http://askubuntu.com/questions/711889/err-apt-get-update-on-ubuntu-14-04
即使给Ubuntu配置了代理，terminal能ping通网络，火狐浏览器能上网，但依然连不上网络安装软件和update，报下面的错，
解决方案是：
Create /etc/apt/apt.conf to add proxy configuration for APT.
sudo nano /etc/apt/apt.conf
Add this line to your /etc/apt/apt.conf file
Acquire::http::Proxy "http://proxyaddress:proxyport";
Save it with Ctrl+X, Y and retry:
sudo apt-get update
sudo apt-get update
Err http://extras.ubuntu.com trusty InRelease
Err http://extras.ubuntu.com trusty Release.gpg
Cannot initiate the connection to extras.ubuntu.com:80 (2001:67c:1360:8c01::23). - connect (101: Network is unreachable) [IP: 2001:67c:1360:8c01::23 80]
Err http://us.archive.ubuntu.com trusty InRelease
Err http://us.archive.ubuntu.com trusty-updates InRelease
Err http://us.archive.ubuntu.com trusty-backports InRelease
Err http://us.archive.ubuntu.com trusty-proposed InRelease
Err http://us.archive.ubuntu.com trusty Release.gpg
Cannot initiate the connection to us.archive.ubuntu.com:80 (2001:67c:1562::15). - connect (101: Network is unreachable) [IP: 2001:67c:1562::15 80]
Err http://us.archive.ubuntu.com trusty-updates Release.gpg
Cannot initiate the connection to us.archive.ubuntu.com:80 (2001:67c:1562::15). - connect (101: Network is unreachable) [IP: 2001:67c:1562::15 80]
Err http://us.archive.ubuntu.com trusty-backports Release.gpg
Cannot initiate the connection to us.archive.ubuntu.com:80 (2001:67c:1562::15). - connect (101: Network is unreachable) [IP: 2001:67c:1562::15 80]
Err http://us.archive.ubuntu.com trusty-proposed Release.gpg
Cannot initiate the connection to us.archive.ubuntu.com:80 (2001:67c:1562::15). - connect (101: Network is unreachable) [IP: 2001:67c:1562::15 80]
Err http://security.ubuntu.com trusty-security InRelease
Err http://security.ubuntu.com trusty-security Release.gpg
Cannot initiate the connection to security.ubuntu.com:80 (2001:67c:1562::17). - connect (101: Network is unreachable) [IP: 2001:67c:1562::17 80]
Reading package lists... Done
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/InRelease
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-updates/InRelease
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-backports/InRelease
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/trusty-security/InRelease
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/trusty/InRelease
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-proposed/InRelease
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/trusty/Release.gpg Cannot initiate the connection to extras.ubuntu.com:80 (2001:67c:1360:8c01::23). - connect (101: Network is unreachable) [IP: 2001:67c:1360:8c01::23 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty/Release.gpg Cannot initiate the connection to us.archive.ubuntu.com:80 (2001:67c:1562::15). - connect (101: Network is unreachable) [IP: 2001:67c:1562::15 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-updates/Release.gpg Cannot initiate the connection to us.archive.ubuntu.com:80 (2001:67c:1562::15). - connect (101: Network is unreachable) [IP: 2001:67c:1562::15 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-backports/Release.gpg Cannot initiate the connection to us.archive.ubuntu.com:80 (2001:67c:1562::15). - connect (101: Network is unreachable) [IP: 2001:67c:1562::15 80]
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/trusty-proposed/Release.gpg Cannot initiate the connection to us.archive.ubuntu.com:80 (2001:67c:1562::15). - connect (101: Network is unreachable) [IP: 2001:67c:1562::15 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/trusty-security/Release.gpg Cannot initiate the connection to security.ubuntu.com:80 (2001:67c:1562::17). - connect (101: Network is unreachable) [IP: 2001:67c:1562::17 80]
W: Some index files failed to download. They have been ignored, or old ones used instead.