make rebind slightly less crufty (#1812)

* tweak it

* GUH

Author: JackDoan

examples/config.yml

@@ -151,7 +151,7 @@ listen:
   # office to home) leaves Nebula sending out an interface that no longer has a route. When true, Nebula watches
   # the routing socket and rebinds the listener once the change settles.
   # iOS does not use this, the host app drives the same rebind itself.
-  # Default true. This setting is reloadable.
+  # Default true. Not reloadable.
   #rebind_on_network_change: true
 
   # By default, Nebula replies to packets it has no tunnel for with a "recv_error" packet. This packet helps speed up reconnection

main.go

@@ -268,8 +268,6 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 
 	attachCommands(l, c, ssh, ifce)
 
-	networkChanges := newNetworkChangeMonitorFromConfig(ctx, l, c)
-
 	control := &Control{
 		state:                  StateReady,
 		f:                      ifce,
@@ -280,13 +278,13 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 		statsStart:             stats.Start,
 		dnsStart:               ds.Start,
 		lighthouseStart:        lightHouse.StartUpdateWorker,
-		networkChangeStart:     networkChanges.Start,
 		connectionManagerStart: connManager.Start,
 	}
 
 	// The monitor's whole job is to trigger a rebind, and Control is what owns rebinding and the state gating
-	// around it, so this can only be wired once Control exists.
-	networkChanges.setRebind(control.RebindUDPServer)
+	// around it, so it is injected the rebind func and can only be built once Control exists.
+	networkChanges := udp.NewNetworkChangeMonitor(ctx, l, c, control.RebindUDPServer)
+	control.networkChangeStart = networkChanges.Start
 
 	return control, nil
 }

network_change.go

@@ -0,0 +1,66 @@
+package udp
+
+import (
+	"context"
+	"log/slog"
+
+	"github.com/slackhq/nebula/config"
+)
+
+// NetworkChangeMonitor rebinds the udp listener when the local network moves out from under it.
+//
+// Detection lives here in the udp package, next to the socket it concerns and the platform matrix that already knows
+// which sockets go stale. What to do about a change — updating the lighthouse, requerying tunnels — is not the udp
+// package's business, so the reaction is injected as a plain function pointer. The monitor calls it once a change
+// has settled and stays ignorant of what it does.
+//
+// On platforms whose sockets do not go stale, watchNetworkChanges hands back a nil channel and Start returns.
+type NetworkChangeMonitor struct {
+	l       *slog.Logger
+	ctx     context.Context
+	enabled bool
+	// rebind is what we call once a change has settled. It is injected because the udp package has no business
+	// knowing what a rebind entails, only when one is warranted.
+	rebind func()
+}
+
+// NewNetworkChangeMonitor builds a monitor that calls rebind whenever the local network moves. The returned monitor
+// is always usable: Start is safe to call unconditionally, it no-ops when disabled or on a platform that does not
+// need it.
+func NewNetworkChangeMonitor(ctx context.Context, l *slog.Logger, c *config.C, rebind func()) *NetworkChangeMonitor {
+	return &NetworkChangeMonitor{
+		l:       l,
+		ctx:     ctx,
+		enabled: c.GetBool("listen.rebind_on_network_change", true),
+		rebind:  rebind,
+	}
+}
+
+// Start watches for network changes until the context is cancelled, calling rebind once per settled change. It
+// blocks, so callers run it in a goroutine, and it no-ops when disabled, unsupported, or with nothing to rebind.
+func (m *NetworkChangeMonitor) Start() {
+	if !m.enabled || m.rebind == nil || m.ctx.Err() != nil {
+		return
+	}
+
+	changes, err := watchNetworkChanges(m.ctx, m.l)
+	if err != nil {
+		// Not fatal. Everything else still works, we just won't notice a network change on our own.
+		m.l.Error("Failed to watch for network changes, will not rebind the udp listener when the network moves",
+			"error", err,
+		)
+		return
+	}
+
+	if changes == nil {
+		// This platform's sockets don't go stale, so there is nothing to watch for.
+		return
+	}
+
+	m.l.Info("Watching for network changes to rebind the udp listener")
+
+	for range changes {
+		m.l.Info("Local network changed, rebinding the udp listener")
+		m.rebind()
+	}
+}