我尝试使用 do-release-upgrade 升级到 15.10，但出现以下错误。我使用“apt-get”没有问题。我的 http_proxy 环境变量已设置---我在 /etc/environment 中设置了它。我该如何进行更新？
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [198 B]
Get:2 Upgrade tool [1,232 kB]
Fetched 1,232 kB in 0s (0 B/s)
authenticate 'wily.tar.gz' against 'wily.tar.gz.gpg'
extracting 'wily.tar.gz'
Reading cache
Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done
Building data structures... Done
Err http://security.ubuntu.com vivid-security InRelease
Err [link] vivid-security Release.gpg
Cannot initiate the connection to [link] (2001:67c:1562::15). - connect (101: Network is unreachable) [IP: 2001:67c:1562::15 80]
Err http://archive.canonical.com vivid InRelease
Err [link] vivid Release.gpg
Cannot initiate the connection to archive.canonical.com:80 (2001:67c:1360:8c01::16). - connect (101: Network is unreachable) [IP: 2001:67c:1360:8c01::16 80]
Err [link] vivid InRelease