feature/conn25/conn25.go

@@ -81,6 +81,7 @@ func init() {
 		}, nil
 	})
 	ipnlocal.RegisterPeerAPIHandler("/v0/connector/transit-ip", handleConnectorTransitIP)
+	ipnlocal.HookReplyToDNSQueries.Add(handleHookReplyToDNSQueries)
 }
 
 func handleConnectorTransitIP(h ipnlocal.PeerAPIHandler, w http.ResponseWriter, r *http.Request) {
@@ -97,6 +98,18 @@ func handleConnectorTransitIP(h ipnlocal.PeerAPIHandler, w http.ResponseWriter,
 	e.handleConnectorTransitIP(h, w, r)
 }
 
+func handleHookReplyToDNSQueries(h ipnlocal.PeerAPIHandler) bool {
+	// TODO(tailscale/corp#39033): Remove for alpha release.
+	if !envknob.UseWIPCode() && !testenv.InTest() {
+		return false
+	}
+	e, ok := ipnlocal.GetExt[*extension](h.LocalBackend())
+	if !ok {
+		return false
+	}
+	return e.handleHookReplyToDNSQueries(h)
+}
+
 // extension is an [ipnext.Extension] managing the connector on platforms
 // that import this package.
 type extension struct {
@@ -343,6 +356,15 @@ func (e *extension) handleConnectorTransitIP(h ipnlocal.PeerAPIHandler, w http.R
 	w.Write(bs)
 }
 
+func (e *extension) handleHookReplyToDNSQueries(h ipnlocal.PeerAPIHandler) bool {
+	if !e.conn25.isConfigured() {
+		return false
+	}
+	// TODO(tailscale/corp#40076): verify the peer has access to the query's
+	// app (if any) domain.
+	return true
+}
+
 // onSelfChange implements the [ipnext.Hooks.OnSelfChange] hook.
 func (e *extension) onSelfChange(selfNode tailcfg.NodeView) {
 	cfg, err := configFromNodeView(selfNode)

ipn/ipnlocal/peerapi.go

@@ -674,6 +674,12 @@ func (h *peerAPIHandler) handleServeDNSFwd(w http.ResponseWriter, r *http.Reques
 	dh.ServeHTTP(w, r)
 }
 
+// HookReplyToDNSQueries allows extensions to register a willingness to allow
+// handling PeerAPI DNS queries for the peer making this request.
+var HookReplyToDNSQueries = feature.Hooks[func(PeerAPIHandler) bool]{
+	offersExitNodeOrAppConnectorAndPeerHasAutogroupInternet,
+}
+
 func (h *peerAPIHandler) replyToDNSQueries() bool {
 	if !buildfeatures.HasDNS {
 		return false
@@ -683,17 +689,36 @@ func (h *peerAPIHandler) replyToDNSQueries() bool {
 		// without further checks.
 		return true
 	}
-	b := h.ps.b
-	if !b.OfferingExitNode() && !b.OfferingAppConnector() {
-		// If we're not an exit node or app connector, there's
-		// no point to being a DNS server for somebody.
-		return false
-	}
 	if !h.remoteAddr.IsValid() {
 		// This should never be the case if the peerAPIHandler
 		// was wired up correctly, but just in case.
 		return false
 	}
+
+	for _, hook := range HookReplyToDNSQueries {
+		if hook(h) {
+			return true
+		}
+	}
+	return false
+}
+
+// offersExitNodeOrAppConnectorAndPeerHasAutogroupInternet is run as part of
+// [HookReplyToDNSQueries] and handles our legacy PeerAPI DNS acceptance
+// criteria:
+//   - When a node is advertising an exit node it will accept DNS queries
+//     from peers that have access to autogroup:internet.
+//   - When a node is advertising an app connector, it will accept DNS queries
+//     to peers that have access to a relevant app.
+//
+// Further details about how these are accomplished are in inline comments.
+func offersExitNodeOrAppConnectorAndPeerHasAutogroupInternet(h PeerAPIHandler) bool {
+	b := h.LocalBackend()
+	if !b.OfferingExitNode() && !b.OfferingAppConnector() {
+		// If we're not an exit node or app connector, this hook
+		// doesn't apply.
+		return false
+	}
 	// Otherwise, we're an exit node but the peer is not us, so
 	// we need to check if they're allowed access to the internet.
 	// As peerapi bypasses wgengine/filter checks, we need to check
@@ -716,7 +741,7 @@ func (h *peerAPIHandler) replyToDNSQueries() bool {
 	// arbitrary. DNS runs over TCP and UDP, so sure... we check
 	// TCP.
 	dstIP := netaddr.IPv4(0, 0, 0, 0)
-	remoteIP := h.remoteAddr.Addr()
+	remoteIP := h.RemoteAddr().Addr()
 	if remoteIP.Is6() {
 		// autogroup:internet for IPv6 is defined to start with 2000::/3,
 		// so use 2000::0 as the probe "the internet" address.