Add stateless DNS hijack

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})
+}