在 Linode (Ubuntu 16.10) 上，通过启用 UFW 防火墙后
sudo ufw allow OpenSSH
sudo ufw enable
sudo ufw allow in "Apache Full"
（虽然我不完全确定防火墙设置是我问题的根源......）
sudo apt-get update正在给我以下信息：
Ign:1 http://mirrors.linode.com/ubuntu yakkety InRelease
Ign:2 http://mirrors.linode.com/ubuntu yakkety-updates InRelease
Ign:3 http://mirrors.linode.com/ubuntu yakkety-backports InRelease
Err:4 http://mirrors.linode.com/ubuntu yakkety Release
404 Not Found [IP: ****:****:*::****:**** 80]
Err:5 http://mirrors.linode.com/ubuntu yakkety-updates Release
404 Not Found [IP: ****:****:*::****:**** 80]
Err:6 http://mirrors.linode.com/ubuntu yakkety-backports Release
404 Not Found [IP: ****:****:*::****:**** 80]
Ign:7 http://security.ubuntu.com/ubuntu yakkety-security InRelease
Err:8 http://security.ubuntu.com/ubuntu yakkety-security Release
404 Not Found [IP: ****:***:****::** 80]
Reading package lists... Done
E: The repository 'http://mirrors.linode.com/ubuntu yakkety Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://mirrors.linode.com/ubuntu yakkety-updates Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://mirrors.linode.com/ubuntu yakkety-backports Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://security.ubuntu.com/ubuntu yakkety-security Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
源文件/etc/apt/source.list：
# deb http://mirrors.linode.com/ubuntu/ yakkety main restricted
# deb http://mirrors.linode.com/ubuntu/ yakkety-updates main restricted
# deb http://security.ubuntu.com/ubuntu yakkety-security main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://mirrors.linode.com/ubuntu/ yakkety main restricted
# deb-src http://mirrors.linode.com/ubuntu/ yakkety main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://mirrors.linode.com/ubuntu/ yakkety-updates main restricted
# deb-src http://mirrors.linode.com/ubuntu/ yakkety-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://mirrors.linode.com/ubuntu/ yakkety universe
# deb-src http://mirrors.linode.com/ubuntu/ yakkety universe
deb http://mirrors.linode.com/ubuntu/ yakkety-updates universe
# deb-src http://mirrors.linode.com/ubuntu/ yakkety-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://mirrors.linode.com/ubuntu/ yakkety multiverse
# deb-src http://mirrors.linode.com/ubuntu/ yakkety multiverse
deb http://mirrors.linode.com/ubuntu/ yakkety-updates multiverse
# deb-src http://mirrors.linode.com/ubuntu/ yakkety-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://mirrors.linode.com/ubuntu/ yakkety-backports main restricted universe multiverse
# deb-src http://mirrors.linode.com/ubuntu/ yakkety-backports main restricted universe multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu yakkety partner
# deb-src http://archive.canonical.com/ubuntu yakkety partner
deb http://security.ubuntu.com/ubuntu yakkety-security main restricted
# deb-src http://security.ubuntu.com/ubuntu yakkety-security main restricted
deb http://security.ubuntu.com/ubuntu yakkety-security universe
# deb-src http://security.ubuntu.com/ubuntu yakkety-security universe
deb http://security.ubuntu.com/ubuntu yakkety-security multiverse
# deb-src http://security.ubuntu.com/ubuntu yakkety-security multiverse
我试过sudo apt-get dist-update无济于事（也给出了404错误）。
我也做过unset http_proxy和unset https_proxy;
apt-get -o Acquire::ForceIPv4=true update- 使用 IPv4 时仍然失败
我还可以ping 8.8.8.8。
...还没有任何工作。有人知道如何解决这个问题吗？