draft: Tun respond ICMP port unreachable when rejecting UDP packets

constant/adapters.go

@@ -274,6 +274,10 @@ func (s *packetAdapter) Key() string {
 	return s.key
 }
 
+func (s *packetAdapter) Upstream() any {
+	return s.UDPPacket
+}
+
 func NewPacketAdapter(packet UDPPacket, metadata *Metadata) PacketAdapter {
 	return &packetAdapter{
 		packet,

constant/rule.go

@@ -1,5 +1,11 @@
 package constant
 
+import (
+	"io"
+
+	"github.com/metacubex/sing/common/exceptions"
+)
+
 // Rule Type
 const (
 	Domain RuleType = iota
@@ -129,3 +135,7 @@ type RuleGroup interface {
 	Rule
 	GetRecodeSize() int
 }
+
+var (
+	ErrResetByRule = exceptions.Cause(io.EOF, "reset by rule") // TODO: replace function from sing
+)