Hosts table allow creating hosts with up to 128 characters in their name and visible name fields, but auto-registration allows only 64 characters maximum.
|Hostname||no||Set by HostnameItem||Unique, case sensitive hostname.
Required for active checks and must match hostname as configured on the server.
Allowed characters: alphanumeric, '.', ' ', '_' and '-'.
Maximum length: 64
The limitation is also on autoreg_host and proxy_autoreg_host tables, where host is defined as host | varchar(64)
The suggestion is to increase these limits to allow hosts with up to 128 chars in their names to be auto registered. This will allow long FQDN names to be used for auto-registration.