DNS-over-TLS requests should default to port 853 (not 53)

[appliedprivacy]

Thank you for adding DoT support!

DNS-over-TLS's is specified to operate on port 853/tcp.

blackbox exporter tries to talk DoT on port 53 by default, which is reserved for plain DNS.

We are proposing to change blackbox_exporter's default port of DoT connections to 853, when dns_over_tls is true.
even though it is easy to workaround this issue by adding the port to the target
eg. dot1.appliedprivacy.net:853

After implementing this change it should still be possible to specify the port via the target (since we want to test our DoT service on port 443 as well).

Host operating system:

4.19.0-9-amd64 #1 SMP Debian 4.19.118-2+deb10u1 (2020-06-07) x86_64 GNU/Linux

blackbox_exporter version:

blackbox_exporter, version 0.17.0 (branch: HEAD, revision: 1bc768014cf6815f7e9d694e0292e77dd10f3235)
  build user:       root@626fb3899f41
  build date:       20200619-11:54:41
  go version:       go1.14.4

What is the blackbox.yml module config.

  dns_tcp:
    prober: dns
    dns:
      preferred_ip_protocol: "ip4" # defaults to "ip6"
      query_name: "www.prometheus.io"
      query_type: "A"
      transport_protocol: "tcp"
      dns_over_tls: true
      tls_config:
        ca_file: "/etc/blackbox_exporter/DSTRootCAX3.crt"
        server_name: "dot1.appliedprivacy.net"

What did you do that produced an error?

curl "http://localhost:9115/probe?target=dot1.appliedprivacy.net&module=dns_tcp&debug=true"

What did you expect to see?

from debug log:

caller=main.go:169 module=dns_tcp target=dot1.appliedprivacy.net level=debug msg="Making DNS query" target=94.130.106.88:853 dial_protocol=tcp4-tls query=www.prometheus.io type=1 class=1

What did you see instead?

from the debug log:

caller=main.go:169 module=dns_tcp target=dot1.appliedprivacy.net level=debug msg="Making DNS query" target=94.130.106.88:53 dial_protocol=tcp4-tls query=www.prometheus.io type=1 class=1

[brian-brazil]

Would you like to send a PR?

[appliedprivacy]

Unfortunately, I don't speak Go

[kkc]

can I work on this one?

[brian-brazil]

Go ahead.