我的计算机上有一个kubuntu 16.04，当我想进行apt更新时，出现403禁止错误。我在互联网上访问了很多网页，但他们并没有帮助我。我该如何解决这个问题。
每次我执行apt-get更新时，我都会得到以下结果：
Ign:1 cdrom://Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1) xenial InRelease
Err:2 cdrom://Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1) xenial Release
Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
Ign:3 http://security.ubuntu.com/ubuntu xenial-security InRelease
Err:4 http://security.ubuntu.com/ubuntu xenial-security Release
503 Service Unavailable
Ign:5 http://archive.ubuntu.com/ubuntu xenial InRelease
Ign:6 http://archive.ubuntu.com/ubuntu xenial-updates InRelease
Err:7 http://archive.ubuntu.com/ubuntu xenial Release
503 DNS error for hostname archive.ubuntu.com: Temporary failure in name resolution. If archive.ubuntu.com refers to a configured cache repository, please check the corresponding configuration file.
Err:8 http://archive.ubuntu.com/ubuntu xenial-updates Release
503 DNS error for hostname archive.ubuntu.com: Temporary failure in name resolution. If archive.ubuntu.com refers to a configured cache repository, please check the corresponding configuration file.
Reading package lists... Done
E: The repository 'cdrom://Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1) xenial Release' does not 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 xenial-security Release' does not 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://archive.ubuntu.com/ubuntu xenial Release' does not 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://archive.ubuntu.com/ubuntu xenial-updates Release' does not 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.