1. DNS/RFC/2181/s11

1.1. 11. Name syntax

Occasionally it is assumed that the Domain Name System serves only the purpose of mapping Internet host names to data, and mapping Internet addresses to host names.

This is not correct, 
the DNS is a general (if somewhat limited) hierarchical database,
and can store almost any kind of data, for almost any purpose.

1.1.1. たったひとつの制限: 名前の長さ

The DNS itself places only one restriction on the particular labels that can be used to identify resource records.

That one restriction relates to the length of the label and the full name.  
  The length of any one label is limited to between 1 and 63 octets.  
  A full domain name is limited to 255 octets (including the separators).  
  The zero length full name is defined as representing the root of the DNS tree,
  and is typically written and displayed as ".".  

Those restrictions aside, any binary string whatever can be used as the label of any resource record.

Similarly, any binary string can serve as the value of any record that includes a domain name as some or all of its value

• (SOA, NS, MX, PTR, CNAME, and any others that may be added).

Implementations of the DNS protocols must not place any restrictions
on the labels that can be used.  

In particular, DNS servers must not refuse to serve a zone because it contains labels that might not be acceptable to some DNS client programs.

A DNS server may be

• configurable to issue warnings when loading, or even to refuse to load, a primary zone containing labels that might be considered questionable, however this should not happen by default.

1.2. 制限

アプリケーションは制限をしていることがある。

Note however, that the various applications that make use of DNS data can have restrictions imposed on what particular values are acceptable in their environment.

For example, that any binary label

• can have an MX record does not imply that any binary name can be used as the host part of an e-mail address.

Clients of the DNS can impose

• whatever restrictions are appropriate to their circumstances on the values they use as keys for DNS lookup requests, and on the values returned by the DNS.

If the client has such restrictions, it is

• solely responsible for validating the data from the DNS to ensure that it conforms before it makes any use of that data.

  See also [RFC1123] section 6.1.3.5. DNS/RFC/1123