.. SPDX-License-Identifier: GPL-3.0-or-later

.. _config-experimental-dot-auth:

Experimental DNS-over-TLS Auto-discovery
========================================

This experimental feature provides automatic discovery of authoritative servers' supporting DNS-over-TLS.
It uses magic NS names to detect SPKI_ fingerprint which is very similar to `dnscurve`_ mechanism.

.. warning:: This protocol and feature is experimental and can be changed or removed at any time. Use at own risk, security properties were not analyzed!

How it works
------------

It will look for NS target names formatted as:
``dot-{base32(sha256(SPKI))}....``

For instance, Knot Resolver will detect NS names formatted like this

.. code-block:: none

  example.com NS dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.example.com

and automatically discover that example.com NS supports DoT with the base64-encoded SPKI digest of ``m+12GgMFIiheEhKvUcOynjbn3WYQUp5tVGDh7Snwj/Q=``
and will associate it with the IPs of ``dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.example.com``.

In that example, the base32 encoded (no padding) version of the sha256 PIN is ``tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a``, which when
converted to base64 translates to ``m+12GgMFIiheEhKvUcOynjbn3WYQUp5tVGDh7Snwj/Q=``.

Generating NS target names
--------------------------

To generate the NS target name, use the following command to generate the base32 encoded string of the SPKI fingerprint:

.. code-block:: bash

  openssl x509 -in /path/to/cert.pem  -pubkey -noout | \
  openssl pkey -pubin -outform der | \
  openssl dgst -sha256 -binary | \
  base32 | tr -d '=' | tr '[:upper:]' '[:lower:]'
  tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a

Then add a target to your NS with: ``dot-${b32}.a.example.com``

Finally, map ``dot-${b32}.a.example.com`` to the right set of IPs.

.. code-block:: bash

  ...
  ...
  ;; QUESTION SECTION:
  ;example.com.      IN      NS

  ;; AUTHORITY SECTION:
  example.com. 3600  IN      NS      dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.a.example.com.
  example.com. 3600  IN      NS      dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.b.example.com.

  ;; ADDITIONAL SECTION:
  dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.a.example.com. 3600 IN A 192.0.2.1
  dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.b.example.com. 3600 IN AAAA 2001:DB8::1
  ...
  ...

You can enable DoT auto-discovery feature in configuration file.

.. code-block:: yaml

   network:
     tls:
       # start an experiment, use with caution
       auto-discovery: true

This feature requires standard ``basexx`` Lua library which is typically provided by ``lua-basexx`` package.

Caveats
-------

The feature relies on seeing the reply of the NS query and as such will not work if Knot Resolver uses data from its cache.
You may need to delete the cache before starting the resolver to work around this.

Auto-discovery also assumes that the NS query answer will return both the NS targets in the Authority section as well as the glue records in the Additional section.

Dependencies
------------

* `lua-basexx <https://github.com/aiq/basexx>`_ available in LuaRocks

.. _dnscurve: https://dnscurve.org/
.. _SPKI: https://en.wikipedia.org/wiki/Simple_public-key_infrastructure
