//Situation
System prompts that:"wget: unable to resolve host address".
//Analysis
Unable to resolve host address means the problem of DNS（Domain Name System）.
//Solution
To fix the problem, the configuration file "resolv.conf" should be modified as follow steps:
1. Get the root permission: su -i
2. Open the file with vi editor: vi /etc/resolv.conf
3. Press "i" into edit mode, and modify the content by:
nameserver 8.8.8.8 #google's DNS
nameserver 8.8.4.4 #google's DNS(backup)