Here's just an excerpt from online man page of "domainname":
hostname - show or set the system's host name
domainname - show or set the system's NIS/YP domain name
dnsdomainname - show the system's DNS domain name
nisdomainname - show or set system's NIS/YP domain name
ypdomainname - show or set the system's NIS/YP domain name
hostname will print the name of the system as returned by the gethost-
domainname, nisdomainname, ypdomainname will print the name of the sys-
tem as returned by the getdomainname(2) function. This is also known as
the YP/NIS domain name of the system.
dnsdomainname will print the domain part of the FQDN (Fully Qualified
Domain Name). The complete FQDN of the system is returned with hostname
Sometime you may find a weird thing that you can use ldap verification to log on a client, but you can not sudo to root. Now you should consider run domainname to check whether it's set to (none). If it does, you should consider set the domainname just using domainname command.