If your system hostname does not resolve in reverse DNS, you'll get this output:

]# ambari-server start
Using python /usr/bin/python2.6
Starting ambari-server
Traceback (most recent call last):
File "/usr/sbin/ambari-server.py", line 4491, in <module>
main()
File "/usr/sbin/ambari-server.py", line 4297, in main
start(options)
File "/usr/sbin/ambari-server.py", line 2453, in start
if not check_reverse_lookup():
File "/usr/sbin/ambari-server.py", line 825, in check_reverse_lookup
host_ip = socket.gethostbyname(host_name)
socket.gaierror: [Errno -2] Name or service not known

This needs to be noted in the documentation. You can fix it by editing /etc/hosts to make the hostname reverse resolve.