Last time, we successfully generated a DNS Query and get the results from the DNS server.
I would like to analyze this result, but this data is compressed.
In this Section. I'll explain about dns compression.
DNS Message Compression
According to (RFC1035)[https://datatracker.ietf.org/doc/html/rfc1035], We can specify the location to be referenced by OFFSET in the following format.
a domain name represented as a sequence of labels, where each label consists of a length octet followed by that number of octets.
The domain name terminates with the zero length octet for the null label of the root.
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| 1 1| OFFSET |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
For example, adding "example.com" to the buffer would look like this
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
13 | 6 | e |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
22 | x | a |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
24 | m | p |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
26 | l | e |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
28 | 3 | c |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
30 | o | m |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
30 | 0 |
+--+--+--+--+--+--+--+--+
From now on, if you return a URL named www.example.com, you can use
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
40 | 3 | w |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
42 | w | w |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
44 | 1 1| 13 |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
You will be able to express yourself in the above way.
Write This In Dart
Let's write a code to compress URLs, which can be written in about 30 lines of code.
// dnsdict.dart
import 'dart:convert';
import 'dart:typed_data' show Uint8List;
class DNSCompressionDictItem {
int index;
}
class DNSCompressionDict {
Map<String, DNSCompressionDictItem> dict = {};
Uint8List add(String item, int index) {
var items = item.split('.');
var buffer = <int>[];
for (var i = 0; i < items.length; i++) {
var key = items.sublist(i).join('.');
if (dict.containsKey(key)) {
// 既に登録されていれば、そのアドレスを返す
var tmp = dict[key].index | 0xC000;
buffer.addAll([(tmp >> 8) & 0xFF, tmp & 0xFF]);
return Uint8List.fromList(buffer);
} else {
// 登録されていないならば、保存する
buffer.add(items[i].length);
buffer.addAll(ascii.encode(items[i]));
dict[key] = DNSCompressionDictItem()..index = index;
index += items[i].length + 1;
}
}
if (buffer.isNotEmpty) {
buffer.add(0);
}
// 登録されていないならば、保存する
return Uint8List.fromList(buffer);
}
}
// dnsdict_test.dart
import 'package:info.kyorohiro.dns/dns.dart';
import 'package:test/test.dart';
void main() {
group('DNSName', () {
setUp(() {});
test('DNSName.encode()', () {
var dict = DNSCompressionDict();
int index = 0;
{
var bufferSrc = dict.add('yahoo.co.jp', 0);
index += bufferSrc.length;
expect(DNSBuffer.fromList(bufferSrc).toHex(), '057961686f6f02636f026a7000');
}
// 057961686f6f02636f026a7000(13)
{
var bufferSrc = dict.add('google.co.jp', index);
index += bufferSrc.length;
expect(DNSBuffer.fromList(bufferSrc).toHex(), '06676f6f676c65c006');
}
// 057961686f6f02636f026a7000(13)
// 06676f6f676c65c006(9)
{
var bufferSrc = dict.add('www.google.co.jp', index);
index += bufferSrc.length;
expect(DNSBuffer.fromList(bufferSrc).toHex(), '03777777c00d');
}
// 057961686f6f02636f026a7000(13)
// 06676f6f676c65c006(9)
// 03777777c00d(6)
{
var bufferSrc = dict.add('www.google.co.jp', index);
index += bufferSrc.length;
expect(DNSBuffer.fromList(bufferSrc).toHex(), 'c016');
}
});
});
}
Decompress In Dart
If you are writing in C, you need to check if you are accessing invalid memory. However, since this is a Dart program, we have not checked for infinite loops.
You may want to check for infinite loops.
This too can be written in about 30 lines of code.
// dnsname.dart
static Tuple2<String, int> createUrlFromName(Uint8List srcBuffer, int index) {
var outBuffer = StringBuffer();
var i = index;
for (; i < srcBuffer.length;) {
var nameLength = srcBuffer[i];
if (nameLength == 0) {
// TEXT END
i++;
return Tuple2<String, int>(outBuffer.toString(), i - index);
} else if ((0xC0 & nameLength) == 0xC0) {
// Compression
var v = ((nameLength & 0x3f) << 8) | srcBuffer[++i];
var r = createUrlFromName(srcBuffer, v);
if (outBuffer.length > 0) {
outBuffer.write('.');
}
outBuffer.write(r.item1);
i++;
return Tuple2<String, int>(outBuffer.toString(), i - index);
} else {
var nameBytes = srcBuffer.sublist(i + 1, i + 1 + nameLength);
if (outBuffer.length > 0) {
outBuffer.write('.');
}
outBuffer.write(ascii.decode(nameBytes, allowInvalid: true));
i = i + 1 + nameLength;
}
}
throw DNSNameException('Not Found Null Char');
}
Next time
Parse the DNS Message retrieved from the DNS server last time and display the result.
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