* * * * *

         Leave it to the phone companies to make simple ideas complex

> These telephone DNS (Domain Name System) records are typically expected to
> contain a record with the type NAPTR (Naming Authority Pointer Record).
> NAPTR, generally speaking, is intended to map addresses (more properly URN
> (Universal Resource Name)s) to other types of addresses. For example, E.164
> to SIP (Session Initiation Protocol). So the NAPTR record, typically, would
> provide an address type of E2U+sip which indicates ENUM (E.164 Number
> Mapping) (E.164 number mapping) to the SIP protocol.
>
> Fascinatingly, the actual payload of an NAPTR record is… a regular
> expression. The regular expression specifies a transformation (with capture
> groups and the whole nine yards) from the original address (the E.164
> number) to the new address type. In theory, this allows optimization of
> NAPTR records at higher levels of the hierarchy if components of the
> original address are also used in the new address. This is one of many
> areas of DNS that are trying perhaps too hard to be clever.
>

“Computers Are Bad: can I get your number domain [1]”

I'm very surprised I haven't mentioned NAPTR records [2] before, because I
have to deal with them at The Corporation. The service we provide to our
customers, the Oligarchic Cell Phone Companies, is to translate a phone
number, such as “867-5309” to “Jenny,” and that is done via DNS using NAPTR
records. I was surprised at first, because I was expecting the Oligarchic
Cell Phone Companies to use some esoteric and proprietary protocol to handle
such information, but nope—it's via DNS. One complication here is that each
phone company keeps its own database of numbers to names, and you have to
negotiate with, and more importantly, pay, each one to query their data. Now,
because The Corporation has all these contracts in place, all an Oligarchic
Cell Phone Company has to do is pay us to get access to the name databases of
the other Oligarchic Cell Phone Companies.

A classic middleman situation here.

But getting back to the article. The author uses the example +18002662278 as
an example of an E.164 global number (where E.164 is a standard from ITU
(International Telecommunication Union)), but the example is a bad one,
because 800 numbers are not global numbers! You can only dial 800 numbers
from North America, so to present +18002662278 as a global number is
incorrect. This is an issue for us, because Project: Sippy-Cup [3] has to
deal with both global and “local” numbers (where “local” means—intra-country
code calling). and yes, we get a ton of 800 numbers sent in as global numbers
(then again, we get an amazing assortment of garbage numbers from the
Oligarchic Cell Phone Companies, including all zeros, all ones, numbers with
international dialing prefixes, short codes, service codes, and it wouldn't
surprise me if we get ZIP (Zone Improvement Plan) codes from time to time—
you'd think the Oligarchic Cell Phone Companies would filter out such
garbage, but hey, they're the Oligarchic Cell Phone Companies, they don't
have to care).

I also agree with the author about the use of regular expressions in the
NAPTR record. It complicates the spec and in the decade I've been working at
The Corporation, I've yet to see anyone use the regular expression
transformation, and in the source code parse NAPTR records is this comment
from the original developer: “Skip the insanely stupid regexp pattern; we are
ignoring it.”

The name lookups are not using the “E2U+sip” addressing, but
“E2U+pstndata:cnam” addressing, based off a draft standard [4] from twelve
years ago. And reading that specification is a hoot in the light of the harsh
reality of … um … reality. It defines the pstndata: scheme as:

-----[ BNF ]-----
pstndatauri  = "pstndata:" datatype ["/" telephone-
subscriber ] ";" content
datatype     = "cnam"
            ; Other datatypes can be defined by adding
            ; alternative values.
content      = [ mediatype ] [ ":base64" ] "," data
mediatype    = [ type "/" subtype ] *( ";" parameter )
data         = *urlchar
parameter    = attribute "=" value

where "telephone-subscriber" is imported from RFC 3966
[19], "urlchar" is imported from RFC 2396 [20], and
"attribute" and "value" are imported from RFC 2045 [21].
-----[ END OF LINE ]-----

LIES! ALL LIES!

Okay, it's not quite that bad, but having dealt with all three RFC (Request
For Comments)s mentioned, I'm not sure the author of the specification
carefully read the them, or considered deeply how they might interact. The
definition of telephone-subscriber contains not only the “phone number” but
also parameters separated by semicolons! I have code to parse telephone-
subscriber [5] but I can't use it here because in reality, it conflicts with
this definition (why yes, the whole notion of parsing this data came up
recently at work, why do you ask?) It's always fun when reality trumps theory
(no, not really).

So as the author states, “DNS does handle phone numbers!” But it's not quite
as simple as IP (Internet Protocol) addresses.

[1] https://computer.rip/2021-03-
[2] https://www.ietf.org/rfc/rfc3403.txt
[3] gopher://gopher.conman.org/0Phlog:2014/03/05.1
[4] https://www.ietf.org/archive/id/draft-ietf-enum-cnam-08.txt
[5] https://github.com/spc476/LPeg-

Email author at [email protected]

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