All,

In previous AD integration tools, the max host name length was customarily 15 chars.  Because of ancient NETBIOS restrictions (16 char restrictions and netbios adds a '$' to the end of host name).

That was like an AD 2003 restriction.

With some slightly (well-documented) trickiness, in newer AD deployments those AD integration tools would allow up to an 18 char host name.

I see in modern AD now, up to a 20 character name is tolerated (although recommendation is to retain the final '$').  So -- 19 char max.

https://social.technet.microsoft.com/Forums/en-US/f2afb2ce-74fb-4434-932b-6b71c3654a98/ad-computer-names-over-15-characters?forum=winserverDS 

(However, that statement that 'common name' (max length 15) has to be unique in the OU is troubling. If you put all your Linux servers in the same OU, you're back to your 15 char limitation.)

Apparently, in 2013 realm join and adcli allowed a15 char host name max.

    https://bugzilla.redhat.com/show_bug.cgi?id=1001667 

Is that still true today, max 15 char host name in realm join / adcli?

Spike