availability detector

README.md

@@ -59,6 +59,8 @@ go build -o /usr/local/bin/tsproxy .
 | `--name`    | `-n` | `tsproxy`     | Hostname advertised on the tailnet. |
 | `--dir`     |      | `~/.config/tsproxy/<name>` | State directory (node identity, keys). |
 | `--verbose` | `-v` | `false`       | Verbose tsnet logging. |
+| `--probe-interval` | | `5s` | How often each target is probed for reachability. |
+| `--probe-timeout`  | | `3s` | How long a probe may take before the target counts as unreachable. |
 
 Target can be any MagicDNS name, short hostname, or tailnet IP.
 
@@ -88,8 +90,22 @@ plist, then remove it once the state directory has been populated.
 
 - **One node, many forwards.** All `--forward` rules share a single tsnet
   identity, so you get one device in the admin console and one ACL subject.
-- **Startup is strict.** If any local listener fails to bind, the process
-  exits — partial success is confusing under a supervisor.
+- **Startup is strict.** Every local address is bound once at startup as a
+  check; if any fails, the process exits — partial success is confusing under
+  a supervisor.
+- **The local port tracks the target.** Each forward dials its target every
+  `--probe-interval` and only keeps the local port bound while that succeeds.
+  So an unavailable target means `connection refused` on the local port, not a
+  connect that immediately EOFs, and clients back off the way they would
+  against a genuinely down service.
+- **Target loss drops live connections.** A tailnet peer can vanish without the
+  userspace TCP stack ever erroring on an established connection, which leaves
+  local sockets hanging and apps waiting on a dead link. When a probe fails,
+  every connection on that forward is closed so clients see the drop and
+  reconnect. Detection takes up to `--probe-interval` + `--probe-timeout`.
+- **Probes are real connections.** Each probe opens and immediately closes a
+  TCP connection to the target. Chatty services may log these; raise
+  `--probe-interval` to quiet them down, at the cost of slower detection.
 - **State directory matters.** Losing `~/.config/tsproxy/<name>/` means the
   node re-registers on next launch and will need a fresh `TS_AUTHKEY`.
 

main.go

@@ -15,6 +15,7 @@ import (
 	"path/filepath"
 	"strings"
 	"sync"
+	"time"
 
 	flag "github.com/spf13/pflag"
 	"tailscale.com/tsnet"
@@ -50,6 +51,10 @@ func main() {
 		hostname = flag.StringP("name", "n", "tsproxy", "hostname this node advertises on the tailnet")
 		dir      = flag.String("dir", "", "state directory (default: ~/.config/tsproxy/<name>)")
 		verbose  = flag.BoolP("verbose", "v", false, "verbose tsnet logging")
+		interval = flag.Duration("probe-interval", 5*time.Second,
+			"how often to probe each target for reachability")
+		timeout = flag.Duration("probe-timeout", 3*time.Second,
+			"how long a target probe may take before the target counts as unreachable")
 	)
 	flag.Parse()
 
@@ -60,6 +65,12 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
+	if *interval <= 0 {
+		log.Fatal("--probe-interval must be positive")
+	}
+	if *timeout <= 0 {
+		log.Fatal("--probe-timeout must be positive")
+	}
 
 	stateDir := *dir
 	if stateDir == "" {
@@ -73,6 +84,18 @@ func main() {
 		log.Fatalf("mkdir state: %v", err)
 	}
 
+	// Bind every local address up front so a typo or a port clash still kills
+	// the process at startup rather than a supervisor restart later. The
+	// listeners are handed straight back; from here on each proxy binds and
+	// unbinds its own port to track target reachability.
+	for _, f := range fwds {
+		ln, err := net.Listen("tcp", f.local)
+		if err != nil {
+			log.Fatalf("listen %s: %v", f.local, err)
+		}
+		ln.Close()
+	}
+
 	srv := &tsnet.Server{
 		Hostname: *hostname,
 		Dir:      stateDir,
@@ -89,36 +112,209 @@ func main() {
 
 	var wg sync.WaitGroup
 	for _, f := range fwds {
-		ln, err := net.Listen("tcp", f.local)
-		if err != nil {
-			log.Fatalf("listen %s: %v", f.local, err)
+		p := &proxy{
+			dial:     srv,
+			local:    f.local,
+			target:   f.target,
+			interval: *interval,
+			timeout:  *timeout,
+			recheck:  make(chan struct{}, 1),
+			conns:    make(map[net.Conn]struct{}),
 		}
 		log.Printf("tsproxy: %s -> %s (via tailnet as %q)", f.local, f.target, *hostname)
 		wg.Add(1)
-		go func(ln net.Listener, target string) {
+		go func() {
 			defer wg.Done()
-			serve(ctx, srv, ln, target)
-		}(ln, f.target)
+			p.run(ctx)
+		}()
 	}
 	wg.Wait()
 }
 
-func serve(ctx context.Context, srv *tsnet.Server, ln net.Listener, target string) {
+// dialer is the subset of *tsnet.Server that a proxy needs.
+type dialer interface {
+	Dial(ctx context.Context, network, addr string) (net.Conn, error)
+}
+
+// target reachability, as last observed by the probe loop.
+type state int
+
+const (
+	stateUnknown state = iota
+	stateUp
+	stateDown
+)
+
+// proxy forwards one local address to one tailnet target, and owns the local
+// listener: the port is only bound while the target is known to be reachable,
+// so clients get a connection refusal (not a connect-then-EOF) while it isn't.
+type proxy struct {
+	dial     dialer
+	local    string
+	target   string
+	interval time.Duration
+	timeout  time.Duration
+	recheck  chan struct{} // nudges the probe loop to re-probe immediately
+
+	mu    sync.Mutex
+	state state
+	ln    net.Listener // nil while the target is down
+	conns map[net.Conn]struct{}
+}
+
+func (p *proxy) run(ctx context.Context) {
+	t := time.NewTicker(p.interval)
+	defer t.Stop()
 	for {
-		c, err := ln.Accept()
-		if err != nil {
-			log.Printf("accept %s: %v", ln.Addr(), err)
-			return
+		if err := p.probe(ctx); err != nil {
+			p.markDown(err)
+		} else {
+			p.markUp(ctx)
+		}
+		select {
+		case <-ctx.Done():
+			p.markDown(ctx.Err())
+			return
+		case <-t.C:
+		case <-p.recheck:
 		}
-		go handle(ctx, srv, c, target)
 	}
 }
 
-func handle(ctx context.Context, srv *tsnet.Server, in net.Conn, target string) {
-	defer in.Close()
-	out, err := srv.Dial(ctx, "tcp", target)
+// probe dials the target and hangs up. It is the only evidence we have that
+// the target is alive: a tailnet peer can disappear without the userspace TCP
+// stack ever reporting an error on an established connection.
+func (p *proxy) probe(ctx context.Context) error {
+	ctx, cancel := context.WithTimeout(ctx, p.timeout)
+	defer cancel()
+	c, err := p.dial.Dial(ctx, "tcp", p.target)
 	if err != nil {
-		log.Printf("dial %s: %v", target, err)
+		return err
+	}
+	return c.Close()
+}
+
+// nudge asks the probe loop to re-probe now rather than at the next tick.
+func (p *proxy) nudge() {
+	select {
+	case p.recheck <- struct{}{}:
+	default:
+	}
+}
+
+// markUp binds the local port if it isn't bound already.
+func (p *proxy) markUp(ctx context.Context) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	was := p.state
+	p.state = stateUp
+	if p.ln != nil {
+		return
+	}
+	ln, err := net.Listen("tcp", p.local)
+	if err != nil {
+		// Someone else holds the port. Stay down and retry on the next tick.
+		p.state = stateDown
+		if was != stateDown {
+			log.Printf("tsproxy: %s -> %s: listen: %v (retrying every %s)", p.local, p.target, err, p.interval)
+		}
+		return
+	}
+	p.ln = ln
+	if was == stateUp {
+		// Listener was lost without the target going down; already logged.
+		log.Printf("tsproxy: %s -> %s: listening again", p.local, p.target)
+	} else {
+		log.Printf("tsproxy: %s -> %s: target reachable, accepting connections", p.local, p.target)
+	}
+	go p.accept(ctx, ln)
+}
+
+// markDown unbinds the local port so further connects are refused, and closes
+// every connection already in flight so clients see the drop and reconnect
+// instead of blocking forever on a socket whose far end is gone.
+func (p *proxy) markDown(cause error) {
+	p.mu.Lock()
+	was := p.state
+	p.state = stateDown
+	ln := p.ln
+	p.ln = nil
+	conns := make([]net.Conn, 0, len(p.conns))
+	for c := range p.conns {
+		conns = append(conns, c)
+	}
+	clear(p.conns)
+	p.mu.Unlock()
+
+	if was != stateDown {
+		log.Printf("tsproxy: %s -> %s: target unreachable: %v (refusing connections, dropped %d in flight)",
+			p.local, p.target, cause, len(conns))
+	}
+	if ln != nil {
+		ln.Close()
+	}
+	for _, c := range conns {
+		c.Close()
+	}
+}
+
+func (p *proxy) accept(ctx context.Context, ln net.Listener) {
+	for {
+		c, err := ln.Accept()
+		if err != nil {
+			// Either markDown closed this listener (expected) or it failed on
+			// its own. Drop it if it's still the live one; the probe loop will
+			// rebind on the next tick.
+			p.mu.Lock()
+			current := p.ln == ln
+			if current {
+				p.ln = nil
+			}
+			p.mu.Unlock()
+			if current {
+				log.Printf("tsproxy: %s -> %s: accept: %v", p.local, p.target, err)
+				ln.Close()
+				p.nudge()
+			}
+			return
+		}
+		if !p.track(c) {
+			// Raced with markDown: the target went away between Accept and
+			// here, so this connection is already condemned.
+			c.Close()
+			continue
+		}
+		go p.handle(ctx, c)
+	}
+}
+
+// track registers a connection for mass close on target loss. It reports false
+// if the target is already down, in which case the connection is not tracked.
+func (p *proxy) track(c net.Conn) bool {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.state != stateUp {
+		return false
+	}
+	p.conns[c] = struct{}{}
+	return true
+}
+
+func (p *proxy) untrack(c net.Conn) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	delete(p.conns, c)
+}
+
+func (p *proxy) handle(ctx context.Context, in net.Conn) {
+	defer p.untrack(in)
+	defer in.Close()
+
+	out, err := p.dial.Dial(ctx, "tcp", p.target)
+	if err != nil {
+		log.Printf("dial %s: %v", p.target, err)
+		p.nudge()
 		return
 	}
 	defer out.Close()

main_test.go

@@ -0,0 +1,208 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"io"
+	"net"
+	"sync"
+	"testing"
+	"time"
+)
+
+// fakeTarget stands in for a tailnet target. Its echo server always runs; the
+// reachable flag only gates *new* dials. That models the failure this is meant
+// to catch: a tailnet peer disappears, new dials fail, but connections already
+// established just hang forever with no error from the userspace TCP stack.
+type fakeTarget struct {
+	ln net.Listener
+
+	mu        sync.Mutex
+	reachable bool
+}
+
+func newFakeTarget(t *testing.T) *fakeTarget {
+	t.Helper()
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("fake target listen: %v", err)
+	}
+	ft := &fakeTarget{ln: ln, reachable: true}
+	t.Cleanup(func() { ln.Close() })
+	go func() {
+		for {
+			c, err := ln.Accept()
+			if err != nil {
+				return
+			}
+			go func() { io.Copy(c, c); c.Close() }()
+		}
+	}()
+	return ft
+}
+
+func (f *fakeTarget) setReachable(v bool) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.reachable = v
+}
+
+func (f *fakeTarget) Dial(ctx context.Context, network, addr string) (net.Conn, error) {
+	f.mu.Lock()
+	ok := f.reachable
+	f.mu.Unlock()
+	if !ok {
+		return nil, errors.New("no route to host")
+	}
+	var d net.Dialer
+	return d.DialContext(ctx, network, f.ln.Addr().String())
+}
+
+// freePort returns an address that is currently bindable.
+func freePort(t *testing.T) string {
+	t.Helper()
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("free port: %v", err)
+	}
+	addr := ln.Addr().String()
+	ln.Close()
+	return addr
+}
+
+func startProxy(t *testing.T, ft *fakeTarget, local string) *proxy {
+	t.Helper()
+	p := &proxy{
+		dial:     ft,
+		local:    local,
+		target:   "target:1234",
+		interval: 20 * time.Millisecond,
+		timeout:  time.Second,
+		recheck:  make(chan struct{}, 1),
+		conns:    make(map[net.Conn]struct{}),
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+	go func() { defer close(done); p.run(ctx) }()
+	t.Cleanup(func() {
+		cancel()
+		<-done
+	})
+	return p
+}
+
+// waitFor polls cond until it holds or the deadline passes.
+func waitFor(t *testing.T, what string, cond func() bool) {
+	t.Helper()
+	deadline := time.Now().Add(3 * time.Second)
+	for time.Now().Before(deadline) {
+		if cond() {
+			return
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	t.Fatalf("timed out waiting for %s", what)
+}
+
+func localPortRefused(local string) bool {
+	c, err := net.DialTimeout("tcp", local, 500*time.Millisecond)
+	if err != nil {
+		return true
+	}
+	c.Close()
+	return false
+}
+
+// An unreachable target must mean a refused connect, not a successful connect
+// followed by an immediate EOF.
+func TestUnreachableTargetRefusesConnections(t *testing.T) {
+	ft := newFakeTarget(t)
+	ft.setReachable(false)
+	local := freePort(t)
+	startProxy(t, ft, local)
+
+	// Give the probe loop several ticks; the port must never come up.
+	time.Sleep(200 * time.Millisecond)
+	if !localPortRefused(local) {
+		t.Fatal("connect succeeded while target was unreachable; want refused")
+	}
+
+	// And it must start accepting once the target comes back.
+	ft.setReachable(true)
+	waitFor(t, "local port to accept after target recovery", func() bool {
+		return !localPortRefused(local)
+	})
+}
+
+func TestForwardsData(t *testing.T) {
+	ft := newFakeTarget(t)
+	local := freePort(t)
+	startProxy(t, ft, local)
+	waitFor(t, "local port to accept", func() bool { return !localPortRefused(local) })
+
+	c, err := net.Dial("tcp", local)
+	if err != nil {
+		t.Fatalf("dial local: %v", err)
+	}
+	defer c.Close()
+
+	if _, err := c.Write([]byte("ping")); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	buf := make([]byte, 4)
+	c.SetReadDeadline(time.Now().Add(3 * time.Second))
+	if _, err := io.ReadFull(c, buf); err != nil {
+		t.Fatalf("read echo: %v", err)
+	}
+	if string(buf) != "ping" {
+		t.Fatalf("echo = %q, want %q", buf, "ping")
+	}
+}
+
+// The reported bug: when the proxy loses the target, an already-established
+// local socket must be closed so the client's read fails and it reconnects,
+// rather than blocking forever on a half-dead connection.
+func TestTargetLossClosesLocalConnection(t *testing.T) {
+	ft := newFakeTarget(t)
+	local := freePort(t)
+	startProxy(t, ft, local)
+	waitFor(t, "local port to accept", func() bool { return !localPortRefused(local) })
+
+	c, err := net.Dial("tcp", local)
+	if err != nil {
+		t.Fatalf("dial local: %v", err)
+	}
+	defer c.Close()
+
+	// Establish that the connection is live and forwarding.
+	if _, err := c.Write([]byte("ping")); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	buf := make([]byte, 4)
+	c.SetReadDeadline(time.Now().Add(3 * time.Second))
+	if _, err := io.ReadFull(c, buf); err != nil {
+		t.Fatalf("read echo: %v", err)
+	}
+
+	// Target vanishes. The echo server still holds the far end open, so
+	// nothing but the probe loop can notice.
+	ft.setReachable(false)
+
+	// The client's blocking read must return, and reasonably promptly.
+	c.SetReadDeadline(time.Now().Add(3 * time.Second))
+	if _, err := c.Read(buf); err == nil {
+		t.Fatal("read succeeded after target loss; want the connection closed")
+	} else if errors.Is(err, io.EOF) || isTimeout(err) {
+		if isTimeout(err) {
+			t.Fatal("read blocked after target loss; local socket was never closed")
+		}
+	}
+
+	// New connects must be refused too.
+	waitFor(t, "local port to be refused", func() bool { return localPortRefused(local) })
+}
+
+func isTimeout(err error) bool {
+	var ne net.Error
+	return errors.As(err, &ne) && ne.Timeout()
+}