DNS Header for KCP (#1672)

* dns header

* fixed domain name encoding for dns header

---------

Co-authored-by: kerry <lvhaiyangkerry@gmail.com>

transport/internet/headers/dns/dns.go

@@ -0,0 +1,57 @@
+package dns
+
+import (
+	"context"
+	"encoding/binary"
+
+	"github.com/miekg/dns"
+	"github.com/xtls/xray-core/common"
+	"github.com/xtls/xray-core/common/dice"
+)
+
+type DNS struct {
+	header []byte
+}
+
+func (d DNS) Size() int32 {
+	return int32(len(d.header))
+}
+
+// Serialize implements PacketHeader.
+func (d DNS) Serialize(b []byte) {
+	copy(b, d.header)
+	binary.BigEndian.PutUint16(b[0:], dice.RollUint16()) // random transaction ID
+}
+
+// NewDNS returns a new DNS instance based on given config.
+func NewDNS(ctx context.Context, config interface{}) (interface{}, error) {
+	var header []byte
+
+	header = binary.BigEndian.AppendUint16(header, 0x0000) // Transaction ID
+	header = binary.BigEndian.AppendUint16(header, 0x0100) // Flags: Standard query
+	header = binary.BigEndian.AppendUint16(header, 0x0001) // Questions
+	header = binary.BigEndian.AppendUint16(header, 0x0000) // Answer RRs
+	header = binary.BigEndian.AppendUint16(header, 0x0000) // Authority RRs
+	header = binary.BigEndian.AppendUint16(header, 0x0000) // Additional RRs
+
+	buf := make([]byte, 0x100)
+
+	off1, err := dns.PackDomainName(dns.Fqdn(config.(*Config).Domain), buf, 0, nil, false)
+
+	if err != nil {
+		return nil, err
+	}
+
+	header = append(header, buf[:off1]...)
+
+	header = binary.BigEndian.AppendUint16(header, 0x0001) // Type: A
+	header = binary.BigEndian.AppendUint16(header, 0x0001) // Class: IN
+
+	return DNS{
+		header: header,
+	}, nil
+}
+
+func init() {
+	common.Must(common.RegisterConfig((*Config)(nil), NewDNS))
+}