Add optional LWIP stack support

stack.go

@@ -1,12 +1,37 @@
 package tun
 
-import E "github.com/sagernet/sing/common/exceptions"
+import (
+	"context"
+
+	E "github.com/sagernet/sing/common/exceptions"
+)
 
 var (
 	ErrGVisorNotIncluded = E.New("gVisor is disabled in current build, try build without -tags `no_gvisor`")
 	ErrGVisorUnsupported = E.New("gVisor stack is unsupported on current platform")
+	ErrLWIPNotIncluded   = E.New("LWIP stack is disabled in current build, try build with -tags `with_lwip` and CGO_ENABLED=1")
 )
 
 type Stack interface {
+	Start() error
 	Close() error
 }
+
+func NewStack(
+	ctx context.Context,
+	stack string,
+	tun Tun,
+	tunMtu uint32,
+	endpointIndependentNat bool,
+	udpTimeout int64,
+	handler Handler,
+) (Stack, error) {
+	switch stack {
+	case "gvisor", "":
+		return NewGVisor(ctx, tun, tunMtu, endpointIndependentNat, udpTimeout, handler)
+	case "lwip":
+		return NewLWIP(ctx, tun, tunMtu, udpTimeout, handler)
+	default:
+		return nil, E.New("unknown stack: ", stack)
+	}
+}