b64

A simple CLI utility for converting captured wire-format DNS requests into Base64

Synopsis

I wrote this since the base64 does not perform URL format encoding.

This format as documented in RFC4648 is slightly different from the regular base64 encoding, and is mandatory to use when using the DoH GET method, as noted in RFC8484

Encoding Comparison

Here's an example on the subtle difference for the two encodings.

Standard Base64 encoding:

Fb4BIAABAAAAAAABAAACAAEAACkQAAAAAAAAAA==

URL-friendly encoding, without padding:

Fb4BIAABAAAAAAABAAACAAEAACkQAAAAAAAAAA

While not so obvious, here's another example, shamelessly stolen from RFC8484:

Standard Base64:

AAABAAABAAAAAAAAAWE+NjJjaGFyYWN0ZXJsYWJlbC1tYWtlcy1iYXNlNjR1cmwtZGlzdGluY3QtZnJvbS1zdGFuZGFyZC1iYXNlNjQHZXhhbXBsZQNjb20AAAEA

URL-friendly encoding:

AAABAAABAAAAAAAAAWE-NjJjaGFyYWN0ZXJsYWJlbC1tYWtlcy1iYXNlNjR1cmwtZGlzdGluY3QtZnJvbS1zdGFuZGFyZC1iYXNlNjQHZXhhbXBsZQNjb20AAAEA

Syntax

The utility does just take two parameters, as shown:

$ ./b64  -h
Usage of ./b64:
  -infile string
      input file name (required)
  -outfile string
      output file name (required)

The b64 utility will try it's best to (very cheaply) detect the input format, and convert from wire-format to base64, and vice-versa.

If you need a proper DNS packet, it's best to capture it using TCPDUMP and store the packet payload as file, then convert it using b64 to the base64 format.

Please note: No validation is done on the binary wire-format input format, so basically anything can be thrown at it. This utility is a quick&dirty helper to assist in the API development, not a fool-proof solid thing