server: MTU probe (MTU_PROBE/MTU_ACK) — path-MTU / black-hole measurement

Server ACKs each DF-flagged probe with a tiny MTU_ACK carrying the size it
received; the client binary-searches the path MTU. Non-amplifying by
construction. Tested.

Also records: v0.3.2 (http-echo + tls-reference) verified live on fmr, and
the finding that upstream trains are already observable via the
observations API (dedicated TRAIN_REPORT deferred — needs an
anti-amplification grant + columnar encoding).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/build-status.md

@@ -254,3 +254,20 @@ caught a finding: **Google applies 0x20 case randomization** (mixed-case qname),
 not — captured via `case_preserved`. Capabilities now: udp-probe, delayed-echo, connect-back,
 tcp-echo, stun-5780, canary-dns. Kept the hand-rolled stdlib DNS (no miekg/dns) — validated
 against independent clients. Deployed via `--self-update` (v0.3.0→v0.3.1, checksum-verified).
+
+## Server v0.3.2 + v0.3.3 (2026-07-31)
+- **v0.3.2 — control-plane security (live on fmr, externally verified):** `POST /v1/echo`
+  reflects the received request head+body (b64) and observed TLS (version/cipher/SNI/ALPN) —
+  captured real SNI `fmr-1.echo-lot.app` and an injected header over public TLS1.3; `GET
+  /v1/tls-reference` returns the served DER chain + pin (cross-checked against the openssl-derived
+  pin). Optional cleartext echo listener (default off). Capability `http-echo`.
+- **v0.3.3 — MTU probe (data plane):** MTU_PROBE (0x09) → small MTU_ACK (0x0A) carrying the
+  received datagram size; client DF-probes increasing sizes to find path MTU / black holes. ACK
+  is tiny → never amplifies. Tested.
+- **Note on trains:** upstream trains (TRAIN_DATA 0x03) are already observable — every HMAC-valid
+  packet is recorded (seq/t_rx/size/type) with no per-packet response, so loss/reordering/inter-
+  arrival are visible via GET observations. The dedicated data-plane TRAIN_REPORT (0x05) is
+  deferred: §3.4 anti-amplification means it needs an asymmetric grant + columnar multi-datagram
+  encoding — a focused batch, not a corner to rush.
+Remaining spec: tls-echo (ClientHello+JA4), TRAIN_REPORT, big/frag-send, throughput, downtrain;
+real admin UI.

server/internal/dataplane/udp.go

@@ -29,6 +29,8 @@ const (
 	TypeEchoResp    = 0x02
 	TypeTimesyncReq = 0x07
 	TypeTimesyncRsp = 0x08
+	TypeMtuProbe    = 0x09
+	TypeMtuAck      = 0x0A
 	TypeDelayedEcho = 0x0B
 )
 
@@ -132,11 +134,24 @@ func (s *Server) handle(conn *net.UDPConn, raddr netip.AddrPort, pkt []byte, tRx
 		s.echoResp(conn, raddr, sess, pkt, seq, tRxNs)
 	case TypeTimesyncReq:
 		s.timesyncResp(conn, raddr, sess, pkt, seq, tRxNs)
+	case TypeMtuProbe:
+		s.mtuAck(conn, raddr, sess, seq, len(pkt))
 	default:
 		slog.Debug("unhandled data-plane type", "type", typ)
 	}
 }
 
+// mtuAck replies to an MTU_PROBE with a small MTU_ACK carrying the total
+// datagram size the server actually received (spec §3.2). The client sends
+// DF-flagged probes of increasing size and binary-searches the path MTU / a
+// black hole from which sizes stop being acknowledged. The ACK is tiny, so it
+// can never amplify regardless of probe size.
+func (s *Server) mtuAck(conn *net.UDPConn, raddr netip.AddrPort, sess *session.Session, seq uint32, received int) {
+	var payload [4]byte
+	binary.BigEndian.PutUint32(payload[:], uint32(received))
+	s.send(conn, raddr, sess, TypeMtuAck, seq, payload[:])
+}
+
 // Observation block (spec §3.3), fixed 40 bytes appended to the RESP header:
 //   0  8  t_rx_ns (server clock, process epoch)
 //   8  8  t_tx_ns

server/internal/dataplane/udp_test.go

@@ -102,6 +102,40 @@ func TestEchoRoundtripObservationAndAntiAmplification(t *testing.T) {
 	}
 }
 
+func TestMtuProbeAckReportsReceivedSizeAndDoesNotAmplify(t *testing.T) {
+	mgr, addr := startServer(t)
+	sess, _, err := mgr.New("dev1", "credential-ikm", netip.MustParseAddr("127.0.0.1"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	client, err := net.DialUDP("udp", nil, net.UDPAddrFromAddrPort(addr))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+	client.SetDeadline(time.Now().Add(2 * time.Second))
+
+	// A large probe: 32 header + 1400 payload.
+	probe := craft(t, sess, TypeMtuProbe, 1, make([]byte, 1400))
+	if _, err := client.Write(probe); err != nil {
+		t.Fatal(err)
+	}
+	buf := make([]byte, 2000)
+	n, err := client.Read(buf)
+	if err != nil {
+		t.Fatalf("no MTU_ACK: %v", err)
+	}
+	if buf[4] != TypeMtuAck {
+		t.Fatalf("type = %#x, want MTU_ACK", buf[4])
+	}
+	if n >= len(probe) {
+		t.Fatalf("MTU_ACK (%d) must be far smaller than the probe (%d)", n, len(probe))
+	}
+	if got := binary.BigEndian.Uint32(buf[HeaderSize:n]); int(got) != len(probe) {
+		t.Fatalf("acked size %d, want %d", got, len(probe))
+	}
+}
+
 func TestDropsReplayBadHmacAndUnknownPrefix(t *testing.T) {
 	mgr, addr := startServer(t)
 	sess, _, err := mgr.New("dev1", "credential-ikm", netip.MustParseAddr("127.0.0.1"))