completed dns response parsing

internal/dns/response.go

@@ -0,0 +1,99 @@
+package dns
+
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+type DNSAnswer struct {
+	Name  string
+	Type  uint16
+	Class uint16
+	TTL   uint32
+	Data  string
+}
+
+func parseHeader(data []byte) DNSHeader {
+	return DNSHeader{
+		ID:      binary.BigEndian.Uint16(data[0:2]),
+		Flags:   binary.BigEndian.Uint16(data[2:4]),
+		QDCount: binary.BigEndian.Uint16(data[4:6]),
+		ANCount: binary.BigEndian.Uint16(data[6:8]),
+		NSCount: binary.BigEndian.Uint16(data[8:10]),
+		ARCount: binary.BigEndian.Uint16(data[10:12]),
+	}
+}
+
+func parseName(data []byte, offset int) (string, int) {
+	name := ""
+	for {
+		length := int(data[offset])
+		if length == 0 {
+			offset++
+			break
+		}
+
+		if length&0xc0 == 0xc0 {
+			pointer := binary.BigEndian.Uint16(data[offset:offset+2]) & 0x3FF
+			offset = offset + 2
+			partial, _ := parseName(data, int(pointer))
+			return name + partial, offset
+		}
+
+		offset++
+		name += string(data[offset:offset+length]) + "."
+		offset += length
+	}
+
+	return name[:len(name)-1], offset
+}
+
+func ParseResponse(response []byte) ([]DNSAnswer, error) {
+	if len(response) < 12 {
+		return nil, fmt.Errorf("response too short to contain header")
+	}
+
+	header := parseHeader(response[:12])
+	if header.Flags&0x8000 == 0 {
+		return nil, fmt.Errorf("not a response")
+	}
+
+	if header.Flags&0x000F != 0 {
+		return nil, fmt.Errorf("response contains errors")
+	}
+
+	offset := 12
+	for i := 0; i < int(header.QDCount); i++ {
+		_, newOffset := parseName(response, offset)
+		offset = newOffset + 4
+	}
+
+	answers := []DNSAnswer{}
+
+	for i := 0; i < int(header.ANCount); i++ {
+		name, newOffset := parseName(response, offset)
+		offset = newOffset
+
+		answer := DNSAnswer{
+			Name: name,
+		}
+
+		// Extract type, class, TTL, and data length
+		answer.Type = binary.BigEndian.Uint16(response[offset : offset+2])
+		answer.Class = binary.BigEndian.Uint16(response[offset+2 : offset+4])
+		answer.TTL = binary.BigEndian.Uint32(response[offset+4 : offset+8])
+		dataLen := binary.BigEndian.Uint16(response[offset+8 : offset+10])
+		offset += 10
+
+		if answer.Type == QTypeA && dataLen == 4 {
+			answer.Data = fmt.Sprintf("%d.%d.%d.%d", response[offset], response[offset+1], response[offset+2], response[offset+3])
+		} else {
+			answer.Data = fmt.Sprintf("unsupported type %d", answer.Type)
+		}
+		offset += int(dataLen)
+
+		answers = append(answers, answer)
+	}
+
+	return answers, nil
+}

main.go

@@ -36,4 +36,15 @@ func main() {
 	}
 
 	fmt.Printf("DNS Response: %x\n", response)
+
+	answers, err := dns.ParseResponse(response)
+	if err != nil {
+		log.Fatalf("Error parsing response: %s", err)
+	}
+
+	for _, answer := range answers {
+		fmt.Printf("Name: %s, Type: %d, Class: %d, TTL: %d, Data: %s\n",
+			answer.Name, answer.Type, answer.Class, answer.TTL, answer.Data)
+	}
+
 }