diff --git a/ucs-school-umc-installer/umc/python/schoolinstaller/__init__.py b/ucs-school-umc-installer/umc/python/schoolinstaller/__init__.py
index c4c182d..dfae54f 100644
--- a/ucs-school-umc-installer/umc/python/schoolinstaller/__init__.py
+++ b/ucs-school-umc-installer/umc/python/schoolinstaller/__init__.py
@@ -217,12 +217,12 @@ def umc(username, password, master, path='', options=None, flavor=None, command=
 
 def get_user_dn(username, password, master):
 	"""Get the LDAP DN for the given username."""
-	result = umc(username, password, master, 'udm/query', {"objectProperty": "username", "objectPropertyValue": username}, 'users/user')
-	result = [ientry.get('$dn$') for ientry in result if ientry.get('username') == username]
-	try:
-		return result[0]
-	except IndexError:
-		pass
+	ssh = get_ssh_connection(username, password, master)
+	stdin, stdout, stderr = ssh.exec_command(' '.join([pipes.quote(x) for x in ['/usr/bin/sudo', '/usr/bin/univention-ldapsearch', '-LLLoldif-wrap=no', filter_format('(&(uid=%s)(objectClass=person))', [username]), 'dn']]))
+	for line in stdout.splitlines():
+		match = re.match('^dn: (.*)\s*$', line)
+		if match:
+			return match.group(1)
 
 
 def create_ou_local(ou, displayName):
@@ -579,6 +579,12 @@ class Instance(Base):
 		except univention.uldap.ldap.LDAPError as err:
 			MODULE.warn('LDAP connection to %s failed: %s' % (master, err))
 			return {'success': False, 'error': _('The LDAP connection to the master system %s failed.') % master}
+		except socket.gaierror as exc:
+			MODULE.warn('Could not connect to master system %s: %s' % (master, exc))
+			return {'success': False, 'error': _('Cannot connect to the master domain controller system %s. Please make sure that the system is reachable. If not this could be due to wrong DNS nameserver settings.') % master}
+		except paramiko.SSHException as exc:
+			MODULE.warn('Could not connect to master system %s: %s' % (master, exc))
+			return {'success': False, 'error': _('Cannot connect to the master domain controller system %s. It seems that the specified domain credentials are not valid.') % master}
 
 		return {'success': True, 'schoolinfo': values}