoutCancel(t.Context()), 5*time.Second)
+			defer cancel()
+			if err := server.ShutdownContext(ctx); err != nil {
+				t.Errorf("stop upstream: %v", err)
+			}
+			select {
+			case err := <-done:
+				if err != nil {
+					t.Errorf("serve upstream: %v", err)
+				}
+			case <-ctx.Done():
+				t.Error("upstream did not stop")
+			}
+		})
+		select {
+		case <-ready:
+		case <-time.After(5 * time.Second):
+			t.Fatal("upstream did not start")
+		}
+	}
+	return listener.Addr().String()
+}

core/dnsserver/example_test.go

@@ -12,6 +12,27 @@ import (
 	"github.com/miekg/dns"
 )
 
+func ExampleSetDirectives() {
+	oldDirectives := dnsserver.Directives
+	defer func() { dnsserver.Directives = oldDirectives }()
+
+	if err := dnsserver.SetDirectives([]string{"bind", "whoami"}); err != nil {
+		panic(err)
+	}
+	fmt.Println(dnsserver.Directives)
+	// Output: [bind whoami]
+}
+
+func ExampleRegister() {
+	// Required before caddy.Start in builds with coredns_manual_registration;
+	// harmless when the default import-time registration has already run.
+	if err := dnsserver.Register(); err != nil {
+		panic(err)
+	}
+	fmt.Println("registered")
+	// Output: registered
+}
+
 func Example_embedding() {
 	oldDirectives := dnsserver.Directives
 	oldCaddyQuiet := caddy.Quiet
@@ -25,6 +46,9 @@ func Example_embedding() {
 	// Import only the plugins the host needs and set their execution order
 	// before starting the first server.
 	dnsserver.Directives = []string{"bind", "whoami"}
+	if err := dnsserver.Register(); err != nil {
+		panic(err)
+	}
 	caddy.Quiet = true
 	dnsserver.Quiet = true
 

core/dnsserver/register.go

@@ -3,6 +3,8 @@ package dnsserver
 import (
 	"fmt"
 	"net"
+	"slices"
+	"sync"
 	"time"
 
 	"github.com/coredns/caddy"
@@ -16,7 +18,22 @@ import (
 
 const serverType = "dns"
 
-func init() {
+// Register registers the DNS server type with Caddy. Repeated calls return the
+// result of the first call without registering again. An existing server type
+// registered by another caller is left unchanged and causes an error.
+//
+// Default builds call Register automatically. When built with the
+// coredns_manual_registration tag, an embedding host must call Register before
+// starting Caddy. Register neither registers plugins nor starts listeners.
+//
+// Concurrent calls to Register are safe, but the first call must not run
+// concurrently with other Caddy configuration or startup operations.
+func Register() error { return registerServerType() }
+
+var registerServerType = sync.OnceValue(func() error {
+	if slices.Contains(caddy.ListPlugins()["server_types"], serverType) {
+		return fmt.Errorf("dnsserver: server type %q already registered", serverType)
+	}
 	caddy.RegisterServerType(serverType, caddy.ServerType{
 		Directives: func() []string { return Directives },
 		DefaultInput: func() caddy.Input {
@@ -28,7 +45,8 @@ func init() {
 		},
 		NewContext: newContext,
 	})
-}
+	return nil
+})
 
 func newContext(_i *caddy.Instance) caddy.Context {
 	return &dnsContext{keysToConfigs: make(map[string]*Config)}

core/dnsserver/register_auto.go

@@ -0,0 +1,9 @@
+//go:build !coredns_manual_registration
+
+package dnsserver
+
+func init() {
+	if err := Register(); err != nil {
+		panic(err)
+	}
+}

core/dnsserver/register_auto_test.go

@@ -0,0 +1,19 @@
+//go:build !coredns_manual_registration
+
+package dnsserver_test
+
+import (
+	"slices"
+	"testing"
+
+	"github.com/coredns/caddy"
+)
+
+// Capture this before any test can explicitly call Register.
+var dnsRegisteredOnImport = slices.Contains(caddy.ListPlugins()["server_types"], "dns")
+
+func TestAutomaticRegistration(t *testing.T) {
+	if !dnsRegisteredOnImport {
+		t.Fatal("default builds must register the DNS server type on import")
+	}
+}

core/dnsserver/register_manual_test.go

@@ -0,0 +1,125 @@
+//go:build coredns_manual_registration
+
+package dnsserver_test
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/coredns/caddy"
+	"github.com/coredns/coredns/core/dnsserver"
+)
+
+const manualRegistrationHelper = "COREDNS_TEST_MANUAL_REGISTRATION"
+
+func TestMain(m *testing.M) {
+	// Most package tests need a registered server type. Fresh subprocesses skip
+	// this setup so their first Register call exercises the import-time state.
+	if os.Getenv(manualRegistrationHelper) == "" {
+		if err := dnsserver.Register(); err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+	}
+	os.Exit(m.Run())
+}
+
+func TestManualRegistrationImport(t *testing.T) {
+	if scenario := os.Getenv(manualRegistrationHelper); scenario != "" {
+		checkManualRegistration(t, scenario)
+		return
+	}
+	executable, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, scenario := range []string{"start", "concurrent", "conflict"} {
+		t.Run(scenario, func(t *testing.T) {
+			ctx, cancel := context.WithTimeout(t.Context(), 20*time.Second)
+			defer cancel()
+			cmd := exec.CommandContext(ctx, executable, "-test.run=^TestManualRegistrationImport$", "-test.count=1")
+			cmd.Env = append(os.Environ(), manualRegistrationHelper+"="+scenario)
+			if output, err := cmd.CombinedOutput(); err != nil {
+				t.Fatalf("fresh-process registration check: %v\n%s", err, output)
+			}
+		})
+	}
+}
+
+func checkManualRegistration(t *testing.T, scenario string) {
+	t.Helper()
+	if slices.Contains(caddy.ListPlugins()["server_types"], "dns") {
+		t.Fatal("importing dnsserver registered the DNS server type in manual mode")
+	}
+	TestEmbeddingDoesNotRegisterCLIFlags(t)
+	configureEmbedding(t)
+	if slices.Contains(caddy.ListPlugins()["server_types"], "dns") {
+		t.Fatal("selecting directives or registering a host plugin registered the server type")
+	}
+
+	switch scenario {
+	case "start":
+		instance, err := caddy.Start(caddy.CaddyfileInput{
+			Filepath:       "Corefile",
+			Contents:       []byte(".:0 {\nbind 127.0.0.1\n}\n"),
+			ServerTypeName: "dns",
+		})
+		if err == nil {
+			stopEmbeddedInstance(t, instance)
+			t.Fatal("starting without Register succeeded")
+		}
+		if !strings.Contains(err.Error(), "no server types plugged in") {
+			t.Fatalf("unexpected unregistered startup error: %v", err)
+		}
+		if len(instance.Servers()) != 0 || len(caddy.Instances()) != 0 {
+			t.Fatal("failed startup left servers or instances behind")
+		}
+	case "concurrent":
+		const callers = 32
+		errs := make(chan error, callers)
+		var wg sync.WaitGroup
+		for range callers {
+			wg.Go(func() { errs <- dnsserver.Register() })
+		}
+		wg.Wait()
+		close(errs)
+		for err := range errs {
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
+	case "conflict":
+		want := []string{"test_foreign_server"}
+		caddy.RegisterServerType("dns", caddy.ServerType{
+			Directives: func() []string { return want },
+		})
+		for range 2 {
+			if err := dnsserver.Register(); err == nil || !strings.Contains(err.Error(), `server type "dns" already registered`) {
+				t.Fatalf("expected registration conflict, got %v", err)
+			}
+			if got := caddy.ValidDirectives("dns"); !slices.Equal(got, want) {
+				t.Fatalf("conflicting server type changed: got %v, want %v", got, want)
+			}
+		}
+		return
+	default:
+		t.Fatalf("unknown registration scenario %q", scenario)
+	}
+
+	for range 2 {
+		if err := dnsserver.Register(); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if got := caddy.ValidDirectives("dns"); !slices.Equal(got, dnsserver.Directives) {
+		t.Fatalf("registration changed the host's directive list: %v", got)
+	}
+	t.Run("forward", TestEmbeddingForward)
+}

core/dnsserver/server.go

@@ -1,15 +1,44 @@
 // Package dnsserver implements CoreDNS as a Caddy server type.
 //
-// Importing this package registers the "dns" server type with Caddy. Programs
+// By default, importing this package registers the "dns" server type with Caddy.
-// embedding CoreDNS can import only the plugins they need, set Directives before
+// Programs embedding CoreDNS can import only the plugins they need, call
-// starting a server, and pass an in-memory Corefile to [caddy.Start]. They should
+// [SetDirectives] before starting a server, and pass an in-memory Corefile to
-// not call coremain.Run, which provides the command-line program behavior such
+// [caddy.Start]. They should not import coremain or the generated all-plugin
-// as flag parsing, signal handling, and blocking until shutdown.
+// bundle: coremain provides command-line behavior such as flag registration,
+// signal handling, and blocking until shutdown, and registers the server type.
 // Before stopping an embedded instance, run its shutdown callbacks so that
 // plugins can release resources.
 //
+// A host can register a custom directive with [plugin.Register] before starting
+// Caddy; it does not need to rebuild CoreDNS or modify plugin.cfg. Include the
+// directive in the list passed to SetDirectives at the desired execution
+// position, and use [GetConfig] and [Config.AddPlugin] in its setup function to
+// add the handler. The setup function can register startup and shutdown callbacks
+// on the Caddy controller.
+// Directives determines execution order, not the order in the Corefile.
+// Each directive must be registered only once per process.
+//
+// SetDirectives copies the supplied list and rejects empty or duplicate names.
+// Direct assignment to Directives remains supported for existing callers.
+// Neither entry point imports plugins or registers them on the host's behalf.
+//
 // Directives and Caddy's plugin registry are process-wide. Configure them
 // before starting any servers and do not mutate them while servers are running.
+// Automatic server-type registration is retained for existing embedding users;
+// it does not start listeners or prevent the host from selecting directives.
+//
+// To control when the DNS server type is registered, build the host with
+// -tags=coredns_manual_registration. This excludes this package's registration
+// init function. After selecting directives and registering host plugins, call
+// [Register] before caddy.Start. Register is idempotent and also works in default
+// builds. It returns an error if another caller already registered a DNS server
+// type, leaving that registration unchanged.
+//
+// The build tag does not disable initialization in Caddy or individual plugins,
+// or make their registries instance-local. Selected plugins must not import
+// coremain, directly or transitively, to avoid its command-line initialization
+// and server-type registration. The CoreDNS command-line program explicitly
+// registers the server type in both build modes.
 package dnsserver
 
 import (

coremain/run.go

@@ -17,6 +17,9 @@ import (
 )
 
 func init() {
+	if err := dnsserver.Register(); err != nil {
+		panic(err)
+	}
 	caddy.DefaultConfigFile = "Corefile"
 	caddy.Quiet = true // don't show init stuff from caddy
 	setVersion()

coremain/run_test.go

@@ -7,11 +7,18 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"slices"
 	"testing"
 
 	"github.com/coredns/caddy"
 )
 
+func TestServerTypeRegistration(t *testing.T) {
+	if !slices.Contains(caddy.ListPlugins()["server_types"], serverType) {
+		t.Fatal("coremain must register the DNS server type in both build modes")
+	}
+}
+
 func TestConfLoader(t *testing.T) {
 	tests := []struct {
 		name          string