The mesh's interface takes over the found tunnel's MTU

Carries MTU from the reported tunnel (mesh-host#28) through inventory,
the overlay graph's TakeOver, into the generated config's [Interface].
A tuned path keeps its MTU across the takeover instead of regressing to
1420 and hanging transfers no ping would reveal. Two emit tests; a
tunnel with no MTU writes no line.

cmd/mesh-controller/network.go

@@ -256,7 +256,7 @@ func network(ctx context.Context, inv *inventory.Inventory, on map[string]bool,
 					"Re-place it — `overlay place %s --hub --endpoint <host>:%d …` — and push again; "+
 					"nothing was composed", p.Name, t.Interface, wrong, p.Name, t.Port)
 			}
-			n.TakesOver = &overlay.TakeOver{Interface: t.Interface, Unit: t.Unit, Config: t.Config, Port: t.Port}
+			n.TakesOver = &overlay.TakeOver{Interface: t.Interface, Unit: t.Unit, Config: t.Config, Port: t.Port, MTU: t.MTU}
 		}
 		if p.Hub {
 			for _, c := range carried {

internal/inventory/tunnel.go

@@ -33,6 +33,9 @@ type Tunnel struct {
 	// Port is the port the found interface listened on — one the hosting provider already lets
 	// through, which is why it is worth taking.
 	Port int `json:"port"`
+	// MTU is the found interface's, when it set one; the mesh's interface takes it over so a
+	// tuned path does not silently regress to the default (novox/hq: a taken tunnel carries its MTU).
+	MTU int `json:"mtu,omitempty"`
 	// Address is the interface's own address with its prefix length, 192.0.2.1/24; Range is the
 	// network that prefix names, 192.0.2.0/24.
 	Address string `json:"address"`

internal/link/enrolment.go

@@ -140,7 +140,7 @@ func (e Enrolment) Enrol(ctx context.Context, request EnrolRequest) (reply Enrol
 		}
 		if err := e.Inventory.RecordTunnel(ctx, node.ID, inventory.Tunnel{
 			Interface: request.Tunnel.Interface, Unit: request.Tunnel.Unit,
-			Config: request.Tunnel.Config, Port: request.Tunnel.Port,
+			Config: request.Tunnel.Config, Port: request.Tunnel.Port, MTU: request.Tunnel.MTU,
 			Address: request.Tunnel.Address, Range: request.Tunnel.Range,
 			PublicKey: request.Tunnel.PublicKey, Peers: peers,
 		}); err != nil {
@@ -199,7 +199,7 @@ func (e Enrolment) rekey(ctx context.Context, node inventory.Node, r Rekey) erro
 		peers = append(peers, inventory.TunnelPeer{PublicKey: p.PublicKey, Address: p.Address})
 	}
 	err := e.Inventory.Rekey(ctx, node.ID, r.Previous, r.OverlayKey, inventory.Tunnel{
-		Interface: r.Tunnel.Interface, Unit: r.Tunnel.Unit, Config: r.Tunnel.Config, Port: r.Tunnel.Port,
+		Interface: r.Tunnel.Interface, Unit: r.Tunnel.Unit, Config: r.Tunnel.Config, Port: r.Tunnel.Port, MTU: r.Tunnel.MTU,
 		Address: r.Tunnel.Address, Range: r.Tunnel.Range, PublicKey: r.Tunnel.PublicKey, Peers: peers,
 	})
 	if err != nil {

internal/link/protocol.go

@@ -93,6 +93,7 @@ type Tunnel struct {
 	Unit      string       `json:"unit"`
 	Config    string       `json:"config"`
 	Port      int          `json:"port"`
+	MTU       int          `json:"mtu,omitempty"`
 	Address   string       `json:"address"`
 	Range     string       `json:"range"`
 	PublicKey string       `json:"public_key"`

internal/overlay/declaration.go

@@ -130,6 +130,12 @@ func config(node Node, peers []Peer, keyPath string) string {
 		// with an endpoint — the guard's suggested remedy — breaks a NAT'd node's path.
 		fmt.Fprintf(&b, "ListenPort = %d\n", node.TakesOver.Port)
 	}
+	// The MTU the found tunnel carried, when it set one: a path tuned to 1380 (say) stalls TLS
+	// and hangs transfers if the mesh's interface comes up at the 1420 default, and no ping shows
+	// it (novox/hq: a taken tunnel carries its MTU).
+	if node.TakesOver != nil && node.TakesOver.MTU != 0 {
+		fmt.Fprintf(&b, "MTU = %d\n", node.TakesOver.MTU)
+	}
 	// The private key is set from a file the node wrote, so it never appears here and never
 	// travelled. Everything else in this file came from the mesh; this one line is the node's.
 	fmt.Fprintf(&b, "PostUp = wg set %%i private-key %s\n", keyPath)

internal/overlay/declaration_test.go

@@ -286,3 +286,26 @@ func TestANodeWithNoTunnelAndNoEndpointStillListensOnNothing(t *testing.T) {
 		t.Fatalf("a dial-only node needs no ListenPort:\n%s", config)
 	}
 }
+
+func TestATakenTunnelCarriesItsMTU(t *testing.T) {
+	// A path tuned to a smaller MTU (1380 here) must survive the takeover — the mesh interface
+	// comes up with the same MTU, or transfers hang silently (novox/hq: a taken tunnel carries
+	// its MTU).
+	config, _ := declarationFor(t, Node{
+		Name: "shanks", Key: "SPOKE", Address: "10.10.0.3",
+		TakesOver: &TakeOver{Interface: "wg0", Port: 51820, MTU: 1380},
+	}, nil)
+	if !strings.Contains(config, "MTU = 1380") {
+		t.Fatalf("the tuned MTU was dropped:\n%s", config)
+	}
+}
+
+func TestNoMTULineWhenTheTunnelSetNone(t *testing.T) {
+	config, _ := declarationFor(t, Node{
+		Name: "shanks", Key: "SPOKE", Address: "10.10.0.3",
+		TakesOver: &TakeOver{Interface: "wg0", Port: 51820},
+	}, nil)
+	if strings.Contains(config, "MTU") {
+		t.Fatalf("no MTU was found, so none should be written:\n%s", config)
+	}
+}

internal/overlay/graph.go

@@ -50,6 +50,8 @@ type TakeOver struct {
 	Interface string
 	Unit      string
 	Config    string
+	// MTU is the found tunnel's, when it set one — emitted so a tuned path keeps its MTU.
+	MTU int
 	// Port is the port the found tunnel listened on. The mesh's interface must listen on it too,
 	// even on a node that is not dialable from the hub: a home node's LAN peers dial it there
 	// (novox/hq: a taken tunnel brings its port). Listening is "a peer dials me here"; it is not