Which encoding does application/dns-message use?

Question

I am writing DNS-over-HTTPS server which should resolve custom names, not just proxy them to some other DoH server, like Google's. I am having trouble properly decoding the body of the request.

For example, I get body of request, that is in binary format, specifically in javascript in Uint8 ArrayBuffer type. I am using the following code to get base64 format of the array:

function _arrayBufferToBase64(buffer) {
    var binary = '';
    var bytes = new Uint8Array(buffer);
    var len = bytes.byteLength;
    for (var i = 0; i < len; i++) {
        binary += String.fromCharCode(bytes[i]);
    }
    return btoa(binary);
}

And I get something like this as a result:

AAABAAABAAAAAAABCmFwbngtbWF0Y2gGZG90b21pA2NvbQAAAQABAAApEAAAAAAAAE4ADABKAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=

Now, per RCF8484 standard this should be decoded as base64url, but when I decode it as such, I get the following:

apnx-matchdotomicom)NJ

I also used this "tutorial" as the reference, but they also decode similarly formatted blob and I get similar nonsense like previously.

There is very little to no information about something like this on the internet and if it is of any help DoH standard uses application/dns-message media type for the body.

If anyone has some insight on what I am doing wrong or how I could edit the question to make it more clear, please help me, cheers :)

Answer 1

As stated in the RFC:

6. Definition of the "application/dns-message" Media Type

The data payload for the "application/dns-message" media type is a single message of the DNS on-the-wire format defined in Section 4.2.1 of [RFC1035], which in turn refers to the full wire format defined in Section 4.1 of that RFC.

So what you get is exactly what is sent on the wire in the normal DNS over 53 case.

I would recommend you use a DNS library that should have a from_wire or similar method to which you can feed this content and get back some structured data.

Showing an example in Python with the content you gave:

In [1]: import base64

In [3]: import dns.message

In [5]: payload = 'AAABAAABAAAAAAABCmFwbngtbWF0Y2gGZG90b21pA2NvbQAAAQABAAApEAAAAAAAAE4ADABKAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA='

In [7]: raw = base64.b64decode(payload)

In [9]: msg = dns.message.from_wire(raw)

In [10]: print msg
id 0
opcode QUERY
rcode NOERROR
flags RD
edns 0
payload 4096
option Generic 12
;QUESTION
apnx-match.dotomi.com. IN A
;ANSWER
;AUTHORITY
;ADDITIONAL

So your message is a DNS query for the A record type on name apnx-match.dotomi.com.

Also about:

I am writing DNS-over-HTTPS server which should resolve custom names,

If you don't do that to learn (which is a fine goal), note that there are already various open source nameservers software that do DOH so you don't need to reinvent it. For example: https://blog.nlnetlabs.nl/dns-over-https-in-unbound/