net/netns: add functionality to bind outgoing sockets based on route table

When turned on via environment variable (off by default), this will use
the BSD routing APIs to query what interface index a socket should be
bound to, rather than binding to the default interface in all cases.

Updates #5719
Updates #5940

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ib4c919471f377b7a08cd3413f8e8caacb29fee0b

net/netns/netns.go

@@ -32,6 +32,17 @@ func SetEnabled(on bool) {
 	disabled.Store(!on)
 }
 
+var bindToInterfaceByRoute atomic.Bool
+
+// SetBindToInterfaceByRoute enables or disables whether we use the system's
+// route information to bind to a particular interface. It is the same as
+// setting the TS_BIND_TO_INTERFACE_BY_ROUTE.
+//
+// Currently, this only changes the behaviour on macOS.
+func SetBindToInterfaceByRoute(v bool) {
+	bindToInterfaceByRoute.Store(v)
+}
+
 // Listener returns a new net.Listener with its Control hook func
 // initialized as necessary to run in logical network namespace that
 // doesn't route back into Tailscale.