expand the reverse DNS documentation

    provider and `$MAC` is the MAC address as found in `ip link show

    up`.

    Make sure reverse DNS is correct as well.

 4. ensure reverse DNS is set for the machine. this can be done either

    in the upstream configuration dashboard (e.g. Hetzner) or in our

    zone files, in the `dns/domains.git` repository.

    Pro tip: `dig -x` will show you an SOA record pointing at the

    authoritative DNS server for the relevant zone, and will even show

    you the right record to create. Since IPv6 records are

    particularly painful to create, you should use this all the time.

    For example, the IP addresses of `chi-node-01` are `38.229.82.104`

    and `2604:8800:5000:82:baca:3aff:fe5d:8774`, so the records to

    create are:

        $ dig -x 2604:8800:5000:82:baca:3aff:fe5d:8774 38.229.82.104

        [...]

        ;; QUESTION SECTION:

        ;4.7.7.8.d.5.e.f.f.f.a.3.a.c.a.b.2.8.0.0.0.0.0.5.0.0.8.8.4.0.6.2.ip6.arpa. IN PTR

        ;; AUTHORITY SECTION:

        2.8.0.0.0.0.0.5.0.0.8.8.4.0.6.2.ip6.arpa. 3552 IN SOA nevii.torproject.org. hostmaster.torproject.org. 2021020201 10800 3600 1814400 3601

        [...]

        ;; QUESTION SECTION:

        ;104.82.229.38.in-addr.arpa.	IN	PTR

        ;; AUTHORITY SECTION:

        82.229.38.in-addr.arpa.	2991	IN	SOA	ns1.cymru.com. noc.cymru.com. 2020110201 21600 3600 604800 7200

        [...]

    In this case, you should add this record to

    `82.229.38.in-addr.arpa.`:

        104.82.229.38.in-addr.arpa.	IN	PTR chi-node-01.torproject.org.

    And this to `2.8.0.0.0.0.0.5.0.0.8.8.4.0.6.2.ip6.arpa.`:

        4.7.7.8.d.5.e.f.f.f.a.3.a.c.a.b.2.8.0.0.0.0.0.5.0.0.8.8.4.0.6.2.ip6.arpa. IN PTR chi-node-01.torproject.org.

 4. the machine has a short hostname (e.g. `test`) which resolves to a

    fully qualified domain name (e.g. `test.torproject.org`) in the