alt Hacker NewsThat's the realm side which should be upper case. The comment reference was for hostname themselves which I've always just done as lower case and have never seen a reason to make it upper case. The krb5.conf has a [domain_realm] section which can map a DNS name/suffix to the actual realm
[domain_realm]
.domain.com = DOMAIN.COM
domain.com = DOMAIN.COM
iirc, on the windows side, workgroups had to be upper case, so initially the krb realm was set to the workgroup name. dns came later