Added parsing of DNS message headers

Author: shadowy-pycoder

Makefile

@@ -14,3 +14,6 @@ create:
 bench: create
 	./mshark.test -test.bench=. -test.benchmem -test.run=^$$ -test.benchtime 1000x \
 	-test.cpuprofile='cpu.prof' -test.memprofile='mem.prof'
+
+clear:
+	rm -v *.txt *.pcap*

layers/dns.go

@@ -1,17 +1,144 @@
 package layers
 
-// https://en.wikipedia.org/wiki/Domain_Name_System
-// port 53
-type DNSMessage struct{}
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+const dnsHeaderSize = 12
+
+type DNSMessage struct {
+	TransactionID uint16 // Used for matching response to queries.
+	Flags         uint16 // Flags specify the requested operation and a response code.
+	Questions     uint16 // Count of entries in the queries section.
+	AnswerRRs     uint16 // Count of entries in the answers section.
+	AuthorityRRs  uint16 // Count of entries in the authority section.
+	AdditionalRRs uint16 // Count of entries in the additional section.
+	payload       []byte
+}
 
 func (d *DNSMessage) String() string {
-	return ""
+	return fmt.Sprintf(`DNS Message:
+- Transaction ID: %#04x
+- Flags: %#04x
+%s
+- Questions: %d
+- Answer RRs: %d
+- Authority RRs: %d
+- Additional RRs: %d
+- Payload: %d bytes
+`,
+		d.TransactionID,
+		d.Flags,
+		d.flags(),
+		d.Questions,
+		d.AnswerRRs,
+		d.AuthorityRRs,
+		d.AdditionalRRs,
+		len(d.payload),
+	)
 }
 
+// Parse parses the given byte data into a DNSMessage struct.
 func (d *DNSMessage) Parse(data []byte) error {
+	if len(data) < dnsHeaderSize {
+		return fmt.Errorf("minimum header size for DNS is %d bytes, got %d bytes", dnsHeaderSize, len(data))
+	}
+	d.TransactionID = binary.BigEndian.Uint16(data[0:2])
+	d.Flags = binary.BigEndian.Uint16(data[2:4])
+	d.Questions = binary.BigEndian.Uint16(data[4:6])
+	d.AnswerRRs = binary.BigEndian.Uint16(data[6:8])
+	d.AuthorityRRs = binary.BigEndian.Uint16(data[8:10])
+	d.AdditionalRRs = binary.BigEndian.Uint16(data[10:dnsHeaderSize])
+	d.payload = data[dnsHeaderSize:]
 	return nil
 }
 
 func (d *DNSMessage) NextLayer() (string, []byte) {
 	return "", nil
 }
+
+func (d *DNSMessage) flags() string {
+	// https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml
+	var flags string
+	opcode := (d.Flags >> 11) & 15
+	var opcodes string
+	switch opcode {
+	case 0:
+		opcodes = "Standard query"
+	case 1:
+		opcodes = "Inverse query"
+	case 2:
+		opcodes = "Server status request"
+	case 4:
+		opcodes = "Notify"
+	case 5:
+		opcodes = "Update"
+	case 6:
+		opcodes = "Stateful operation"
+	default:
+		opcodes = "Unknown"
+	}
+	qr := (d.Flags >> 15) & 1
+	var qrs string
+	switch qr {
+	case 0:
+		qrs = "query"
+		tc := (d.Flags >> 9) & 1
+		rd := (d.Flags >> 8) & 1
+		z := (d.Flags >> 6) & 1
+		na := (d.Flags >> 4) & 1
+		flags = fmt.Sprintf(`  - Response: Message is a %s (%d)
+  - Opcode: %s (%d)
+  - Truncated: %d
+  - Recursion desired: %d
+  - Reserved: %d
+  - Non-authenticated data: %d`, qrs, qr, opcodes, opcode, tc, rd, z, na)
+	case 1:
+		qrs = "reply"
+		a := (d.Flags >> 10) & 1
+		tc := (d.Flags >> 9) & 1
+		rd := (d.Flags >> 8) & 1
+		ra := (d.Flags >> 7) & 1
+		z := (d.Flags >> 6) & 1
+		aa := (d.Flags >> 5) & 1
+		na := (d.Flags >> 4) & 1
+		rcode := d.Flags & 15
+		var rcodes string
+		switch rcode {
+		case 0:
+			rcodes = "No error"
+		case 1:
+			rcodes = "Format error"
+		case 2:
+			rcodes = "Server failed to complete the DNS request"
+		case 3:
+			rcodes = "Domain name does not exist"
+		case 4:
+			rcodes = "Function not implemented"
+		case 5:
+			rcodes = "The server refused to answer for the query"
+		case 6:
+			rcodes = "Name that should not exist, does exist"
+		case 7:
+			rcodes = "RRset that should not exist, does exist"
+		case 8:
+			rcodes = "Server not authoritative for the zone"
+		case 9:
+			rcodes = "Name not in zone"
+		default:
+			rcodes = "Unknown"
+		}
+		flags = fmt.Sprintf(`  - Response: Message is a %s (%d)
+  - Opcode: %s (%d)
+  - Authoritative: %d
+  - Truncated: %d
+  - Recursion desired: %d
+  - Recursion available: %d
+  - Reserved: %d
+  - Answer authenticated: %d
+  - Non-authenticated data: %d
+  - Reply code: %s (%d)`, qrs, qr, opcodes, opcode, a, tc, rd, ra, z, aa, na, rcodes, rcode)
+	}
+	return flags
+}