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

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:

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.

...
...
;; 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.

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