Valid Names and Labels
Host Names (or 'labels' in DNS jargon) were traditionally defined by RFC 952 and RFC 1123 and may be composed of the following valid characters.
A to Z ; upper case characters
a to z ; lower case characters
0 to 9 ; numeric characters 0 to 9
- ; dash
The rules say:
- A host name (label) can start or end with a letter or a number
- A host name (label) MUST NOT start or end with a '-' (dash)
- A host name (label) MUST NOT consist of all numeric values
- A host name (label) can be up to 63 characters
RFC 2181 significantly liberalized the valid character set including the use of "_" (underscore) essentially saying that anything goes and its up to the client to validate in context. If you want to be safe stick with the rules above if you need the expanded capabilities (e.g. SRV RRs) use them. However you are taking a risk that one vital system will not talk to you at 3AM in the morning due to an upgrade!
The named.conf statement check-names allows control over the names accepted.
Domain names are defined to be case insensitive (essentially so you don't have to register every possible variant of your domain name) but the rule is that case should be preserved since this may change in the future. Host names seem to obey the same rules - essentially as an artifact of subdomains.
Host Name Examples
; for clarity we show a host name in A records
www IN A 192.168.0.3 ; valid
wWw IN A 192.168.0.3 ; valid
my www IN A 192.168.0.3 ; invalid
my-www IN A 192.168.0.3 ; valid
my_www IN A 192.168.0.3 ; invalid but may work
3www IN A 192.168.0.3 ; valid
-www IN A 192.168.0.3 ; invalid
5512 IN A 192.168.0.3 ; invalid
host-5512 IN A 192.168.0.3 ; valid
@www IN A 192.168.0.3 ; invalid
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