derp/server: resolve the live DERP map in /bootstrap-dns

The handler captured the startup map, so hostnames added by
derp.auto_update never resolved.

hscontrol/app.go

@@ -501,7 +501,7 @@ func (h *Headscale) createRouter(apiV1Mux, apiV2Mux http.Handler) *chi.Mux {
 		r.HandleFunc("/derp", h.DERPServer.DERPHandler)
 		r.HandleFunc("/derp/probe", derpServer.DERPProbeHandler)
 		r.HandleFunc("/derp/latency-check", derpServer.DERPProbeHandler)
-		r.HandleFunc("/bootstrap-dns", derpServer.DERPBootstrapDNSHandler(h.state.DERPMap()))
+		r.HandleFunc("/bootstrap-dns", derpServer.DERPBootstrapDNSHandler(h.state.DERPMap))
 	}
 
 	// Auth is enforced inside each Huma mux per-operation, so the whole API

hscontrol/derp/server/derp_server.go

@@ -334,8 +334,9 @@ func DERPProbeHandler(
 // They have a cache, but not clear if that is really necessary at Headscale, uh, scale.
 // An example implementation is found here https://derp.tailscale.com/bootstrap-dns
 // Coordination server is included automatically, since local DERP is using the same DNS Name in d.serverURL.
+// derpMap is called per request so DERP map updates are served.
 func DERPBootstrapDNSHandler(
-	derpMap tailcfg.DERPMapView,
+	derpMap func() tailcfg.DERPMapView,
 ) func(http.ResponseWriter, *http.Request) {
 	return func(
 		writer http.ResponseWriter,
@@ -348,7 +349,7 @@ func DERPBootstrapDNSHandler(
 
 		var resolver net.Resolver
 
-		for _, region := range derpMap.Regions().All() { //nolint:unqueryvet // not SQLBoiler, tailcfg iterator
+		for _, region := range derpMap().Regions().All() { //nolint:unqueryvet // not SQLBoiler, tailcfg iterator
 			for _, node := range region.Nodes().All() { //nolint:unqueryvet // not SQLBoiler, tailcfg iterator
 				addrs, err := resolver.LookupIP(resolvCtx, "ip", node.HostName())
 				if err != nil {

hscontrol/derp/server/derp_server_test.go

@@ -1,12 +1,18 @@
 package server
 
 import (
+	"encoding/json"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"sync/atomic"
 	"testing"
 
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"tailscale.com/envknob"
+	"tailscale.com/tailcfg"
 )
 
 // TestGenerateRegionInsecureTLS pins the contract nix/testkit.nix relies on:
@@ -58,3 +64,26 @@ func TestGenerateRegionInsecureTLS(t *testing.T) {
 		})
 	}
 }
+
+// TestDERPBootstrapDNSHandlerFollowsDERPMapUpdates guards against the handler
+// resolving a DERP map captured at startup: hostnames that a later
+// auto-update adds must be served.
+func TestDERPBootstrapDNSHandlerFollowsDERPMapUpdates(t *testing.T) {
+	var current atomic.Pointer[tailcfg.DERPMap]
+	current.Store(&tailcfg.DERPMap{})
+
+	handler := DERPBootstrapDNSHandler(func() tailcfg.DERPMapView {
+		return current.Load().View()
+	})
+
+	current.Store(&tailcfg.DERPMap{Regions: map[tailcfg.DERPRegionID]*tailcfg.DERPRegion{
+		1: {RegionID: 1, Nodes: []*tailcfg.DERPNode{{Name: "1a", RegionID: 1, HostName: "localhost"}}},
+	}})
+
+	rec := httptest.NewRecorder()
+	handler(rec, httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/bootstrap-dns", nil))
+
+	var got map[string][]net.IP
+	require.NoError(t, json.NewDecoder(rec.Body).Decode(&got))
+	assert.Contains(t, got, "localhost")
+}