mDNS Documentation

This document is currently used and update for testing purposes of DNS packets with Daplie applications. Please make note of any errata, as the organization of this document is based on the step by step process of debugging current issues regarding DNS, and not necessarily a demonstration on how to fix those issues. This document is for learning purposes and meant to assist future developers avoid similar bugs.

Objective

Create a robust DNS library that checks all possible combinations of DNS flags and messages in order to debug current DNS state for Daplie system and potentially develope DNS library with built in linting for use of Daplie, inc and community.

How to duplicate DNS crash:

>> cd ~/dns_test
>> node listen.jss

Then in another terminal enter:

>> dig @224.0.0.251 -p 5353 -t PTR _cloud._tcp.local

The listener then crashes with an output of:

START DNS PACKET
/home/daplie/dns_test/node_modules/dns-js/lib/bufferconsumer.js:52
      throw new Error('Buffer overflow')
      ^

Error: Buffer overflow
    at BufferConsumer.slice (/home/daplie/dns_test/node_modules/dns-js/lib/bufferconsumer.js:52:13)
  s  at Function.DNSRecord.parse (/home/daplie/dns_test/node_modules/dns-js/lib/dnsrecord.js:237:46)
    at /home/daplie/dns_test/node_modules/dns-js/lib/dnspacket.js:164:30
    at Array.forEach (native)
    at Function.DNSPacket.parse (/home/daplie/dns_test/node_modules/dns-js/lib/dnspacket.js:159:17)
    at /home/daplie/dns_test/cloud-respond.js:86:31
    at Array.forEach (native)
    at /home/daplie/dns_test/cloud-respond.js:11:21
    at Array.forEach (native)
    at Object.module.exports.respond (/home/daplie/dns_test/cloud-respond.js:10:11)

After commenting out lines 45-53 in dns_test/node_modules/dns-js/lib/bufferconsumer.js and rerunning the previous commands, the result is a new error:

START DNS PACKET
buffer.js:829
    throw new RangeError('Index out of range');
    ^

RangeError: Index out of range
    at checkOffset (buffer.js:829:11)
    at Buffer.readUInt8 (buffer.js:867:5)
    at BufferConsumer.byte (/home/daplie/dns_test/node_modules/dns-js/lib/bufferconsumer.js:67:22)
    at BufferConsumer.name (/home/daplie/dns_test/node_modules/dns-js/lib/bufferconsumer.js:120:14)
    at Function.DNSRecord.parse (/home/daplie/dns_test/node_modules/dns-js/lib/dnsrecord.js:187:14)
    at /home/daplie/dns_test/node_modules/dns-js/lib/dnspacket.js:164:30
    at Array.forEach (native)
    at Function.DNSPacket.parse (/home/daplie/dns_test/node_modules/dns-js/lib/dnspacket.js:159:17)
    at /home/daplie/dns_test/cloud-respond.js:86:31
    at Array.forEach (native)

which is located in the node.js buffer module. The API is here.

However, the error we are working with will most likely be dealt with by parsing through the binary and putting it in a format that is acceptable to a custom buffer, since the current buffer.js does doesn't seem to do the trick.

Using

function pad(str, len, ch) {

  while (str.length < len) {
    str = ch + str;
  }

  return str;
}

the binary output comes out as:

11100001
10001000
00000001
00100000
00000000
00000001
00000000
00000000
00000000
00000000
00000000
00000001
00000110
01011111
01100011
01101100
01101111
01110101
01100100
00000100
01011111
01110100
01100011
01110000
00000101
01101100
01101111
01100011
01100001
01101100
00000000
00000000
00001100
00000000
00000001
00000000
00000000
00101001
00010000
00000000
00000000
00000000
00000000
00000000
00000000
00000000

What are the possible problems?

How to print out hex values of the DNS message in node.js?

socket.on('message', function (message, rinfo) {
  console.log('Received %d bytes from %s:%d\n',
    message.length, rinfo.address, rinfo.port);
  //console.log(msg.toString('utf8'));

  console.log(message.toString('hex'));

DNS sec: security puts a signature on a DNS packet and imprints a signature so that the sender of the packet is confirmed