diff --git a/outside.go b/outside.go
index 28be3520..a19f2bdb 100644
--- a/outside.go
+++ b/outside.go
@@ -231,7 +231,7 @@ func (f *Interface) handleOutsideRelayPacket(hostinfo *HostInfo, via ViaSender,
 			case ForwardingType:
 				// Forward this packet through the relay tunnel, rebuilding it in place.
 				// Encode overwrites the old outer header, and the new AEAD tag lands where the old one was
-				fwdBuf := packet[:0:len(packet)] // Cap to len(packet) to protect memory from a larger parent buffer
+				fwdBuf := packet[:0]
 				//todo it would potentially be nice to batch these
 				f.SendVia(targetHI, targetRelay, signedPayload, nb, fwdBuf, true, q)
 			case TerminalType:
diff --git a/udp/udp_darwin.go b/udp/udp_darwin.go
index b381920f..e64f1330 100644
--- a/udp/udp_darwin.go
+++ b/udp/udp_darwin.go
@@ -195,7 +195,7 @@ func (u *StdConn) ListenOut(r EncReader, flush func()) error {
 			continue
 		}
 
-		r(netip.AddrPortFrom(rua.Addr().Unmap(), rua.Port()), buffer[:n])
+		r(netip.AddrPortFrom(rua.Addr().Unmap(), rua.Port()), buffer[:n:n])
 		flush()
 	}
 }
diff --git a/udp/udp_generic.go b/udp/udp_generic.go
index 9735592c..119b5d7d 100644
--- a/udp/udp_generic.go
+++ b/udp/udp_generic.go
@@ -106,7 +106,7 @@ func (u *GenericConn) ListenOut(r EncReader, flush func()) error {
 			continue
 		}
 
-		r(netip.AddrPortFrom(rua.Addr().Unmap(), rua.Port()), buffer[:n])
+		r(netip.AddrPortFrom(rua.Addr().Unmap(), rua.Port()), buffer[:n:n])
 		flush()
 	}
 }
diff --git a/udp/udp_linux.go b/udp/udp_linux.go
index 001555a3..398baf3f 100644
--- a/udp/udp_linux.go
+++ b/udp/udp_linux.go
@@ -305,7 +305,7 @@ func (u *StdConn) ListenOut(r EncReader, flush func()) error {
 // deliverSegments hands a received superdatagram to r, splitting it back into pre-coalesce packets
 func deliverSegments(r EncReader, from netip.AddrPort, payload []byte, segSize int) {
 	if segSize <= 0 || segSize >= len(payload) { //avoid bogus values
-		r(from, payload)
+		r(from, payload[:len(payload):len(payload)])
 		return
 	}
 	for off := 0; off < len(payload); off += segSize {
@@ -313,7 +313,7 @@ func deliverSegments(r EncReader, from netip.AddrPort, payload []byte, segSize i
 		if end > len(payload) {
 			end = len(payload)
 		}
-		r(from, payload[off:end])
+		r(from, payload[off:end:end])
 	}
 }
 
diff --git a/udp/udp_linux_fixes_test.go b/udp/udp_linux_fixes_test.go
index 4ed3514d..414c5864 100644
--- a/udp/udp_linux_fixes_test.go
+++ b/udp/udp_linux_fixes_test.go
@@ -210,8 +210,10 @@ func TestParseRecvCmsgCorruptLenNoPanic(t *testing.T) {
 // error here shreds encrypted packets and every decrypt downstream fails.
 func TestDeliverSegments(t *testing.T) {
 	from := netip.MustParseAddrPort("192.0.2.1:4242")
+	// Spare backing capacity mimics the recvmmsg row a real payload sits in;
+	// the cap checks below prove none of it leaks to a delivered segment.
 	pay := func(n int) []byte {
-		b := make([]byte, n)
+		b := make([]byte, n, n+512)
 		for i := range b {
 			b[i] = byte(i)
 		}
@@ -262,6 +264,11 @@ func TestDeliverSegments(t *testing.T) {
 				if len(seg) != wantLens[i] {
 					t.Fatalf("segment %d len=%d want %d", i, len(seg), wantLens[i])
 				}
+				if cap(seg) != len(seg) {
+					// EncReader contract: an append into spare capacity would
+					// scribble into the next segment of the shared row.
+					t.Errorf("segment %d cap=%d, want %d (capacity must not reach into the row)", i, cap(seg), len(seg))
+				}
 				if len(seg) > 0 && &seg[0] != &c.payload[off] {
 					t.Errorf("segment %d does not alias payload at offset %d", i, off)
 				}
diff --git a/udp/udp_rio_windows.go b/udp/udp_rio_windows.go
index 566a44d9..b04770af 100644
--- a/udp/udp_rio_windows.go
+++ b/udp/udp_rio_windows.go
@@ -161,7 +161,7 @@ func (u *RIOConn) ListenOut(r EncReader, flush func()) error {
 			continue
 		}
 
-		r(netip.AddrPortFrom(netip.AddrFrom16(rua.Addr).Unmap(), (rua.Port>>8)|((rua.Port&0xff)<<8)), buffer[:n])
+		r(netip.AddrPortFrom(netip.AddrFrom16(rua.Addr).Unmap(), (rua.Port>>8)|((rua.Port&0xff)<<8)), buffer[:n:n])
 		flush()
 	}
 }
diff --git a/udp/udp_tester.go b/udp/udp_tester.go
index 13d0bc15..bb1a5afe 100644
--- a/udp/udp_tester.go
+++ b/udp/udp_tester.go
@@ -189,7 +189,7 @@ func (u *TesterConn) ListenOut(r EncReader, flush func()) error {
 		case <-u.done:
 			return os.ErrClosed
 		case p := <-u.RxPackets:
-			r(p.From, p.Data)
+			r(p.From, p.Data[:len(p.Data):len(p.Data)])
 			// The batcher borrows plaintext decrypted in place inside p.Data
 			// until Flush, so the packet must stay alive across flush()
 			flush()