Flushing DNS Cache in CentOS

If you need to flush dns, you can use the following command to:

# service nscd restart

Usually you do not need to modify the /etc/nscd.conf and the current default should suffice for most situation.

According to Linux Manual Page
Nscd  is a daemon that provides a cache for the most common name  service  requests.  The  default  configuration file,  /etc/nscd.conf,  determines  the  behavior  of the cache daemon. See nscd.conf(5).
 
Nscd provides cacheing for  accesses  of  the  passwd(5), group(5),  and  hosts(5)  databases through standard libc interfaces,  such  as  getpwnam(3),  getpwuid(3),  getgrnam(3), getgrgid(3), gethostbyname(3), and others.......