[Freedombox-discuss] Can't get android phone to connect to radicale.

Daniel Gnoutcheff gnoutchd at softwarefreedom.org
Wed Feb 8 18:16:43 UTC 2017

On 02/07/2017 11:51 PM, A. F. Cano wrote:
> ping <domain>.freedombox.rocks
> PING <domain>.freedombox.rocks ( 56(84) bytes of data.
> 64 bytes from 229.sub-75-226-115.myvzw.com ( icmp_req=1
> ttl=64 time=0.247 ms
> 64 bytes from 229.sub-75-226-115.myvzw.com ( icmp_req=2
> ttl=64 time=0.468 ms
> I wonder why the IP is correct but the domain name has been changed by
> verizon.  If I were the suspicious type I might conclude that verizon
> is trying to prevent access from the outside by re-writing the domain
> name.  Is this what is happening?

No.  This is typical, and it won't have any impact on HTTP(S) or Let's

The reason you see the `229.sub-75-226-115.myvzw.com` is that ping(1)
performs a reverse DNS lookup on the IP you're pinging.  We call it
"reverse DNS" because it takes us from an IP to a hostname, rather than
the other way around.

Reverse DNS records are distinct from your normal ("forward") DNS
records and are subject to a different control structure.  For a given
IP address (say,, the corresponding reverse DNS record
(``) is usually controlled by whoever owns
that IP (in this case, Verizon).

Most Internet applications don't use reverse DNS for anything important,
because (as you've observed) it's often wrong.  For sure, web browsers
and the Let's Encrypt validation servers don't use it at all.

E-mail is the only application I know of where reverse DNS matters.  An
email server must have a correct reverse DNS record, or other servers
will assume it's a spammer.

"Business class" ISPs often allow you to customize the reverse DNS
records for the IP addresses they've given you.  Residential ISPs
usually do not.

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