draft: Tun respond ICMP port unreachable when rejecting UDP packets

common/net/sing.go

@@ -33,6 +33,9 @@ type WriterWithUpstream = network.WriterWithUpstream
 type WithUpstreamWriter = network.WithUpstreamWriter
 type WithUpstream = common.WithUpstream
 
+type HandshakeSuccess = network.HandshakeSuccess
+type HandshakeFailure = network.HandshakeFailure
+
 var UnwrapReader = network.UnwrapReader
 var UnwrapWriter = network.UnwrapWriter
 

common/utils/cast.go

@@ -0,0 +1,31 @@
+package utils
+
+import (
+	"fmt"
+	"net"
+)
+
+type WithUpstream interface {
+	Upstream() any
+}
+
+type stdWithUpstreamNetConn interface {
+	NetConn() net.Conn
+}
+
+func Cast[T any](obj any) (_ T, _ bool) {
+	if c, ok := obj.(T); ok {
+		fmt.Printf("Got 1: %T\n", obj) // TODO
+		return c, true
+	}
+	if u, ok := obj.(WithUpstream); ok {
+		fmt.Printf("Upstream 2: %T\n", obj) // TODO
+		return Cast[T](u.Upstream())
+	}
+	if u, ok := obj.(stdWithUpstreamNetConn); ok {
+		fmt.Printf("Std 3: %T\n", obj) // TODO
+		return Cast[T](u.NetConn())
+	}
+	fmt.Printf("Failed: %T\n", obj) // TODO
+	return
+}