RFC-952 (last sentence of point 1 under Assumptions) prohibits single-character host names and I have had experiences (
over 7 years ago summer of 2002) where some services would refuse to work with single-character host names (because such names were not standards-compliant), but I have seen a number of single-character host names in use in the past few years. Are single-character host names now valid? (If so, what is the proper validation reference?)
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. [ ... ] See also [RFC1123] section 188.8.131.52.
From RFC-1123 section 184.108.40.206:
The DNS defines domain name syntax very generally -- a string of labels each containing up to 63 8-bit octets, separated by dots, and with a maximum total of 255 octets. Particular applications of the DNS are permitted to further constrain the syntax of the domain names they use, although the DNS deployment has led to some applications allowing more general names. In particular, Section 2.1 of this document liberalizes slightly the syntax of a legal Internet host name that was defined in RFC-952 [DNS:4].
From RFC-1123 section 2.1:
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.
And finally, as originally referenced, from RFC-952:
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. [ ... ] Single character names or nicknames are not allowed.
It is from following this chain that I originally came to say that RFC-952 prohibits single-character host names.