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kyorohiro (kiyohiro kawamura)
kyorohiro (kiyohiro kawamura)

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DNS Compression In Dart

#dart #dartlang #dns

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.
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+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1 1| OFFSET | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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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 | +--+--+--+--+--+--+--+--+
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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 | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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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); } }
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// 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'); } }); }); }
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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'); }
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Next time

Parse the DNS Message retrieved from the DNS server last time and display the result.

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