AD Client allow login without domain name

If it was been build sssd

you would edit /etc/sssd/sssd.conf

#vi /etc/sssd/sssd/conf
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use_fully_qualified_names false
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.

restart sssd

If it was build likewise open

you would edit /etc/samba/lwiauthd.conf

#vi /etc/samba/lwiauthd.conf
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winbind use default domain = yes
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restart likewise open