[idn] hostname history hell

["Eric A. Hall" <ehall@ehsco.com>]

The earliest definition I know of for hostnames is rfc608 from 1974.

| in which <host-name> will be the official Host Name, a
| string obtained through negotiation between the Host and the
| NIC, governed by these constraints:
| 
|    up to 48 characters drawn from the alphabet (A-Z),
|    digits (0-9), and the minus sign (-) ... specifically,
|    no blank or space characters allowed;
| 
|    no distinction between upper and lower case letters;
| 
|    the first character is a letter;
| 
|    the last character is NOT a minus sign;
| 
|    no other restrictions on content or syntax.

Minimum 1 alpha character, maximum 48 alphanumeric chars, first char alpha
(ergo not hyphen), last char alphanumeric. The exclusion of beginning and
ending hyphens had to do with syntax of multi-part service-specific names
as is more clear later.

One side-note is that hyphen is one of the only punctuation characters
that is common between all implementations of the ECMA-6 charset.

Updated by rfc810

| 1. A "name" (Net, Host, Gateway, or Domain name) is a text string up
| to 24 characters drawn from the alphabet (A-Z), digits (0-9), and the
| minus sign (-) and period (.).  No blank or space characters are
| permitted as part of a name.  No distinction is made between upper
| and lower case.  The first character must be a letter.  The last
| character must not be a minus sign or period.  A host which serves as
| a GATEWAY should have "-GATEWAY" or "-GW" as part of its name.  A
| host which is a TIP or a TAC should have  "-TIP" or "-TAC" as part of
| its host name, if it is an ARPANET or DoD host.

Minimum 1 alpha character, maximum 24 alphanumeric chars (half the
original length from rfc608), first char alpha (ergo not hyphen), last
char alphanumeric. No explicit FQDN lengths defined.

Updated by rfc952

| 1. A "name" (Net, Host, Gateway, or Domain name) is a text string up
| to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus
| sign (-), and period (.).  Note that periods are only allowed when
| they serve to delimit components of "domain style names". (See
| RFC-921, "Domain Name System Implementation Schedule", for
| background).  No blank or space characters are permitted as part of a
| name. No distinction is made between upper and lower case.  The first
| character must be an alpha character.  The last character must not be
| a minus sign or period.  A host which serves as a GATEWAY should have
| "-GATEWAY" or "-GW" as part of its name.  Hosts which do not serve as
| Internet gateways should not use "-GATEWAY" and "-GW" as part of
| their names. A host which is a TAC should have "-TAC" as the last
| part of its host name, if it is a DoD host.  Single character names
| or nicknames are not allowed.

Minimum 2 alphanumeric characters (double the original minimum), maximum
24 alphanumeric chars, first char alpha (ergo not hyphen), last char
alphanumeric. No explicit FQDN lengths defined.

All of the above specifies host names as they appear in hosts databases.
These rules were often enforced by /etc/hosts implementations as well (eg,
some unix systems have maximum entry lengths of 24 chars in the local
/etc/hosts database).

rfc1035 expanded the hostname rules implicitly

| However, when assigning a domain name for an object, the prudent user
| will select a name which satisfies both the rules of the domain system
| and any existing rules for the object, whether these rules are published
| or implied by existing programs.

| The labels must follow the rules for ARPANET host names.  They must
| start with a letter, end with a letter or digit, and have as interior
| characters only letters, digits, and hyphen.  There are also some
| restrictions on the length.  Labels must be 63 characters or less.

Note that rfc1035 allows labels (and thus hostnames) to have minimum
length of 1 character, with a maximum of 63 characters for each label, and
with a fully-qualified domain name (and thus hostnames) to be 255
characters including the separators.

Minimum 2 alphanumeric character (inherited from rfc952), maximum 63
alphanumeric chars, first char alpha (ergo not hyphen), last char
alphanumeric. Maximum 255 for FQDN.

Some unification and clarification by rfc1123

| The syntax of a legal Internet host name was specified in RFC-952
| [DNS:4].  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.

[first char alphanumeric, but not hyphen]

| Host software MUST handle host names of up to 63 characters and
| SHOULD handle host names of up to 255 characters.

[unified label length of 63 max, unified FQDN of 255, no unified minimum
length so minimum of 2 inherited from rfc952 prevails]

| Whenever a user inputs the identity of an Internet host, it SHOULD
| be possible to enter either (1) a host domain name or (2) an IP
| address in dotted-decimal ("#.#.#.#") form.  The host SHOULD check
| the string syntactically for a dotted-decimal number before
| looking it up in the Domain Name System.
| 
| DISCUSSION:
|      This last requirement is not intended to specify the complete
|      syntactic form for entering a dotted-decimal host number;
|      that is considered to be a user-interface issue.  For
|      example, a dotted-decimal number must be enclosed within
|      "[ ]" brackets for SMTP mail (see Section 5.2.17).  This
|      notation could be made universal within a host system,
|      simplifying the syntactic checking for a dotted-decimal
|      number.
|      
|      If a dotted-decimal number can be entered without such
|      identifying delimiters, then a full syntactic check must be
|      made, because a segment of a host domain name is now allowed
|      to begin with a digit and could legally be entirely numeric
|      (see Section 6.1.2.4).  However, a valid host name can never
|      have the dotted-decimal form #.#.#.#, since at least the
|      highest-level component label will be alphabetic.

Note that the last sentence says that the trailing label must contain
alpha chars, probably under the assumption that TLDs will be used. The
rule is not explicity stated but it is implied strongly.

Minimum 2 alphanumeric characters, maximum 63 alphanumeric chars, first
char alphanumeric, last char alphanumeric. Maximum 255 for FQDN. FQDN must
contain at least one alpha.

-- 
Eric A. Hall                                        http://www.ehsco.com/
Internet Core Protocols          http://www.oreilly.com/catalog/coreprot/