diff --git a/server/cmd/echolot-server/main.go b/server/cmd/echolot-server/main.go
index c0e1929..771c2c7 100644
--- a/server/cmd/echolot-server/main.go
+++ b/server/cmd/echolot-server/main.go
@@ -38,7 +38,9 @@ import (
 	"echo-lot.app/server/internal/selfupdate"
 	"echo-lot.app/server/internal/session"
 	"echo-lot.app/server/internal/store"
+	"echo-lot.app/server/internal/stun"
 	"echo-lot.app/server/internal/system"
+	"echo-lot.app/server/internal/tcpecho"
 )
 
 // Version is stamped via -ldflags "-X main.Version=v1.2.3" in CI.
@@ -95,9 +97,20 @@ func serve(cfg *config.Config) error {
 	slog.Info("control-plane certificate", "pin-sha256", pin)
 
 	sessions := session.NewManager(15 * time.Minute)
+	dp := &dataplane.Server{Sessions: sessions}
+	tcpSrv := &tcpecho.Server{}
+
+	caps := []string{"udp-probe", "delayed-echo", "connect-back"}
+	if len(config.Addrs(cfg.TCPListen)) > 0 {
+		caps = append(caps, "tcp-echo")
+	}
+
 	ctl := &control.Server{
 		Store: st, Sessions: sessions, Name: cfg.Name,
-		UDPPort: mustPort(firstAddr(cfg.UDPListen)), TCPPort: mustPort(firstAddr(cfg.TCPListen)), PinB64: pin,
+		UDPPort: mustPort(firstAddr(cfg.UDPListen)), TCPPort: mustPort(firstAddr(cfg.TCPListen)),
+		StunPort: mustPort(firstAddr(cfg.StunListen)), PinB64: pin,
+		DelayedEcho: dp.SendDelayedEcho,
+		TCPRecent:   func(ip string) any { return tcpSrv.RecentFor(ip) },
 	}
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
@@ -156,14 +169,45 @@ func serve(cfg *config.Config) error {
 			return fmt.Errorf("udp listen %s: %w", addr, err)
 		}
 		udpConns = append(udpConns, conn)
-		dp := &dataplane.Server{Sessions: sessions}
 		go func(a string, c *net.UDPConn) {
 			errCh <- fmt.Errorf("udp %s: %w", a, dp.Serve(c))
 		}(addr, conn)
 	}
 
+	// TCP echo (spec §4)
+	var tcpLns []net.Listener
+	for _, addr := range config.Addrs(cfg.TCPListen) {
+		ln, err := net.Listen("tcp", addr)
+		if err != nil {
+			return fmt.Errorf("tcp listen %s: %w", addr, err)
+		}
+		tcpLns = append(tcpLns, ln)
+		go func(a string, l net.Listener) {
+			errCh <- fmt.Errorf("tcp %s: %w", a, tcpSrv.Serve(l))
+		}(addr, ln)
+	}
+
+	// STUN (spec §4) — advertises stun-5780 only with ≥2 same-family addrs.
+	var stunSrv *stun.Server
+	if stunAddrs := config.Addrs(cfg.StunListen); len(stunAddrs) > 0 {
+		stunSrv, err = stun.Listen(stunAddrs)
+		if err != nil {
+			return fmt.Errorf("stun listen: %w", err)
+		}
+		go func() { errCh <- fmt.Errorf("stun: %w", stunSrv.Serve()) }()
+		if stunSrv.Has5780() {
+			ctl.Capabilities = append(caps, "stun-5780")
+		} else {
+			ctl.Capabilities = append(caps, "stun-basic")
+		}
+	} else {
+		ctl.Capabilities = caps
+	}
+
 	slog.Info("listening",
-		"control", ctlAddrs, "admin", cfg.AdminListen, "udp", udpAddrs)
+		"control", ctlAddrs, "admin", cfg.AdminListen, "udp", udpAddrs,
+		"tcp", config.Addrs(cfg.TCPListen), "stun", config.Addrs(cfg.StunListen),
+		"capabilities", ctl.Capabilities)
 
 	select {
 	case <-ctx.Done():
@@ -175,6 +219,12 @@ func serve(cfg *config.Config) error {
 		for _, c := range udpConns {
 			_ = c.Close()
 		}
+		for _, l := range tcpLns {
+			_ = l.Close()
+		}
+		if stunSrv != nil {
+			stunSrv.Close()
+		}
 		return nil
 	case err := <-errCh:
 		return err
diff --git a/server/internal/config/config.go b/server/internal/config/config.go
index 4e68f5c..374baab 100644
--- a/server/internal/config/config.go
+++ b/server/internal/config/config.go
@@ -24,8 +24,9 @@ type Config struct {
 	TLSKey        string // ECHOLOT_TLS_KEY / --tls-key
 
 	// Data plane
-	UDPListen string // ECHOLOT_UDP_LISTEN / --udp-listen (spec default port 8442)
-	TCPListen string // ECHOLOT_TCP_LISTEN / --tcp-listen (spec default port 8441)
+	UDPListen  string // ECHOLOT_UDP_LISTEN / --udp-listen (spec default port 8442)
+	TCPListen  string // ECHOLOT_TCP_LISTEN / --tcp-listen (spec default port 8441)
+	StunListen string // ECHOLOT_STUN_LISTEN / --stun-listen (spec default 3478; empty disables)
 
 	// Admin UI / health listener (spec §7: localhost-only by default)
 	AdminListen string // ECHOLOT_ADMIN_LISTEN / --admin-listen
@@ -65,6 +66,7 @@ func Load(args []string) (*Config, *Actions, error) {
 	fs.StringVar(&c.TLSKey, "tls-key", envOr("TLS_KEY", ""), "TLS key path (empty: self-signed in state dir)")
 	fs.StringVar(&c.UDPListen, "udp-listen", envOr("UDP_LISTEN", ":8442"), "UDP data-plane listen address(es), comma-separated")
 	fs.StringVar(&c.TCPListen, "tcp-listen", envOr("TCP_LISTEN", ":8441"), "TCP echo listen address(es), comma-separated")
+	fs.StringVar(&c.StunListen, "stun-listen", envOr("STUN_LISTEN", ":3478"), "STUN listen address(es), comma-separated; empty disables (spec §4)")
 	fs.StringVar(&c.AdminListen, "admin-listen", envOr("ADMIN_LISTEN", "127.0.0.1:8444"), "admin/health listen address (keep localhost)")
 	fs.StringVar(&c.StateDir, "state-dir", envOr("STATE_DIR", defaultStateDir()), "state directory (device store, generated TLS)")
 	fs.StringVar(&c.Name, "name", envOr("NAME", "echolot"), "server profile name")
diff --git a/server/internal/control/control.go b/server/internal/control/control.go
index e4a177e..bfd43ab 100644
--- a/server/internal/control/control.go
+++ b/server/internal/control/control.go
@@ -7,14 +7,19 @@
 package control
 
 import (
+	"crypto/rand"
 	"crypto/sha256"
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/base64"
+	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"log/slog"
+	"net"
 	"net/http"
 	"net/netip"
+	"strconv"
 	"strings"
 	"time"
 
@@ -29,12 +34,19 @@ type Server struct {
 	Store    *store.Store
 	Sessions *session.Manager
 	Name     string
-	// Targets/capabilities for the profile response. The skeleton offers only
+	// Targets/capabilities for the profile response. The server offers only
 	// what it actually implements; the registry grows with the code.
-	UDPPort int
-	TCPPort int
+	UDPPort  int
+	TCPPort  int
+	StunPort int
 	// SPKI pin of the serving cert, for the profile's pins[] field.
 	PinB64 string
+	// Capabilities as computed at startup from what is actually wired up.
+	Capabilities []string
+	// TCPRecent returns recent TCP-echo connections for a source IP (may be nil).
+	TCPRecent func(ip string) any
+	// DelayedEcho schedules/sends a DELAYED_ECHO for a session (may be nil).
+	DelayedEcho func(sess *session.Session, actionID string) error
 }
 
 func (s *Server) Handler() http.Handler {
@@ -43,11 +55,131 @@ func (s *Server) Handler() http.Handler {
 	mux.HandleFunc("GET /v1/profile", s.profile)
 	mux.HandleFunc("POST /v1/sessions", s.newSession)
 	mux.HandleFunc("DELETE /v1/sessions/{id}", s.deleteSession)
-	// TODO(spec §5, §6): /v1/sessions/{id}/actions, /v1/sessions/{id}/observations
-	// TODO(spec §4): POST /v1/echo, GET /v1/tls-reference
+	mux.HandleFunc("GET /v1/sessions/{id}/observations", s.observations)
+	mux.HandleFunc("POST /v1/sessions/{id}/actions", s.actions)
+	// TODO(spec §4): POST /v1/echo, GET /v1/tls-reference; TLS-echo/JA4
+	// TODO(spec §5): downtrain, big_send, frag_send, throughput
 	return mux
 }
 
+// sessionAuth resolves {id} and requires the bearer to be the owning device.
+func (s *Server) sessionAuth(w http.ResponseWriter, r *http.Request) *session.Session {
+	dev := s.Store.DeviceByCredential(bearer(r))
+	if dev == nil {
+		writeJSON(w, http.StatusUnauthorized, map[string]string{"error": "unknown credential"})
+		return nil
+	}
+	sess := s.Sessions.ByID(r.PathValue("id"))
+	if sess == nil || sess.Device != dev.ID {
+		writeJSON(w, http.StatusNotFound, map[string]string{"error": "no such session"})
+		return nil
+	}
+	return sess
+}
+
+// observations is spec §6 — everything the server witnessed for a session.
+func (s *Server) observations(w http.ResponseWriter, r *http.Request) {
+	sess := s.sessionAuth(w, r)
+	if sess == nil {
+		return
+	}
+	packetsSeen, udp, cb := sess.Observations()
+	var tcp any
+	if s.TCPRecent != nil {
+		// Correlate by source IP: TCP echo carries no session id on the wire.
+		if ds := sess.DataSource(); ds.IsValid() {
+			tcp = s.TCPRecent(ds.Addr().Unmap().String())
+		} else if sess.ControlSource.IsValid() {
+			tcp = s.TCPRecent(sess.ControlSource.Unmap().String())
+		}
+	}
+	writeJSON(w, http.StatusOK, map[string]any{
+		"udp":          map[string]any{"packets_seen": packetsSeen, "packets": udp},
+		"tcp":          tcp,
+		"connect_back": cb,
+		// TODO(spec §6): http echo records, dns_canary
+	})
+}
+
+// actions is spec §5 — authenticated asymmetric operations. Implemented:
+// delayed_echo, connect_back. Destination is ALWAYS the session's observed
+// source (data-plane source; connect_back uses the control-plane source).
+func (s *Server) actions(w http.ResponseWriter, r *http.Request) {
+	sess := s.sessionAuth(w, r)
+	if sess == nil {
+		return
+	}
+	var req struct {
+		Action   string `json:"action"`
+		DelayS   int    `json:"delay_s"`
+		Protocol string `json:"protocol"`
+		Port     int    `json:"port"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "bad body"})
+		return
+	}
+	actionID := randomID()
+	switch req.Action {
+	case "delayed_echo":
+		if s.DelayedEcho == nil {
+			writeJSON(w, http.StatusNotImplemented, map[string]string{"error": "delayed_echo not wired"})
+			return
+		}
+		delay := min(max(req.DelayS, 1), 600)
+		if !sess.DataSource().IsValid() {
+			writeJSON(w, http.StatusConflict, map[string]string{"error": "no data-plane traffic seen yet — send an ECHO first"})
+			return
+		}
+		time.AfterFunc(time.Duration(delay)*time.Second, func() {
+			if err := s.DelayedEcho(sess, actionID); err != nil {
+				slog.Debug("delayed echo failed", "err", err)
+			}
+		})
+		writeJSON(w, http.StatusAccepted, map[string]any{"action_id": actionID, "delay_s": delay})
+	case "connect_back":
+		if req.Port < 1 || req.Port > 65535 || (req.Protocol != "tcp" && req.Protocol != "udp") {
+			writeJSON(w, http.StatusBadRequest, map[string]string{"error": "connect_back needs protocol tcp|udp and a port"})
+			return
+		}
+		target := net.JoinHostPort(sess.ControlSource.Unmap().String(), strconv.Itoa(req.Port))
+		go func() {
+			start := time.Now()
+			conn, err := net.DialTimeout(req.Protocol, target, 5*time.Second)
+			res := session.ConnectBackResult{ActionID: actionID, RttMs: float64(time.Since(start).Microseconds()) / 1000}
+			switch {
+			case err == nil:
+				res.Result = "connected"
+				if req.Protocol == "udp" {
+					// UDP "dial" always succeeds locally; send one datagram
+					// so the client actually observes something.
+					_, _ = conn.Write([]byte("echolot-connect-back " + actionID))
+				}
+				conn.Close()
+			case isTimeout(err):
+				res.Result = "timeout"
+			default:
+				res.Result = "refused"
+			}
+			sess.RecordConnectBack(res)
+		}()
+		writeJSON(w, http.StatusAccepted, map[string]any{"action_id": actionID, "target": target})
+	default:
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "unknown or unimplemented action"})
+	}
+}
+
+func isTimeout(err error) bool {
+	var ne net.Error
+	return errors.As(err, &ne) && ne.Timeout()
+}
+
+func randomID() string {
+	var b [8]byte
+	_, _ = rand.Read(b[:])
+	return hex.EncodeToString(b[:])
+}
+
 func bearer(r *http.Request) string {
 	h := r.Header.Get("Authorization")
 	if v, ok := strings.CutPrefix(h, "Bearer "); ok {
@@ -103,12 +235,13 @@ func (s *Server) profile(w http.ResponseWriter, r *http.Request) {
 		// source_url makes GPL §6 compliance mechanical for operators of
 		// modified builds and gives clients provenance for the measurement.
 		"source_url":   "", // TODO: stamp from build metadata
-		"capabilities": []string{"udp-probe"},
+		"capabilities": s.Capabilities,
 		"targets": []map[string]any{{
-			"id":       s.Name,
-			"ip4":      host, // TODO: explicit configured addresses, v6, second STUN addr
-			"udp_port": s.UDPPort,
-			"tcp_port": s.TCPPort,
+			"id":        s.Name,
+			"ip4":       host, // TODO: explicit configured addresses, v6, second STUN addr
+			"udp_port":  s.UDPPort,
+			"tcp_port":  s.TCPPort,
+			"stun_port": s.StunPort,
 		}},
 		"pins":      []string{"pin-sha256:" + s.PinB64},
 		"next_pins": []string{},
diff --git a/server/internal/dataplane/udp.go b/server/internal/dataplane/udp.go
index 28c5c05..18236e8 100644
--- a/server/internal/dataplane/udp.go
+++ b/server/internal/dataplane/udp.go
@@ -11,9 +11,11 @@ import (
 	"crypto/hmac"
 	"crypto/sha256"
 	"encoding/binary"
+	"fmt"
 	"log/slog"
 	"net"
 	"net/netip"
+	"sync"
 	"time"
 
 	"echo-lot.app/server/internal/session"
@@ -27,6 +29,7 @@ const (
 	TypeEchoResp    = 0x02
 	TypeTimesyncReq = 0x07
 	TypeTimesyncRsp = 0x08
+	TypeDelayedEcho = 0x0B
 )
 
 type Server struct {
@@ -34,13 +37,22 @@ type Server struct {
 	// Epoch for server-side t_rx/t_tx: process start; observation consumers
 	// only need differences plus the timesync exchange, not absolute time.
 	start time.Time
+
+	mu    sync.Mutex
+	conns []*net.UDPConn
 }
 
+// Serve runs the read loop for one socket; call once per bound address.
+// The socket is retained so actions (delayed echo) can pick a family-matching
+// sender later.
 func (s *Server) Serve(conn *net.UDPConn) error {
-	s.start = time.Now()
+	s.mu.Lock()
+	if s.start.IsZero() {
+		s.start = time.Now()
+	}
+	s.conns = append(s.conns, conn)
+	s.mu.Unlock()
 	buf := make([]byte, 65535)
-	oob := make([]byte, 0)
-	_ = oob // TODO: recvmsg w/ IP_RECVTOS+IP_RECVTTL via golang.org/x/net for TTL/DSCP/ECN observation
 	for {
 		n, raddr, err := conn.ReadFromUDPAddrPort(buf)
 		if err != nil {
@@ -51,6 +63,36 @@ func (s *Server) Serve(conn *net.UDPConn) error {
 	}
 }
 
+// connFor picks a retained socket whose family matches the target.
+func (s *Server) connFor(target netip.AddrPort) *net.UDPConn {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	want4 := target.Addr().Unmap().Is4()
+	for _, c := range s.conns {
+		la := c.LocalAddr().(*net.UDPAddr).AddrPort()
+		if la.Addr().Unmap().Is4() == want4 {
+			return c
+		}
+	}
+	return nil
+}
+
+// SendDelayedEcho fires one DELAYED_ECHO packet at the session's observed
+// data-plane source (spec §5: the NAT-mapping-lifetime primitive). The
+// payload carries the action id for correlation.
+func (s *Server) SendDelayedEcho(sess *session.Session, actionID string) error {
+	target := sess.DataSource()
+	if !target.IsValid() {
+		return fmt.Errorf("session has no observed data-plane source yet")
+	}
+	conn := s.connFor(target)
+	if conn == nil {
+		return fmt.Errorf("no data-plane socket matches target family")
+	}
+	s.send(conn, target, sess, TypeDelayedEcho, 0, []byte(actionID))
+	return nil
+}
+
 // handle enforces spec §3.1/§3.4: unknown prefix, bad HMAC, expired session,
 // replayed seq → silent drop, never a response.
 func (s *Server) handle(conn *net.UDPConn, raddr netip.AddrPort, pkt []byte, tRxNs int64) {
@@ -80,6 +122,10 @@ func (s *Server) handle(conn *net.UDPConn, raddr netip.AddrPort, pkt []byte, tRx
 		return
 	}
 	sess.NoteDataSource(raddr)
+	sess.RecordUDP(session.UDPObservation{
+		Seq: seq, TRxNs: tRxNs, TTxNs: time.Since(s.start).Nanoseconds(),
+		Src: raddr.String(), Size: len(pkt), Type: typ,
+	})
 
 	switch typ {
 	case TypeEchoReq:
diff --git a/server/internal/session/session.go b/server/internal/session/session.go
index 099ca44..3a39f24 100644
--- a/server/internal/session/session.go
+++ b/server/internal/session/session.go
@@ -31,6 +31,64 @@ type Session struct {
 	// a bitmask of the 1024 preceding.
 	maxSeq uint32
 	window [16]uint64
+
+	// Observations (spec §6): per-packet UDP view + connect-back results.
+	packetsSeen uint64
+	udpObs      []UDPObservation // ring, newest last, cap obsCap
+	connectBack []ConnectBackResult
+}
+
+const obsCap = 4096
+
+// UDPObservation is the server's witnessed view of one data-plane packet.
+type UDPObservation struct {
+	Seq   uint32 `json:"seq"`
+	TRxNs int64  `json:"t_rx_ns"`
+	TTxNs int64  `json:"t_tx_ns"`
+	Src   string `json:"src"`
+	Size  int    `json:"size"`
+	Type  uint8  `json:"type"`
+}
+
+// ConnectBackResult records one connect-back action outcome.
+type ConnectBackResult struct {
+	ActionID string  `json:"action_id"`
+	Result   string  `json:"result"` // connected | refused | timeout
+	RttMs    float64 `json:"rtt_ms"`
+}
+
+// RecordUDP appends a packet observation (ring-capped).
+func (s *Session) RecordUDP(o UDPObservation) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.packetsSeen++
+	if len(s.udpObs) >= obsCap {
+		s.udpObs = s.udpObs[1:]
+	}
+	s.udpObs = append(s.udpObs, o)
+}
+
+// RecordConnectBack appends a connect-back outcome.
+func (s *Session) RecordConnectBack(r ConnectBackResult) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.connectBack = append(s.connectBack, r)
+}
+
+// Observations returns a copy of everything witnessed so far.
+func (s *Session) Observations() (packetsSeen uint64, udp []UDPObservation, cb []ConnectBackResult) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.packetsSeen, append([]UDPObservation(nil), s.udpObs...),
+		append([]ConnectBackResult(nil), s.connectBack...)
+}
+
+// DataSource returns the last verified data-plane source (invalid when the
+// session has not sent data-plane traffic yet).
+func (s *Session) DataSource() netip.AddrPort {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.dataSource
 }
 
 // KeySalt returns nothing — the salt is not retained after derivation; it is
@@ -87,6 +145,20 @@ func (m *Manager) ByWirePrefix(prefix [8]byte) *Session {
 	return s
 }
 
+// ByID resolves a full session id (sessions are keyed by their wire prefix).
+func (m *Manager) ByID(id string) *Session {
+	if len(id) < 16 {
+		return nil
+	}
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	s := m.byPrefix[id[:16]]
+	if s == nil || s.ID != id || time.Now().After(s.Expires) {
+		return nil
+	}
+	return s
+}
+
 func (m *Manager) Delete(id string) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
diff --git a/server/internal/stun/stun.go b/server/internal/stun/stun.go
new file mode 100644
index 0000000..de75080
--- /dev/null
+++ b/server/internal/stun/stun.go
@@ -0,0 +1,266 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Package stun implements an unmodified RFC 5389 STUN binding responder with
+// the RFC 5780 NAT-behavior-discovery attributes (OTHER-ADDRESS,
+// RESPONSE-ORIGIN, CHANGE-REQUEST) when alternate addresses are available.
+// No custom framing — interop with existing STUN tooling is a feature
+// (spec §4). Each configured primary address gets two sockets: the given
+// port and port+1 (the RFC 5780 alternate-port convention).
+package stun
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+	"log/slog"
+	"net"
+	"net/netip"
+)
+
+const (
+	magicCookie = 0x2112A442
+
+	typeBindingRequest = 0x0001
+	typeBindingSuccess = 0x0101
+
+	attrChangeRequest  = 0x0003
+	attrXorMapped      = 0x0020
+	attrSoftware       = 0x8022
+	attrResponseOrigin = 0x802B
+	attrOtherAddress   = 0x802C
+
+	changeIP   = 0x04
+	changePort = 0x02
+)
+
+// sock is one bound socket, addressable by (address index, port index).
+type sock struct {
+	conn *net.UDPConn
+	addr netip.AddrPort
+}
+
+// Server holds the socket grid: addrs × {primary, alternate} ports.
+type Server struct {
+	// socks[i][0] = primary port, socks[i][1] = alt port for address i.
+	socks [][2]*sock
+}
+
+// Listen binds primary+alternate sockets for every address. Addresses are
+// "ip:port" specs; the alternate port is port+1.
+func Listen(addrs []string) (*Server, error) {
+	s := &Server{}
+	for _, spec := range addrs {
+		ap, err := netip.ParseAddrPort(spec)
+		if err != nil {
+			return nil, err
+		}
+		var pair [2]*sock
+		for i, port := range []uint16{ap.Port(), ap.Port() + 1} {
+			bind := netip.AddrPortFrom(ap.Addr(), port)
+			conn, err := net.ListenUDP("udp", net.UDPAddrFromAddrPort(bind))
+			if err != nil {
+				s.Close()
+				return nil, err
+			}
+			pair[i] = &sock{conn: conn, addr: bind}
+		}
+		s.socks = append(s.socks, pair)
+	}
+	return s, nil
+}
+
+func (s *Server) Close() {
+	for _, pair := range s.socks {
+		for _, sk := range pair {
+			if sk != nil {
+				sk.conn.Close()
+			}
+		}
+	}
+}
+
+// Has5780 reports whether any address family has ≥2 addresses — the
+// prerequisite for full NAT behavior discovery.
+func (s *Server) Has5780() bool {
+	var v4, v6 int
+	for _, pair := range s.socks {
+		if pair[0].addr.Addr().Is4() || pair[0].addr.Addr().Is4In6() {
+			v4++
+		} else {
+			v6++
+		}
+	}
+	return v4 >= 2 || v6 >= 2
+}
+
+// Serve starts one read loop per socket and blocks until the first error.
+func (s *Server) Serve() error {
+	errCh := make(chan error, len(s.socks)*2)
+	for ai := range s.socks {
+		for pi := range s.socks[ai] {
+			go func(ai, pi int) { errCh <- s.loop(ai, pi) }(ai, pi)
+		}
+	}
+	return <-errCh
+}
+
+func (s *Server) loop(ai, pi int) error {
+	sk := s.socks[ai][pi]
+	buf := make([]byte, 1500)
+	for {
+		n, raddr, err := sk.conn.ReadFromUDPAddrPort(buf)
+		if err != nil {
+			return err
+		}
+		s.handle(ai, pi, buf[:n], raddr)
+	}
+}
+
+// otherAddr finds the "diagonal" alternate for RFC 5780: different address
+// (same family), different port. Returns nil when there is none.
+func (s *Server) other(ai int, sameFamily bool, fam4 bool) int {
+	for i, pair := range s.socks {
+		if i == ai {
+			continue
+		}
+		is4 := pair[0].addr.Addr().Is4() || pair[0].addr.Addr().Is4In6()
+		if !sameFamily || is4 == fam4 {
+			return i
+		}
+	}
+	return -1
+}
+
+func (s *Server) handle(ai, pi int, pkt []byte, raddr netip.AddrPort) {
+	if len(pkt) < 20 || binary.BigEndian.Uint16(pkt[0:2]) != typeBindingRequest {
+		return
+	}
+	if binary.BigEndian.Uint32(pkt[4:8]) != magicCookie {
+		return
+	}
+	msgLen := int(binary.BigEndian.Uint16(pkt[2:4]))
+	if 20+msgLen > len(pkt) {
+		return
+	}
+	var txid [12]byte
+	copy(txid[:], pkt[8:20])
+
+	// Parse CHANGE-REQUEST if present (RFC 5780 §7.2).
+	var change byte
+	for off := 20; off+4 <= 20+msgLen; {
+		at := binary.BigEndian.Uint16(pkt[off : off+2])
+		al := int(binary.BigEndian.Uint16(pkt[off+2 : off+4]))
+		if off+4+al > len(pkt) {
+			break
+		}
+		if at == attrChangeRequest && al >= 4 {
+			change = pkt[off+7]
+		}
+		off += 4 + al + (4-al%4)%4 // attributes are 32-bit aligned
+	}
+
+	// Pick the responding socket per CHANGE-REQUEST.
+	fam4 := raddr.Addr().Is4() || raddr.Addr().Is4In6()
+	rai, rpi := ai, pi
+	if change&changeIP != 0 {
+		if o := s.other(ai, true, fam4); o >= 0 {
+			rai = o
+		} else {
+			return // cannot honor — RFC says error response; silence is safer for a probe target
+		}
+	}
+	if change&changePort != 0 {
+		rpi = 1 - pi
+	}
+	responder := s.socks[rai][rpi]
+
+	resp := buildResponse(txid, raddr, responder.addr, s.otherAddress(ai, fam4))
+	if _, err := responder.conn.WriteToUDPAddrPort(resp, raddr); err != nil {
+		slog.Debug("stun write failed", "to", raddr, "err", err)
+	}
+}
+
+// otherAddress computes the OTHER-ADDRESS attribute value (alt IP, alt port)
+// for the client's family, or an invalid AddrPort when unavailable.
+func (s *Server) otherAddress(ai int, fam4 bool) netip.AddrPort {
+	if o := s.other(ai, true, fam4); o >= 0 {
+		return s.socks[o][1].addr
+	}
+	return netip.AddrPort{}
+}
+
+func buildResponse(txid [12]byte, mapped, origin, other netip.AddrPort) []byte {
+	attrs := xorMappedAttr(attrXorMapped, mapped, txid)
+	attrs = append(attrs, addrAttr(attrResponseOrigin, origin)...)
+	if other.IsValid() {
+		attrs = append(attrs, addrAttr(attrOtherAddress, other)...)
+	}
+	sw := []byte("echolot")
+	attrs = append(attrs, attrHeader(attrSoftware, len(sw))...)
+	attrs = append(attrs, pad4(sw)...)
+
+	msg := make([]byte, 20, 20+len(attrs))
+	binary.BigEndian.PutUint16(msg[0:2], typeBindingSuccess)
+	binary.BigEndian.PutUint16(msg[2:4], uint16(len(attrs)))
+	binary.BigEndian.PutUint32(msg[4:8], magicCookie)
+	copy(msg[8:20], txid[:])
+	return append(msg, attrs...)
+}
+
+func attrHeader(typ uint16, valLen int) []byte {
+	h := make([]byte, 4)
+	binary.BigEndian.PutUint16(h[0:2], typ)
+	binary.BigEndian.PutUint16(h[2:4], uint16(valLen))
+	return h
+}
+
+func pad4(b []byte) []byte {
+	for len(b)%4 != 0 {
+		b = append(b, 0)
+	}
+	return b
+}
+
+// addrValue encodes the RFC 5389 address structure (family, port, address).
+func addrValue(ap netip.AddrPort) []byte {
+	addr := ap.Addr().Unmap()
+	if addr.Is4() {
+		v := make([]byte, 8)
+		v[1] = 0x01
+		binary.BigEndian.PutUint16(v[2:4], ap.Port())
+		a4 := addr.As4()
+		copy(v[4:], a4[:])
+		return v
+	}
+	v := make([]byte, 20)
+	v[1] = 0x02
+	binary.BigEndian.PutUint16(v[2:4], ap.Port())
+	a16 := addr.As16()
+	copy(v[4:], a16[:])
+	return v
+}
+
+func addrAttr(typ uint16, ap netip.AddrPort) []byte {
+	v := addrValue(ap)
+	return append(attrHeader(typ, len(v)), v...)
+}
+
+// xorMappedAttr encodes XOR-MAPPED-ADDRESS (RFC 5389 §15.2).
+func xorMappedAttr(typ uint16, ap netip.AddrPort, txid [12]byte) []byte {
+	v := addrValue(ap)
+	binary.BigEndian.PutUint16(v[2:4], ap.Port()^uint16(magicCookie>>16))
+	var key [16]byte
+	binary.BigEndian.PutUint32(key[0:4], magicCookie)
+	copy(key[4:], txid[:])
+	for i := 4; i < len(v); i++ {
+		v[i] ^= key[i-4]
+	}
+	return append(attrHeader(typ, len(v)), v...)
+}
+
+// NewTxID is exported for tests and client code.
+func NewTxID() [12]byte {
+	var t [12]byte
+	_, _ = rand.Read(t[:])
+	return t
+}
diff --git a/server/internal/stun/stun_test.go b/server/internal/stun/stun_test.go
new file mode 100644
index 0000000..43de78f
--- /dev/null
+++ b/server/internal/stun/stun_test.go
@@ -0,0 +1,137 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package stun
+
+import (
+	"encoding/binary"
+	"net"
+	"net/netip"
+	"testing"
+	"time"
+)
+
+// bindingRequest builds a minimal RFC 5389 binding request.
+func bindingRequest(txid [12]byte, change byte) []byte {
+	var attrs []byte
+	if change != 0 {
+		attrs = append(attrs, attrHeader(attrChangeRequest, 4)...)
+		attrs = append(attrs, 0, 0, 0, change)
+	}
+	msg := make([]byte, 20, 20+len(attrs))
+	binary.BigEndian.PutUint16(msg[0:2], typeBindingRequest)
+	binary.BigEndian.PutUint16(msg[2:4], uint16(len(attrs)))
+	binary.BigEndian.PutUint32(msg[4:8], magicCookie)
+	copy(msg[8:20], txid[:])
+	return append(msg, attrs...)
+}
+
+// parseXorMapped extracts XOR-MAPPED-ADDRESS from a binding success.
+func parseXorMapped(t *testing.T, resp []byte, txid [12]byte) netip.AddrPort {
+	t.Helper()
+	if binary.BigEndian.Uint16(resp[0:2]) != typeBindingSuccess {
+		t.Fatalf("type = %#x, want binding success", resp[0:2])
+	}
+	msgLen := int(binary.BigEndian.Uint16(resp[2:4]))
+	for off := 20; off+4 <= 20+msgLen; {
+		at := binary.BigEndian.Uint16(resp[off : off+2])
+		al := int(binary.BigEndian.Uint16(resp[off+2 : off+4]))
+		if at == attrXorMapped {
+			v := append([]byte(nil), resp[off+4:off+4+al]...)
+			port := binary.BigEndian.Uint16(v[2:4]) ^ uint16(magicCookie>>16)
+			var key [16]byte
+			binary.BigEndian.PutUint32(key[0:4], magicCookie)
+			copy(key[4:], txid[:])
+			for i := 4; i < len(v); i++ {
+				v[i] ^= key[i-4]
+			}
+			if v[1] == 0x01 {
+				return netip.AddrPortFrom(netip.AddrFrom4([4]byte(v[4:8])), port)
+			}
+			return netip.AddrPortFrom(netip.AddrFrom16([16]byte(v[4:20])), port)
+		}
+		off += 4 + al + (4-al%4)%4
+	}
+	t.Fatal("no XOR-MAPPED-ADDRESS in response")
+	return netip.AddrPort{}
+}
+
+func TestBindingAndChangePort(t *testing.T) {
+	// Two loopback "addresses" is not possible portably, so exercise one
+	// address (basic binding + change-port); the change-IP path needs the
+	// two-address grid of a real deployment.
+	srv, err := Listen([]string{"127.0.0.1:0"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	// port 0 twice would collide at 1; rebind explicitly on free ports
+	srv.Close()
+	base := freePort(t)
+	srv, err = Listen([]string{netip.AddrPortFrom(netip.MustParseAddr("127.0.0.1"), base).String()})
+	if err != nil {
+		t.Skipf("cannot bind %d/%d: %v", base, base+1, err)
+	}
+	defer srv.Close()
+	go srv.Serve()
+
+	client, err := net.DialUDP("udp", nil, srv.socks[0][0].conn.LocalAddr().(*net.UDPAddr))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+	client.SetDeadline(time.Now().Add(2 * time.Second))
+
+	txid := NewTxID()
+	client.Write(bindingRequest(txid, 0))
+	buf := make([]byte, 1500)
+	n, err := client.Read(buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	mapped := parseXorMapped(t, buf[:n], txid)
+	want := client.LocalAddr().(*net.UDPAddr).AddrPort()
+	if mapped.Port() != want.Port() {
+		t.Fatalf("mapped port %d, want %d", mapped.Port(), want.Port())
+	}
+
+	// CHANGE-REQUEST(port): response must come from the alternate port.
+	// Dial-connected sockets drop packets from other sources, so use an
+	// unconnected socket and inspect the reply's source.
+	uc, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4(127, 0, 0, 1)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer uc.Close()
+	uc.SetDeadline(time.Now().Add(2 * time.Second))
+	txid2 := NewTxID()
+	uc.WriteToUDPAddrPort(bindingRequest(txid2, changePort), srv.socks[0][0].addr)
+	n, from, err := uc.ReadFromUDPAddrPort(buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if from.Port() != srv.socks[0][1].addr.Port() {
+		t.Fatalf("change-port reply came from %v, want alt port %d", from, srv.socks[0][1].addr.Port())
+	}
+	parseXorMapped(t, buf[:n], txid2)
+}
+
+func freePort(t *testing.T) uint16 {
+	t.Helper()
+	// Find two adjacent free ports for the primary/alternate pair.
+	for tries := 0; tries < 20; tries++ {
+		l, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4(127, 0, 0, 1)})
+		if err != nil {
+			continue
+		}
+		p := l.LocalAddr().(*net.UDPAddr).Port
+		l.Close()
+		l2, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4(127, 0, 0, 1), Port: p + 1})
+		if err != nil {
+			continue
+		}
+		l2.Close()
+		return uint16(p)
+	}
+	t.Skip("no adjacent free UDP ports found")
+	return 0
+}
diff --git a/server/internal/tcpecho/tcpecho.go b/server/internal/tcpecho/tcpecho.go
new file mode 100644
index 0000000..d0b6f84
--- /dev/null
+++ b/server/internal/tcpecho/tcpecho.go
@@ -0,0 +1,91 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Package tcpecho implements the spec §4 TCP echo: after connect the server
+// sends one JSON line with what it observed (source address/port, negotiated
+// MSS and TCP options from TCP_INFO), then byte-echoes until FIN. This is the
+// evidence source for mtu.mss_observed.
+//
+// The TLS/ALPN "elt-echo" variant (ClientHello capture + JA4) is not
+// implemented yet.
+package tcpecho
+
+import (
+	"encoding/json"
+	"io"
+	"net"
+	"sync"
+	"time"
+)
+
+// ConnRecord is what the observations API reports per connection (spec §6).
+type ConnRecord struct {
+	ConnectedAt time.Time `json:"connected_at"`
+	Src         string    `json:"src"`
+	MSS         int       `json:"mss"`
+	Options     []string  `json:"options"`
+}
+
+type Server struct {
+	mu     sync.Mutex
+	recent []ConnRecord // ring, newest last
+}
+
+const recentCap = 1024
+
+func (s *Server) record(r ConnRecord) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if len(s.recent) >= recentCap {
+		s.recent = s.recent[1:]
+	}
+	s.recent = append(s.recent, r)
+}
+
+// RecentFor returns records whose source IP matches ip.
+func (s *Server) RecentFor(ip string) []ConnRecord {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	var out []ConnRecord
+	for _, r := range s.recent {
+		if h, _, err := net.SplitHostPort(r.Src); err == nil && h == ip {
+			out = append(out, r)
+		}
+	}
+	return out
+}
+
+func (s *Server) Serve(ln net.Listener) error {
+	for {
+		conn, err := ln.Accept()
+		if err != nil {
+			return err
+		}
+		go s.handle(conn)
+	}
+}
+
+func (s *Server) handle(conn net.Conn) {
+	defer conn.Close()
+	_ = conn.SetDeadline(time.Now().Add(5 * time.Minute))
+
+	info := tcpInfo(conn) // platform-specific; zero values off-Linux
+	rec := ConnRecord{
+		ConnectedAt: time.Now().UTC(),
+		Src:         conn.RemoteAddr().String(),
+		MSS:         info.MSS,
+		Options:     info.Options,
+	}
+	s.record(rec)
+
+	greeting, _ := json.Marshal(map[string]any{
+		"observed_src": rec.Src,
+		"mss":          rec.MSS,
+		"options":      rec.Options,
+	})
+	if _, err := conn.Write(append(greeting, '\n')); err != nil {
+		return
+	}
+	// Byte-echo until FIN; the client's data is its own to interpret.
+	_, _ = io.Copy(conn, conn)
+}
diff --git a/server/internal/tcpecho/tcpinfo_linux.go b/server/internal/tcpecho/tcpinfo_linux.go
new file mode 100644
index 0000000..da4cb0a
--- /dev/null
+++ b/server/internal/tcpecho/tcpinfo_linux.go
@@ -0,0 +1,69 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build linux
+
+package tcpecho
+
+import (
+	"encoding/binary"
+	"net"
+	"syscall"
+	"unsafe"
+)
+
+type connInfo struct {
+	MSS     int
+	Options []string
+}
+
+// tcpInfo reads TCP_INFO via getsockopt. Only the head of struct tcp_info is
+// needed: 8 header bytes (state..wscale flags) then u32 rto, ato, snd_mss,
+// rcv_mss — layout is part of the kernel ABI and stable.
+func tcpInfo(conn net.Conn) connInfo {
+	tc, ok := conn.(*net.TCPConn)
+	if !ok {
+		return connInfo{}
+	}
+	raw, err := tc.SyscallConn()
+	if err != nil {
+		return connInfo{}
+	}
+	var buf [104]byte
+	var got bool
+	_ = raw.Control(func(fd uintptr) {
+		l := uint32(len(buf))
+		_, _, errno := syscall.Syscall6(syscall.SYS_GETSOCKOPT, fd,
+			uintptr(syscall.SOL_TCP), uintptr(syscall.TCP_INFO),
+			uintptr(unsafe.Pointer(&buf[0])), uintptr(unsafe.Pointer(&l)), 0)
+		got = errno == 0 && l >= 24
+	})
+	if !got {
+		return connInfo{}
+	}
+	// tcpi_options bit flags (include/uapi/linux/tcp.h)
+	const (
+		optTimestamps = 1
+		optSACK       = 2
+		optWscale     = 4
+		optECN        = 8
+	)
+	var opts []string
+	ob := buf[5]
+	if ob&optTimestamps != 0 {
+		opts = append(opts, "timestamps")
+	}
+	if ob&optSACK != 0 {
+		opts = append(opts, "sack")
+	}
+	if ob&optWscale != 0 {
+		opts = append(opts, "wscale")
+	}
+	if ob&optECN != 0 {
+		opts = append(opts, "ecn")
+	}
+	return connInfo{
+		MSS:     int(binary.LittleEndian.Uint32(buf[16:20])), // tcpi_snd_mss
+		Options: opts,
+	}
+}
diff --git a/server/internal/tcpecho/tcpinfo_other.go b/server/internal/tcpecho/tcpinfo_other.go
new file mode 100644
index 0000000..e800c0d
--- /dev/null
+++ b/server/internal/tcpecho/tcpinfo_other.go
@@ -0,0 +1,17 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build !linux
+
+package tcpecho
+
+import "net"
+
+type connInfo struct {
+	MSS     int
+	Options []string
+}
+
+// tcpInfo: TCP_INFO is Linux-only; other platforms report zero values and
+// the greeting says mss:0 — honest absence rather than a guess.
+func tcpInfo(net.Conn) connInfo { return connInfo{} }