cmd/tsidp: add funnel support (#12591)

* cmd/tsidp: add funnel support

Updates #10263.

Signed-off-by: Naman Sood <mail@nsood.in>

* look past funnel-ingress-node to see who we're authenticating

Signed-off-by: Naman Sood <mail@nsood.in>

* fix comment typo

Signed-off-by: Naman Sood <mail@nsood.in>

* address review feedback, support Basic auth for /token

Turns out you need to support Basic auth if you do client ID/secret
according to OAuth.

Signed-off-by: Naman Sood <mail@nsood.in>

* fix typos

Signed-off-by: Naman Sood <mail@nsood.in>

* review fixes

Signed-off-by: Naman Sood <mail@nsood.in>

* remove debugging log

Signed-off-by: Naman Sood <mail@nsood.in>

* add comments, fix header

Signed-off-by: Naman Sood <mail@nsood.in>

---------

Signed-off-by: Naman Sood <mail@nsood.in>

cmd/tsidp/tsidp.go

@@ -7,6 +7,7 @@
 package main
 
 import (
+	"bytes"
 	"context"
 	crand "crypto/rand"
 	"crypto/rsa"
@@ -16,6 +17,7 @@
 	"encoding/binary"
 	"encoding/json"
 	"encoding/pem"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -25,6 +27,7 @@
 	"net/netip"
 	"net/url"
 	"os"
+	"os/signal"
 	"strconv"
 	"strings"
 	"sync"
@@ -35,6 +38,7 @@
 	"tailscale.com/client/tailscale"
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/envknob"
+	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tsnet"
@@ -44,13 +48,22 @@
 	"tailscale.com/util/mak"
 	"tailscale.com/util/must"
 	"tailscale.com/util/rands"
+	"tailscale.com/version"
 )
 
+// ctxConn is a key to look up a net.Conn stored in an HTTP request's context.
+type ctxConn struct{}
+
+// funnelClientsFile is the file where client IDs and secrets for OIDC clients
+// accessing the IDP over Funnel are persisted.
+const funnelClientsFile = "oidc-funnel-clients.json"
+
 var (
 	flagVerbose            = flag.Bool("verbose", false, "be verbose")
 	flagPort               = flag.Int("port", 443, "port to listen on")
 	flagLocalPort          = flag.Int("local-port", -1, "allow requests from localhost")
 	flagUseLocalTailscaled = flag.Bool("use-local-tailscaled", false, "use local tailscaled instead of tsnet")
+	flagFunnel             = flag.Bool("funnel", false, "use Tailscale Funnel to make tsidp available on the public internet")
 )
 
 func main() {
@@ -61,9 +74,11 @@ func main() {
 	}
 
 	var (
-		lc  *tailscale.LocalClient
+		lc          *tailscale.LocalClient
-		st  *ipnstate.Status
+		st          *ipnstate.Status
-		err error
+		err         error
+		watcherChan chan error
+		cleanup     func()
 
 		lns []net.Listener
 	)
@@ -90,6 +105,18 @@ func main() {
 		if !anySuccess {
 			log.Fatalf("failed to listen on any of %v", st.TailscaleIPs)
 		}
+
+		// tailscaled needs to be setting an HTTP header for funneled requests
+		// that older versions don't provide.
+		// TODO(naman): is this the correct check?
+		if *flagFunnel && !version.AtLeast(st.Version, "1.71.0") {
+			log.Fatalf("Local tailscaled not new enough to support -funnel. Update Tailscale or use tsnet mode.")
+		}
+		cleanup, watcherChan, err = serveOnLocalTailscaled(ctx, lc, st, uint16(*flagPort), *flagFunnel)
+		if err != nil {
+			log.Fatalf("could not serve on local tailscaled: %v", err)
+		}
+		defer cleanup()
 	} else {
 		ts := &tsnet.Server{
 			Hostname: "idp",
@@ -105,7 +132,15 @@ func main() {
 		if err != nil {
 			log.Fatalf("getting local client: %v", err)
 		}
-		ln, err := ts.ListenTLS("tcp", fmt.Sprintf(":%d", *flagPort))
+		var ln net.Listener
+		if *flagFunnel {
+			if err := ipn.CheckFunnelAccess(uint16(*flagPort), st.Self); err != nil {
+				log.Fatalf("%v", err)
+			}
+			ln, err = ts.ListenFunnel("tcp", fmt.Sprintf(":%d", *flagPort))
+		} else {
+			ln, err = ts.ListenTLS("tcp", fmt.Sprintf(":%d", *flagPort))
+		}
 		if err != nil {
 			log.Fatal(err)
 		}
@@ -113,13 +148,26 @@ func main() {
 	}
 
 	srv := &idpServer{
-		lc: lc,
+		lc:          lc,
+		funnel:      *flagFunnel,
+		localTSMode: *flagUseLocalTailscaled,
 	}
 	if *flagPort != 443 {
 		srv.serverURL = fmt.Sprintf("https://%s:%d", strings.TrimSuffix(st.Self.DNSName, "."), *flagPort)
 	} else {
 		srv.serverURL = fmt.Sprintf("https://%s", strings.TrimSuffix(st.Self.DNSName, "."))
 	}
+	if *flagFunnel {
+		f, err := os.Open(funnelClientsFile)
+		if err == nil {
+			srv.funnelClients = make(map[string]*funnelClient)
+			if err := json.NewDecoder(f).Decode(&srv.funnelClients); err != nil {
+				log.Fatalf("could not parse %s: %v", funnelClientsFile, err)
+			}
+		} else if !errors.Is(err, os.ErrNotExist) {
+			log.Fatalf("could not open %s: %v", funnelClientsFile, err)
+		}
+	}
 
 	log.Printf("Running tsidp at %s ...", srv.serverURL)
 
@@ -134,35 +182,129 @@ func main() {
 	}
 
 	for _, ln := range lns {
-		go http.Serve(ln, srv)
+		server := http.Server{
+			Handler: srv,
+			ConnContext: func(ctx context.Context, c net.Conn) context.Context {
+				return context.WithValue(ctx, ctxConn{}, c)
+			},
+		}
+		go server.Serve(ln)
 	}
-	select {}
+	// need to catch os.Interrupt, otherwise deferred cleanup code doesn't run
+	exitChan := make(chan os.Signal, 1)
+	signal.Notify(exitChan, os.Interrupt)
+	select {
+	case <-exitChan:
+		log.Printf("interrupt, exiting")
+		return
+	case <-watcherChan:
+		if errors.Is(err, io.EOF) || errors.Is(err, context.Canceled) {
+			log.Printf("watcher closed, exiting")
+			return
+		}
+		log.Fatalf("watcher error: %v", err)
+		return
+	}
+}
+
+// serveOnLocalTailscaled starts a serve session using an already-running
+// tailscaled instead of starting a fresh tsnet server, making something
+// listening on clientDNSName:dstPort accessible over serve/funnel.
+func serveOnLocalTailscaled(ctx context.Context, lc *tailscale.LocalClient, st *ipnstate.Status, dstPort uint16, shouldFunnel bool) (cleanup func(), watcherChan chan error, err error) {
+	// In order to support funneling out in local tailscaled mode, we need
+	// to add a serve config to forward the listeners we bound above and
+	// allow those forwarders to be funneled out.
+	sc, err := lc.GetServeConfig(ctx)
+	if err != nil {
+		return nil, nil, fmt.Errorf("could not get serve config: %v", err)
+	}
+	if sc == nil {
+		sc = new(ipn.ServeConfig)
+	}
+
+	// We watch the IPN bus just to get a session ID. The session expires
+	// when we stop watching the bus, and that auto-deletes the foreground
+	// serve/funnel configs we are creating below.
+	watcher, err := lc.WatchIPNBus(ctx, ipn.NotifyInitialState|ipn.NotifyNoPrivateKeys)
+	if err != nil {
+		return nil, nil, fmt.Errorf("could not set up ipn bus watcher: %v", err)
+	}
+	defer func() {
+		if err != nil {
+			watcher.Close()
+		}
+	}()
+	n, err := watcher.Next()
+	if err != nil {
+		return nil, nil, fmt.Errorf("could not get initial state from ipn bus watcher: %v", err)
+	}
+	if n.SessionID == "" {
+		err = fmt.Errorf("missing sessionID in ipn.Notify")
+		return nil, nil, err
+	}
+	watcherChan = make(chan error)
+	go func() {
+		for {
+			_, err = watcher.Next()
+			if err != nil {
+				watcherChan <- err
+				return
+			}
+		}
+	}()
+
+	// Create a foreground serve config that gets cleaned up when tsidp
+	// exits and the session ID associated with this config is invalidated.
+	foregroundSc := new(ipn.ServeConfig)
+	mak.Set(&sc.Foreground, n.SessionID, foregroundSc)
+	serverURL := strings.TrimSuffix(st.Self.DNSName, ".")
+	fmt.Printf("setting funnel for %s:%v\n", serverURL, dstPort)
+
+	foregroundSc.SetFunnel(serverURL, dstPort, shouldFunnel)
+	foregroundSc.SetWebHandler(&ipn.HTTPHandler{
+		Proxy: fmt.Sprintf("https://%s", net.JoinHostPort(serverURL, strconv.Itoa(int(dstPort)))),
+	}, serverURL, uint16(*flagPort), "/", true)
+	err = lc.SetServeConfig(ctx, sc)
+	if err != nil {
+		return nil, watcherChan, fmt.Errorf("could not set serve config: %v", err)
+	}
+
+	return func() { watcher.Close() }, watcherChan, nil
 }
 
 type idpServer struct {
 	lc          *tailscale.LocalClient
 	loopbackURL string
 	serverURL   string // "https://foo.bar.ts.net"
+	funnel      bool
+	localTSMode bool
 
 	lazyMux        lazy.SyncValue[*http.ServeMux]
 	lazySigningKey lazy.SyncValue[*signingKey]
 	lazySigner     lazy.SyncValue[jose.Signer]
 
-	mu          sync.Mutex              // guards the fields below
+	mu            sync.Mutex               // guards the fields below
-	code        map[string]*authRequest // keyed by random hex
+	code          map[string]*authRequest  // keyed by random hex
-	accessToken map[string]*authRequest // keyed by random hex
+	accessToken   map[string]*authRequest  // keyed by random hex
+	funnelClients map[string]*funnelClient // keyed by client ID
 }
 
 type authRequest struct {
 	// localRP is true if the request is from a relying party running on the
-	// same machine as the idp server. It is mutually exclusive with rpNodeID.
+	// same machine as the idp server. It is mutually exclusive with rpNodeID
+	// and funnelRP.
 	localRP bool
 
 	// rpNodeID is the NodeID of the relying party (who requested the auth, such
 	// as Proxmox or Synology), not the user node who is being authenticated. It
-	// is mutually exclusive with localRP.
+	// is mutually exclusive with localRP and funnelRP.
 	rpNodeID tailcfg.NodeID
 
+	// funnelRP is non-nil if the request is from a relying party outside the
+	// tailnet, via Tailscale Funnel. It is mutually exclusive with rpNodeID
+	// and localRP.
+	funnelRP *funnelClient
+
 	// clientID is the "client_id" sent in the authorized request.
 	clientID string
 
@@ -181,9 +323,12 @@ type authRequest struct {
 	validTill time.Time
 }
 
-func (ar *authRequest) allowRelyingParty(ctx context.Context, remoteAddr string, lc *tailscale.LocalClient) error {
+// allowRelyingParty validates that a relying party identified either by a
+// known remoteAddr or a valid client ID/secret pair is allowed to proceed
+// with the authorization flow associated with this authRequest.
+func (ar *authRequest) allowRelyingParty(r *http.Request, lc *tailscale.LocalClient) error {
 	if ar.localRP {
-		ra, err := netip.ParseAddrPort(remoteAddr)
+		ra, err := netip.ParseAddrPort(r.RemoteAddr)
 		if err != nil {
 			return err
 		}
@@ -192,7 +337,18 @@ func (ar *authRequest) allowRelyingParty(ctx context.Context, remoteAddr string,
 		}
 		return nil
 	}
-	who, err := lc.WhoIs(ctx, remoteAddr)
+	if ar.funnelRP != nil {
+		clientID, clientSecret, ok := r.BasicAuth()
+		if !ok {
+			clientID = r.FormValue("client_id")
+			clientSecret = r.FormValue("client_secret")
+		}
+		if ar.funnelRP.ID != clientID || ar.funnelRP.Secret != clientSecret {
+			return fmt.Errorf("tsidp: invalid client credentials")
+		}
+		return nil
+	}
+	who, err := lc.WhoIs(r.Context(), r.RemoteAddr)
 	if err != nil {
 		return fmt.Errorf("tsidp: error getting WhoIs: %w", err)
 	}
@@ -203,24 +359,60 @@ func (ar *authRequest) allowRelyingParty(ctx context.Context, remoteAddr string,
 }
 
 func (s *idpServer) authorize(w http.ResponseWriter, r *http.Request) {
-	who, err := s.lc.WhoIs(r.Context(), r.RemoteAddr)
+	// This URL is visited by the user who is being authenticated. If they are
+	// visiting the URL over Funnel, that means they are not part of the
+	// tailnet that they are trying to be authenticated for.
+	if isFunnelRequest(r) {
+		http.Error(w, "tsidp: unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	uq := r.URL.Query()
+
+	redirectURI := uq.Get("redirect_uri")
+	if redirectURI == "" {
+		http.Error(w, "tsidp: must specify redirect_uri", http.StatusBadRequest)
+		return
+	}
+
+	var remoteAddr string
+	if s.localTSMode {
+		// in local tailscaled mode, the local tailscaled is forwarding us
+		// HTTP requests, so reading r.RemoteAddr will just get us our own
+		// address.
+		remoteAddr = r.Header.Get("X-Forwarded-For")
+	} else {
+		remoteAddr = r.RemoteAddr
+	}
+	who, err := s.lc.WhoIs(r.Context(), remoteAddr)
 	if err != nil {
 		log.Printf("Error getting WhoIs: %v", err)
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 
-	uq := r.URL.Query()
-
 	code := rands.HexString(32)
 	ar := &authRequest{
 		nonce:       uq.Get("nonce"),
 		remoteUser:  who,
-		redirectURI: uq.Get("redirect_uri"),
+		redirectURI: redirectURI,
 		clientID:    uq.Get("client_id"),
 	}
 
-	if r.URL.Path == "/authorize/localhost" {
+	if r.URL.Path == "/authorize/funnel" {
+		s.mu.Lock()
+		c, ok := s.funnelClients[ar.clientID]
+		s.mu.Unlock()
+		if !ok {
+			http.Error(w, "tsidp: invalid client ID", http.StatusBadRequest)
+			return
+		}
+		if ar.redirectURI != c.RedirectURI {
+			http.Error(w, "tsidp: redirect_uri mismatch", http.StatusBadRequest)
+			return
+		}
+		ar.funnelRP = c
+	} else if r.URL.Path == "/authorize/localhost" {
 		ar.localRP = true
 	} else {
 		var ok bool
@@ -237,8 +429,10 @@ func (s *idpServer) authorize(w http.ResponseWriter, r *http.Request) {
 
 	q := make(url.Values)
 	q.Set("code", code)
-	q.Set("state", uq.Get("state"))
+	if state := uq.Get("state"); state != "" {
-	u := uq.Get("redirect_uri") + "?" + q.Encode()
+		q.Set("state", state)
+	}
+	u := redirectURI + "?" + q.Encode()
 	log.Printf("Redirecting to %q", u)
 
 	http.Redirect(w, r, u, http.StatusFound)
@@ -251,6 +445,7 @@ func (s *idpServer) newMux() *http.ServeMux {
 	mux.HandleFunc("/authorize/", s.authorize)
 	mux.HandleFunc("/userinfo", s.serveUserInfo)
 	mux.HandleFunc("/token", s.serveToken)
+	mux.HandleFunc("/clients/", s.serveClients)
 	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.Path == "/" {
 			io.WriteString(w, "<html><body><h1>Tailscale OIDC IdP</h1>")
@@ -284,11 +479,6 @@ func (s *idpServer) serveUserInfo(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "tsidp: invalid token", http.StatusBadRequest)
 		return
 	}
-	if err := ar.allowRelyingParty(r.Context(), r.RemoteAddr, s.lc); err != nil {
-		log.Printf("Error allowing relying party: %v", err)
-		http.Error(w, err.Error(), http.StatusForbidden)
-		return
-	}
 
 	if ar.validTill.Before(time.Now()) {
 		http.Error(w, "tsidp: token expired", http.StatusBadRequest)
@@ -348,7 +538,7 @@ func (s *idpServer) serveToken(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "tsidp: code not found", http.StatusBadRequest)
 		return
 	}
-	if err := ar.allowRelyingParty(r.Context(), r.RemoteAddr, s.lc); err != nil {
+	if err := ar.allowRelyingParty(r, s.lc); err != nil {
 		log.Printf("Error allowing relying party: %v", err)
 		http.Error(w, err.Error(), http.StatusForbidden)
 		return
@@ -581,7 +771,9 @@ func (s *idpServer) serveOpenIDConfig(w http.ResponseWriter, r *http.Request) {
 	}
 	var authorizeEndpoint string
 	rpEndpoint := s.serverURL
-	if who, err := s.lc.WhoIs(r.Context(), r.RemoteAddr); err == nil {
+	if isFunnelRequest(r) {
+		authorizeEndpoint = fmt.Sprintf("%s/authorize/funnel", s.serverURL)
+	} else if who, err := s.lc.WhoIs(r.Context(), r.RemoteAddr); err == nil {
 		authorizeEndpoint = fmt.Sprintf("%s/authorize/%d", s.serverURL, who.Node.ID)
 	} else if ap.Addr().IsLoopback() {
 		rpEndpoint = s.loopbackURL
@@ -611,6 +803,148 @@ func (s *idpServer) serveOpenIDConfig(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// funnelClient represents an OIDC client/relying party that is accessing the
+// IDP over Funnel.
+type funnelClient struct {
+	ID          string `json:"client_id"`
+	Secret      string `json:"client_secret,omitempty"`
+	Name        string `json:"name,omitempty"`
+	RedirectURI string `json:"redirect_uri"`
+}
+
+// /clients is a privileged endpoint that allows the visitor to create new
+// Funnel-capable OIDC clients, so it is only accessible over the tailnet.
+func (s *idpServer) serveClients(w http.ResponseWriter, r *http.Request) {
+	if isFunnelRequest(r) {
+		http.Error(w, "tsidp: not found", http.StatusNotFound)
+		return
+	}
+
+	path := strings.TrimPrefix(r.URL.Path, "/clients/")
+
+	if path == "new" {
+		s.serveNewClient(w, r)
+		return
+	}
+
+	if path == "" {
+		s.serveGetClientsList(w, r)
+		return
+	}
+
+	s.mu.Lock()
+	c, ok := s.funnelClients[path]
+	s.mu.Unlock()
+	if !ok {
+		http.Error(w, "tsidp: not found", http.StatusNotFound)
+		return
+	}
+
+	switch r.Method {
+	case "DELETE":
+		s.serveDeleteClient(w, r, path)
+	case "GET":
+		json.NewEncoder(w).Encode(&funnelClient{
+			ID:          c.ID,
+			Name:        c.Name,
+			Secret:      "",
+			RedirectURI: c.RedirectURI,
+		})
+	default:
+		http.Error(w, "tsidp: method not allowed", http.StatusMethodNotAllowed)
+	}
+}
+
+func (s *idpServer) serveNewClient(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "POST" {
+		http.Error(w, "tsidp: method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	redirectURI := r.FormValue("redirect_uri")
+	if redirectURI == "" {
+		http.Error(w, "tsidp: must provide redirect_uri", http.StatusBadRequest)
+		return
+	}
+	clientID := rands.HexString(32)
+	clientSecret := rands.HexString(64)
+	newClient := funnelClient{
+		ID:          clientID,
+		Secret:      clientSecret,
+		Name:        r.FormValue("name"),
+		RedirectURI: redirectURI,
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	mak.Set(&s.funnelClients, clientID, &newClient)
+	if err := s.storeFunnelClientsLocked(); err != nil {
+		log.Printf("could not write funnel clients db: %v", err)
+		http.Error(w, "tsidp: could not write funnel clients to db", http.StatusInternalServerError)
+		// delete the new client to avoid inconsistent state between memory
+		// and disk
+		delete(s.funnelClients, clientID)
+		return
+	}
+	json.NewEncoder(w).Encode(newClient)
+}
+
+func (s *idpServer) serveGetClientsList(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "GET" {
+		http.Error(w, "tsidp: method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	s.mu.Lock()
+	redactedClients := make([]funnelClient, 0, len(s.funnelClients))
+	for _, c := range s.funnelClients {
+		redactedClients = append(redactedClients, funnelClient{
+			ID:          c.ID,
+			Name:        c.Name,
+			Secret:      "",
+			RedirectURI: c.RedirectURI,
+		})
+	}
+	s.mu.Unlock()
+	json.NewEncoder(w).Encode(redactedClients)
+}
+
+func (s *idpServer) serveDeleteClient(w http.ResponseWriter, r *http.Request, clientID string) {
+	if r.Method != "DELETE" {
+		http.Error(w, "tsidp: method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.funnelClients == nil {
+		http.Error(w, "tsidp: client not found", http.StatusNotFound)
+		return
+	}
+	if _, ok := s.funnelClients[clientID]; !ok {
+		http.Error(w, "tsidp: client not found", http.StatusNotFound)
+		return
+	}
+	deleted := s.funnelClients[clientID]
+	delete(s.funnelClients, clientID)
+	if err := s.storeFunnelClientsLocked(); err != nil {
+		log.Printf("could not write funnel clients db: %v", err)
+		http.Error(w, "tsidp: could not write funnel clients to db", http.StatusInternalServerError)
+		// restore the deleted value to avoid inconsistent state between memory
+		// and disk
+		s.funnelClients[clientID] = deleted
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
+// storeFunnelClientsLocked writes the current mapping of OIDC client ID/secret
+// pairs for RPs that access the IDP over funnel. s.mu must be held while
+// calling this.
+func (s *idpServer) storeFunnelClientsLocked() error {
+	var buf bytes.Buffer
+	if err := json.NewEncoder(&buf).Encode(s.funnelClients); err != nil {
+		return err
+	}
+	return os.WriteFile(funnelClientsFile, buf.Bytes(), 0600)
+}
+
 const (
 	minimumRSAKeySize = 2048
 )
@@ -700,3 +1034,24 @@ func parseID[T ~int64](input string) (_ T, ok bool) {
 	}
 	return T(i), true
 }
+
+// isFunnelRequest checks if an HTTP request is coming over Tailscale Funnel.
+func isFunnelRequest(r *http.Request) bool {
+	// If we're funneling through the local tailscaled, it will set this HTTP
+	// header.
+	if r.Header.Get("Tailscale-Funnel-Request") != "" {
+		return true
+	}
+
+	// If the funneled connection is from tsnet, then the net.Conn will be of
+	// type ipn.FunnelConn.
+	netConn := r.Context().Value(ctxConn{})
+	// if the conn is wrapped inside TLS, unwrap it
+	if tlsConn, ok := netConn.(*tls.Conn); ok {
+		netConn = tlsConn.NetConn()
+	}
+	if _, ok := netConn.(*ipn.FunnelConn); ok {
+		return true
+	}
+	return false
+}

ipn/ipnlocal/local.go

@@ -3781,7 +3781,7 @@ func (b *LocalBackend) TCPHandlerForDst(src, dst netip.AddrPort) (handler func(c
 			return nil
 		}, opts
 	}
-	if handler := b.tcpHandlerForServe(dst.Port(), src); handler != nil {
+	if handler := b.tcpHandlerForServe(dst.Port(), src, nil); handler != nil {
 		return handler, opts
 	}
 	return nil, nil

ipn/ipnlocal/serve.go

@@ -56,6 +56,16 @@
 type serveHTTPContext struct {
 	SrcAddr  netip.AddrPort
 	DestPort uint16
+
+	// provides funnel-specific context, nil if not funneled
+	Funnel *funnelFlow
+}
+
+// funnelFlow represents a funneled connection initiated via IngressPeer
+// to Host.
+type funnelFlow struct {
+	Host        string
+	IngressPeer tailcfg.NodeView
 }
 
 // localListener is the state of host-level net.Listen for a specific (Tailscale IP, port)
@@ -91,7 +101,7 @@ func (b *LocalBackend) newServeListener(ctx context.Context, ap netip.AddrPort,
 
 		handler: func(conn net.Conn) error {
 			srcAddr := conn.RemoteAddr().(*net.TCPAddr).AddrPort()
-			handler := b.tcpHandlerForServe(ap.Port(), srcAddr)
+			handler := b.tcpHandlerForServe(ap.Port(), srcAddr, nil)
 			if handler == nil {
 				b.logf("[unexpected] local-serve: no handler for %v to port %v", srcAddr, ap.Port())
 				conn.Close()
@@ -382,7 +392,7 @@ func (b *LocalBackend) HandleIngressTCPConn(ingressPeer tailcfg.NodeView, target
 		return
 	}
 
-	_, port, err := net.SplitHostPort(string(target))
+	host, port, err := net.SplitHostPort(string(target))
 	if err != nil {
 		logf("got ingress conn for bad target %q; rejecting", target)
 		sendRST()
@@ -407,9 +417,10 @@ func (b *LocalBackend) HandleIngressTCPConn(ingressPeer tailcfg.NodeView, target
 			return
 		}
 	}
-	// TODO(bradfitz): pass ingressPeer etc in context to tcpHandlerForServe,
+	handler := b.tcpHandlerForServe(dport, srcAddr, &funnelFlow{
-	// extend serveHTTPContext or similar.
+		Host:        host,
-	handler := b.tcpHandlerForServe(dport, srcAddr)
+		IngressPeer: ingressPeer,
+	})
 	if handler == nil {
 		logf("[unexpected] no matching ingress serve handler for %v to port %v", srcAddr, dport)
 		sendRST()
@@ -424,8 +435,9 @@ func (b *LocalBackend) HandleIngressTCPConn(ingressPeer tailcfg.NodeView, target
 }
 
 // tcpHandlerForServe returns a handler for a TCP connection to be served via
-// the ipn.ServeConfig.
+// the ipn.ServeConfig. The funnelFlow can be nil if this is not a funneled
-func (b *LocalBackend) tcpHandlerForServe(dport uint16, srcAddr netip.AddrPort) (handler func(net.Conn) error) {
+// connection.
+func (b *LocalBackend) tcpHandlerForServe(dport uint16, srcAddr netip.AddrPort, f *funnelFlow) (handler func(net.Conn) error) {
 	b.mu.Lock()
 	sc := b.serveConfig
 	b.mu.Unlock()
@@ -444,6 +456,7 @@ func (b *LocalBackend) tcpHandlerForServe(dport uint16, srcAddr netip.AddrPort)
 			Handler: http.HandlerFunc(b.serveWebHandler),
 			BaseContext: func(_ net.Listener) context.Context {
 				return serveHTTPContextKey.WithValue(context.Background(), &serveHTTPContext{
+					Funnel:   f,
 					SrcAddr:  srcAddr,
 					DestPort: dport,
 				})
@@ -712,15 +725,20 @@ func (b *LocalBackend) addTailscaleIdentityHeaders(r *httputil.ProxyRequest) {
 	r.Out.Header.Del("Tailscale-User-Login")
 	r.Out.Header.Del("Tailscale-User-Name")
 	r.Out.Header.Del("Tailscale-User-Profile-Pic")
+	r.Out.Header.Del("Tailscale-Funnel-Request")
 	r.Out.Header.Del("Tailscale-Headers-Info")
 
 	c, ok := serveHTTPContextKey.ValueOk(r.Out.Context())
 	if !ok {
 		return
 	}
+	if c.Funnel != nil {
+		r.Out.Header.Set("Tailscale-Funnel-Request", "?1")
+		return
+	}
 	node, user, ok := b.WhoIs("tcp", c.SrcAddr)
 	if !ok {
-		return // traffic from outside of Tailnet (funneled)
+		return // traffic from outside of Tailnet (funneled or local machine)
 	}
 	if node.IsTagged() {
 		// 2023-06-14: Not setting identity headers for tagged nodes.