[root@localhost network-scripts]# ifup wlan0
SIOCSIFFLAGS: No such file or directory

Determining IP information for wlan0...dhclient(5922) is already running - exiting. 
This version of ISC DHCP is based on the release available
on ftp.isc.org.  Features have been added and other changes
have been made to the base software release in order to make
it work better with this distribution.
Please report for this software via the Red Hat Bugzilla site:
    http://bugzilla.redhat.com
exiting.
 failed.
[root@localhost network-scripts]#