fix(udp): read WSJT-X packets that arrive through a relay

A forwarder (W&P, seen in the field in front of MSHV) prepends the origin as
plain text before re-broadcasting:

    "127.0.0.1:2237|" + <the original, untouched WSJT-X packet>

That puts the magic 15 bytes in, so every datagram failed on "bad magic
0x3132372e" — those four bytes being ASCII "127." — and an operator running
MSHV behind the relay saw no decodes, no callsigns and no auto-logged QSOs.

No new service type: what follows the header IS a WSJT-X packet, so the parser
and everything downstream apply unchanged, and a separate type would duplicate
decode, status and logged-ADIF handling to strip 15 bytes. ParseWSJT skips the
header instead, which also covers any other relay that wraps traffic this way.

The match is deliberately narrow — the magic must fall within the first 64
bytes AND every byte before it must be printable ASCII. A corrupt or truncated
packet that merely contains those four bytes somewhere is not resurrected into
a QSO; it fails exactly as it did before.

Test data is the real captured datagram, header included.

internal/integrations/udp/forwarder_test.go

@@ -0,0 +1,63 @@
+package udp
+
+import (
+	"bytes"
+	"testing"
+)
+
+// Bytes captured from a real W&P relay in front of MSHV: the origin as text,
+// a '|', then the untouched WSJT-X packet. This exact datagram produced
+// "bad magic 0x3132372e" — 0x3132372e being ASCII "127.".
+var wpDecode = []byte{
+	// "127.0.0.1:2237|"
+	0x31, 0x32, 0x37, 0x2e, 0x30, 0x2e, 0x30, 0x2e, 0x31, 0x3a, 0x32, 0x32, 0x33, 0x37, 0x7c,
+	// magic, schema 3, type 2 (Decode), id "MSHV"
+	0xad, 0xbc, 0xcb, 0xda, 0x00, 0x00, 0x00, 0x03, 0x00, 0x00, 0x00, 0x02,
+	0x00, 0x00, 0x00, 0x04, 0x4d, 0x53, 0x48, 0x56,
+	0x01, 0x01, 0x4b, 0x31, 0x28, 0x00, 0x00, 0x00, 0x16,
+	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x8a,
+	0x00, 0x00, 0x00, 0x03, 0x46, 0x54, 0x38, // "FT8"
+	0x00, 0x00, 0x00, 0x0e, 0x54, 0x4e, 0x38, 0x47, 0x44, 0x20, 0x39, 0x41, 0x31, 0x4d, 0x4d, 0x20, 0x37, 0x33, // "TN8GD 9A1MM 73"
+	0x00, 0x00,
+}
+
+func TestParseWSJTBehindAForwarder(t *testing.T) {
+	if _, _, err := ParseWSJT(wpDecode); err != nil {
+		t.Fatalf("relayed packet still fails: %v", err)
+	}
+	// And the header is what was in the way: without it the same bytes parse.
+	if _, _, err := ParseWSJT(wpDecode[15:]); err != nil {
+		t.Fatalf("bare packet fails: %v", err)
+	}
+}
+
+func TestStripForwarderHeader(t *testing.T) {
+	bare := wpDecode[15:]
+
+	if got := stripForwarderHeader(bare); !bytes.Equal(got, bare) {
+		t.Error("a packet with no header must be returned untouched")
+	}
+	if got := stripForwarderHeader(wpDecode); !bytes.Equal(got, bare) {
+		t.Error("the text header was not stripped")
+	}
+
+	// A BINARY prefix is not a forwarder header — refusing it is what stops a
+	// corrupt packet that merely contains the magic from becoming a QSO.
+	binPrefix := append([]byte{0x00, 0x01, 0x02}, bare...)
+	if got := stripForwarderHeader(binPrefix); !bytes.Equal(got, binPrefix) {
+		t.Error("a non-printable prefix must not be treated as a header")
+	}
+
+	// Beyond the window, the magic is ignored however printable the prefix is.
+	far := append(bytes.Repeat([]byte("A"), maxFwdHeader+1), bare...)
+	if got := stripForwarderHeader(far); !bytes.Equal(got, far) {
+		t.Error("magic past maxFwdHeader must not be trusted")
+	}
+
+	// No magic anywhere, and runt packets, must not panic or invent anything.
+	for _, junk := range [][]byte{[]byte("127.0.0.1:2237|hello"), {}, {0xad}, {0xad, 0xbc, 0xcb}} {
+		if got := stripForwarderHeader(junk); !bytes.Equal(got, junk) {
+			t.Errorf("junk %q was altered", junk)
+		}
+	}
+}

internal/integrations/udp/wsjt.go

@@ -56,12 +56,61 @@ type WSJTEvent struct {
 	IsCQ        bool   // the decode was a CQ call
 }
 
+// maxFwdHeader bounds how far into a packet the WSJT-X magic may sit behind a
+// forwarder's header. The one seen in the field ("127.0.0.1:2237|") is 15 bytes;
+// 64 leaves room for a longer address without ever scanning a real payload.
+const maxFwdHeader = 64
+
+// stripForwarderHeader removes the origin header a UDP relay prepends.
+//
+// A relay that re-broadcasts WSJT-X traffic has to say where each datagram came
+// from, and it does so as plain text in front of the payload:
+//
+//	"127.0.0.1:2237|" + <the original, untouched WSJT-X packet>
+//
+// The magic then sits 15 bytes in, every packet fails on "bad magic", and an
+// operator running MSHV behind such a relay gets nothing at all. There is no
+// need for a separate service type: what follows the header IS a WSJT-X packet,
+// so the whole parser and everything downstream apply unchanged.
+//
+// Deliberately narrow. The magic must appear within maxFwdHeader bytes AND
+// everything before it must be printable ASCII — a truncated or corrupt packet
+// that happens to contain those four bytes somewhere is not resurrected into a
+// QSO. Anything else is returned untouched, and still fails as it did.
+func stripForwarderHeader(pkt []byte) []byte {
+	if len(pkt) < 4 {
+		return pkt
+	}
+	if binary.BigEndian.Uint32(pkt) == wsjtMagic {
+		return pkt // no header — the overwhelmingly common case
+	}
+	limit := len(pkt) - 4
+	if limit > maxFwdHeader {
+		limit = maxFwdHeader
+	}
+	for i := 1; i <= limit; i++ {
+		if binary.BigEndian.Uint32(pkt[i:]) != wsjtMagic {
+			continue
+		}
+		for _, b := range pkt[:i] {
+			if b < 0x20 || b >= 0x7f {
+				return pkt // not a text header — leave it alone
+			}
+		}
+		return pkt[i:]
+	}
+	return pkt
+}
+
 // ParseWSJT decodes one UDP packet. Returns ok=false for messages we
 // don't care about (heartbeat, clears, etc.).
 func ParseWSJT(pkt []byte) (WSJTEvent, bool, error) {
 	if len(pkt) < 12 {
 		return WSJTEvent{}, false, fmt.Errorf("packet too short")
 	}
+	// A relay (W&P and friends) puts its own origin header in front — skip it so
+	// the packet parses exactly as if it had arrived from WSJT-X directly.
+	pkt = stripForwarderHeader(pkt)
 	r := bytes.NewReader(pkt)
 	var magic, schema, mtype uint32
 	if err := binary.Read(r, binary.BigEndian, &magic); err != nil {