How to resolve DNS Error if you got a "wget: unable to resolve host address" or "ping: unknown host" error?

wget http://sourceforge.net/projects/autodl-irssi/files/autodl-setup/download
Resolving sourceforge.net (sourceforge.net)... failed: Temporary failure in name resolution.
wget: unable to resolve host address `sourceforge.net'

wget http://www.google.com/
Resolving www.google.com (www.google.com)... failed: Temporary failure in name resolution.
wget: unable to resolve host address `www.google.com'

ping google.com
ping: unknown host google.com

This is a normal DNS setting error, please ping 8.8.8.8 to find out your network works or not first

If ping 8.8.8.8 works, then you should run below command via ssh root:

echo 'nameserver 8.8.8.8' > /etc/resolv.conf;

After that, please try wget again, it should no such error in future.