After mountain lion is upgraded, WiFi always displays an exclamation point

After the system is upgraded to mountaion lion, the WiFi icon always displays an exclamation point, indicating that there is no Internet connection, even if the Internet connection is good, it is useless.

I tried to follow the method shown above: delete all WiFi connections and Ethernet connections, and then re-establish them. The exclamation point will indeed disappear. However, after switching to wifi, the fault will reappear.

Finally, here we can see the solution,

/Library/preferences/systemconfiguration

Delete under directory

Networkinterfaces. plist
Preferences. plist

(Backup is recommended before deletion ).
After deletion, restart and the fault is rectified. After several Wi-Fi switches, the exclamation point is not reproduced.