RFCs
======

Nomenclature
-------------

The definitions below will refer to this example::

    bar.foo.example

hostname
    A hostname is a DNS name used to identify a "host" (although the term "host" no longer refers to a single server). It does not include IP addresses. So ``192.168.0.1`` does not count as a hostname for our purposes here, even though it does identify a host.

label
    Roughly speaking, a label is a portion of a hostname between the dots.
    In the example,
    we see three [#noroot]_ labels, ``bar``, ``foo``, and ``exmaple``.
    Note that a valid hostname need not contain any dots if it is a relative hostname.

rightmost (most significant) label
    In our example ``example`` is the rightmost or most signification label. It may also be referred to as the **last** label. There are conditions that apply to only the rightmost label which is why this term is useful.

leftmost (least significant) label
    In our example ``bar`` is the lefttmost label. It may also be referred to as the **first** label. We may optionally relax some conditions on the leftmost label, which is why this term is useful

DNS
    The Domain Name System. Hostnames are a proper subset of DNS names, and so many of the constraints on valid hostnames arise from the definitions of a valid DNS name. Of course not all valid domain names are valid hostnames.
    After all, ``*&.!!+1.()`` is a valid domain name, but is certainly not a valid hostname.

The primary basis for what is and is not a valid hostname comes from :rfc:`952` (with updates from :rfc:`1123` and :rfc:`5890`).


RFC952: Hosts, etc
------------------

:rfc:`952` codifies hostname conventions which emerged prior to the Domain Name System.
It tells us that a valid hostname ("hname" in the grammar given) conforms to

.. productionlist::
  hname:  `name` *[ "." `name` ]
  name: `let` [ *[ `let_or_digit_or_hyphen` ] `let_or_digit` ]


Roughly that is a string of labels dot separated labels ("name" in that grammar.
The first character of a label
(and there must be a first character)
must be a letter.
The requirement that it be a letter was later dropped.
The last character of a label must be a letter or a digit.
Internal characters must be a letter, or a digit, or a hyphen.

Length restrictions
-------------------

:rfc:`1034` and :rfc:`1123` introduce additional requirements on hostnames.

:rfc:`1123` makes it clear that a hostname must also be a valid domain name.
So all requirements on domain names must apply to hostnames.


Section 3.1 of :rfc:`1034` specifies a length limits of 63 bytes on labels,
and 255 bytes for the entire domain name. That 255 includes a root "." and root domain.
And so domain names without the trailing dot can have a maximum length of 253 bytes.

The Hostname package uses the excellent [dnspython] to validate that its input is a valid domain name.

Digits first
-------------

Section 2.1 of :rfc:`1123` amends the restriction on the first character, allowing it to be a digit as well as a letter.

  The syntax of a legal Internet host name was specified in :rfc:`952`.
  One aspect of host name syntax is hereby changed: the
  restriction on the first character is relaxed to allow either a
  letter or a digit.  Host software MUST support this more liberal syntax.

This allowed hostnames such as `3Com.net`. [#3]_

Internationalization
---------------------

Internationalized hostnames add a number of complications.

.. note::
    This section is not written yet.

.. rubric:: Footnotes

.. [#noroot] I am not counting the invisible root label, "", in this discussion.

.. [#3] 3Com was a a leading network technology company at the time that these standards were developed.