git や yum update、 ./run.sh のコマンドを実行すると、以下のような「Couldn't resolve host ... 」というエラーが発生します。どうすればよいでしょうか？

[vagrant@localhost centos6]$ git clone https://github.com/dotinstallres/centos6.git
Initialized empty Git repository in /home/vagrant/centos6/centos6/.git/
error: Couldn't resolve host 'github.com' while accessing https://github.com/dotinstallres/centos6.git/info/refs

fatal: HTTP request failed

Loading mirror speeds from cached hostfile
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=6&arch=x86_64&repo=os&infra=stock error was
14: PYCURL ERROR 6 - "Couldn't resolve host 'mirrorlist.centos.org'"
エラー: Cannot find a valid baseurl for repo: base