lib/dns: detect invalid header earlier

Previously, we unpack the header and then question without
detecting whether the header itself is valid or not, for
example the op-code, the response code.
This cause the unpacking question return an error like

  label length overflow at index xxx

One of the case is when someone sent random or HTTP request
to DoT port.

1 files changed, 18 insertions, 3 deletions

diff --git a/lib/dns/message_header.go b/lib/dns/message_header.go
index c3c6f9cb..aa505aec 100644
--- a/lib/dns/message_header.go
+++ b/lib/dns/message_header.go
@@ -5,6 +5,9 @@
 package dns
 
 import (
+	"errors"
+	"fmt"
+
 	libbytes "git.sr.ht/~shulhan/pakakeh.go/lib/bytes"
 )
 
@@ -161,19 +164,31 @@ func (hdr *MessageHeader) pack() []byte {
 }
 
 // unpack the DNS header section.
-func (hdr *MessageHeader) unpack(packet []byte) {
+func (hdr *MessageHeader) unpack(packet []byte) (err error) {
+	if len(packet) < sectionHeaderSize {
+		return errors.New(`header too small`)
+	}
+	hdr.Op = OpCode((packet[2] & headerMaskOpCode) >> 3)
+	if hdr.Op < 0 || hdr.Op > OpCodeStatus {
+		return fmt.Errorf(`unknown op code=%d`, hdr.Op)
+	}
+	hdr.RCode = ResponseCode(headerMaskRCode & packet[3])
+	if hdr.RCode < 0 || hdr.RCode > RCodeRefused {
+		return fmt.Errorf(`unknown response code=%d`, hdr.RCode)
+	}
+
 	hdr.ID = libbytes.ReadUint16(packet, 0)
 
 	hdr.IsQuery = packet[2]&headerIsResponse != headerIsResponse
-	hdr.Op = OpCode((packet[2] & headerMaskOpCode) >> 3)
 	hdr.IsAA = packet[2]&headerIsAA == headerIsAA
 	hdr.IsTC = packet[2]&headerIsTC == headerIsTC
 	hdr.IsRD = packet[2]&headerIsRD == headerIsRD
 	hdr.IsRA = packet[3]&headerIsRA == headerIsRA
-	hdr.RCode = ResponseCode(headerMaskRCode & packet[3])
 
 	hdr.QDCount = libbytes.ReadUint16(packet, 4)
 	hdr.ANCount = libbytes.ReadUint16(packet, 6)
 	hdr.NSCount = libbytes.ReadUint16(packet, 8)
 	hdr.ARCount = libbytes.ReadUint16(packet, 10)
+
+	return nil
 }