'realm join' is failing with error "Failed to join domain: Failed to set machine spn: Constraint violation"
 SOLUTION VERIFIED - Updated March 19 2018 at 10:42 AM - English
Environment
Red Hat Enterprise Linux(RHEL) 7
realmd
Issue
'realm join' is failing with the following error even if user is member of "Domain Admins" group:
Raw
# realm join -v example.com --computer-ou OU=Servers,OU=DEV -U ad_user
 * Resolving: _ldap._tcp.example.com
 * Performing LDAP DSE lookup on: 192.168.0.1
 * Successfully discovered: EXAMPLE.COM
Password for ad_user:
 * Required files: /usr/sbin/oddjobd, /usr/libexec/oddjob/mkhomedir, /usr/sbin/sssd, /usr/bin/net
 * LANG=C LOGNAME=root /usr/bin/net -s /var/cache/realmd/realmd-smb-conf.GVUCFZ -U ad_user ads join EXAMPLE.COM createcomputer=DEV/Servers
Enter  ad_user's password:
Failed to join domain: Failed to set machine spn: Constraint violation   <-----
Do you have sufficient permissions to create machine accounts?           <-----
 ! Joining the domain EXAMPLE.COM failed
realm: Couldn't join realm: Joining the domain EXAMPLE.COM failed
Resolution
Check if machine account for the system is already available/present in AD.

If it's already there, delete/remove the existing account in AD or choose different hostname for the system.

Then re-attempt realm join.

Root Cause
Turned out the hostname was conflicting with another hostname in a different domain in the same forest thus the SPN was not unique in the forest. Changing the hostname should resolve this issue.
Diagnostic Steps
Check if AD user has sufficient permissions in AD to join system into the domain.

Before attempting a join using realm join confirm on AD side if machine account of the system is available/present or not.

Product(s)  Red Hat Enterprise Linux Category  Troubleshoot Tags  active_directory rhel_7
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