Add stateless DNS hijack

flow.go

@@ -21,6 +21,7 @@ const (
 	ActionReject
 	ActionDrop
 	ActionBypass
+	ActionHijackDNS
 )
 
 type FlowTracker interface {

flow_dispatch.go

@@ -291,6 +291,13 @@ func (d *ForwardDispatcher) judgeAndInstall(key flowKey, packet *forwardPacket,
 	case ActionDrop:
 		d.installSimple(key, ActionDrop, packet.protocol, now)
 		return true
+	case ActionHijackDNS:
+		if packet.protocol == uint8(header.UDPProtocolNumber) {
+			d.hijackDNSPacket(packet)
+			return true
+		}
+		d.installSimple(key, ActionAccept, packet.protocol, now)
+		return false
 	default:
 		d.installSimple(key, ActionAccept, packet.protocol, now)
 		return false

flow_dns.go

@@ -0,0 +1,89 @@
+package tun
+
+import (
+	"net/netip"
+
+	"github.com/sagernet/sing-tun/gtcpip/checksum"
+	"github.com/sagernet/sing-tun/gtcpip/header"
+	"github.com/sagernet/sing/common/buf"
+	E "github.com/sagernet/sing/common/exceptions"
+	M "github.com/sagernet/sing/common/metadata"
+	N "github.com/sagernet/sing/common/network"
+)
+
+func (d *ForwardDispatcher) hijackDNSPacket(packet *forwardPacket) {
+	writer := &dnsResponseWriter{
+		writeback: d.writeback,
+		source:    packet.source,
+	}
+	d.handler.NewDNSPacket(header.UDP(packet.transport).Payload(), M.SocksaddrFromNetIP(packet.source), M.SocksaddrFromNetIP(packet.destination), writer)
+}
+
+var _ N.PacketWriter = (*dnsResponseWriter)(nil)
+
+type dnsResponseWriter struct {
+	writeback ForwardWriteback
+	source    netip.AddrPort
+}
+
+func (w *dnsResponseWriter) WritePacket(buffer *buf.Buffer, destination M.Socksaddr) error {
+	defer buffer.Release()
+	if !destination.IsIP() {
+		return E.New("invalid destination: ", destination)
+	}
+	sourceAddr := w.source.Addr().Unmap()
+	destinationAddr := destination.Addr.Unmap()
+	headroom := w.writeback.ReturnHeadroom()
+	udpLen := header.UDPMinimumSize + buffer.Len()
+	var (
+		packet []byte
+		udpHdr header.UDP
+		ipHdr  header.Network
+	)
+	if sourceAddr.Is4() {
+		if !destinationAddr.Is4() {
+			return E.New("send IPv6 packet to IPv4 connection")
+		}
+		size := header.IPv4MinimumSize + udpLen
+		packet = make([]byte, headroom+size)
+		inet4Hdr := header.IPv4(packet[headroom:])
+		inet4Hdr.Encode(&header.IPv4Fields{
+			TotalLength: uint16(size),
+			TTL:         synthesizedTTL,
+			Protocol:    uint8(header.UDPProtocolNumber),
+			SrcAddr:     destinationAddr,
+			DstAddr:     sourceAddr,
+		})
+		udpHdr = header.UDP(inet4Hdr.Payload())
+		ipHdr = inet4Hdr
+	} else {
+		if destinationAddr.Is4() {
+			destinationAddr = netip.AddrFrom16(destinationAddr.As16())
+		}
+		size := header.IPv6MinimumSize + udpLen
+		packet = make([]byte, headroom+size)
+		inet6Hdr := header.IPv6(packet[headroom:])
+		inet6Hdr.Encode(&header.IPv6Fields{
+			PayloadLength:     uint16(udpLen),
+			TransportProtocol: header.UDPProtocolNumber,
+			HopLimit:          synthesizedTTL,
+			SrcAddr:           destinationAddr,
+			DstAddr:           sourceAddr,
+		})
+		udpHdr = header.UDP(inet6Hdr.Payload())
+		ipHdr = inet6Hdr
+	}
+	udpHdr.Encode(&header.UDPFields{
+		SrcPort: destination.Port,
+		DstPort: w.source.Port(),
+		Length:  uint16(udpLen),
+	})
+	copy(udpHdr.Payload(), buffer.Bytes())
+	udpHdr.SetChecksum(^checksum.Checksum(udpHdr.Payload(), udpHdr.CalculateChecksum(
+		header.PseudoHeaderChecksum(header.UDPProtocolNumber, ipHdr.SourceAddressSlice(), ipHdr.DestinationAddressSlice(), uint16(udpLen)),
+	)))
+	if inet4Hdr, isInet4 := ipHdr.(header.IPv4); isInet4 {
+		inet4Hdr.SetChecksum(^inet4Hdr.CalculateChecksum())
+	}
+	return w.writeback.WriteReturnPackets([][]byte{packet})
+}

stack_gvisor_udp.go

@@ -71,18 +71,26 @@ func (f *UDPForwarder) PreparePacketConnection(source M.Socksaddr, destination M
 			firstPacket = append(firstPacket[:len(firstPacket):len(firstPacket)], view.AsSlice()...)
 		}
 	})
+	var sourceNetwork tcpip.NetworkProtocolNumber
+	if source.Addr.Is4() {
+		sourceNetwork = header.IPv4ProtocolNumber
+	} else {
+		sourceNetwork = header.IPv6ProtocolNumber
+	}
 	switch f.handler.JudgeFlow(uint8(header.UDPProtocolNumber), source.AddrPort(), destination.AddrPort(), firstPacket).Action {
 	case ActionReject:
 		gWriteUnreachable(f.stack, userData.(*stack.PacketBuffer))
 		return false, nil, nil, nil
 	case ActionDrop:
 		return false, nil, nil, nil
-	}
-	var sourceNetwork tcpip.NetworkProtocolNumber
-	if source.Addr.Is4() {
-		sourceNetwork = header.IPv4ProtocolNumber
-	} else {
-		sourceNetwork = header.IPv6ProtocolNumber
+	case ActionHijackDNS:
+		f.handler.NewDNSPacket(firstPacket, source, destination, &UDPBackWriter{
+			stack:         f.stack,
+			source:        AddressFromAddr(source.Addr),
+			sourcePort:    source.Port,
+			sourceNetwork: sourceNetwork,
+		})
+		return false, nil, nil, nil
 	}
 	writer := &UDPBackWriter{
 		stack:         f.stack,

tun.go

@@ -14,12 +14,14 @@ import (
 	E "github.com/sagernet/sing/common/exceptions"
 	F "github.com/sagernet/sing/common/format"
 	"github.com/sagernet/sing/common/logger"
+	M "github.com/sagernet/sing/common/metadata"
 	N "github.com/sagernet/sing/common/network"
 	"github.com/sagernet/sing/common/ranges"
 )
 
 type Handler interface {
 	JudgeFlow(network uint8, source netip.AddrPort, destination netip.AddrPort, firstPacket []byte) FlowVerdict
+	NewDNSPacket(payload []byte, source M.Socksaddr, destination M.Socksaddr, writer N.PacketWriter)
 	N.TCPConnectionHandlerEx
 	N.UDPConnectionHandlerEx
 }