Error some index files failed to download

I want install ubuntu desktop but after run sudo apt-get update get error

i did read all article in google but can't solve my problem

root@a:~# sudo apt-get update sudo: unable to resolve host a.com: Resource temporarily unavailable Err:1 http://security.ubuntu.com/ubuntu cosmic-security InRelease Temporary failure resolving 'security.ubuntu.com' Err:2 http://us.archive.ubuntu.com/ubuntu cosmic InRelease Temporary failure resolving 'us.archive.ubuntu.com' Err:3 http://us.archive.ubuntu.com/ubuntu cosmic-updates InRelease Temporary failure resolving 'us.archive.ubuntu.com' Err:4 http://us.archive.ubuntu.com/ubuntu cosmic-backports InRelease Temporary failure resolving 'us.archive.ubuntu.com' Reading package lists... Done W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/cosmic/InRelease Temporary failure resolving 'us.archive.ubuntu.com' W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/cosmic-updates/InRelease Temporary failure resolving 'us.archive.ubuntu.com' W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/cosmic-backports/InRelease Temporary failure resolving 'us.archive.ubuntu.com' W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/cosmic-security/InRelease Temporary failure resolving 'security.ubuntu.com' W: Some index files failed to download. They have been ignored, or old ones used instead. root@a:~# editor /etc/apt/sources.list root@a:~# - run: curl -Lo- "https://raw.githubusercontent.com/bpkg/bpkg/master/setup.sh" | sudo bash sudo: unable to resolve host a.com: Resource temporarily unavailable -: command not found root@a:~# ubuntu-support-status Support status summary of 'a.com':

You have 1578 packages (100.0%) that can not/no-longer be downloaded You have 0 packages (0.0%) that are unsupported

Run with --show-unsupported, --show-supported or --show-all to see more details root@a:~# cat /etc/dpkg/dpkg.cfg.d/multiarch cat: /etc/dpkg/dpkg.cfg.d/multiarch: No such file or directory root@a:~# sudo apt-get update sudo: unable to resolve host a.com: Resource temporarily unavailable Err:1 http://us.archive.ubuntu.com/ubuntu cosmic InRelease Temporary failure resolving 'us.archive.ubuntu.com' Err:2 http://security.ubuntu.com/ubuntu cosmic-security InRelease Temporary failure resolving 'security.ubuntu.com' Err:3 http://us.archive.ubuntu.com/ubuntu cosmic-updates InRelease Temporary failure resolving 'us.archive.ubuntu.com' Err:4 http://us.archive.ubuntu.com/ubuntu cosmic-backports InRelease Temporary failure resolving 'us.archive.ubuntu.com' Reading package lists... Done W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/cosmic/InRelease Temporary failure resolving 'us.archive.ubuntu.com' W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/cosmic-updates/InRelease Temporary failure resolving 'us.archive.ubuntu.com' W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/cosmic-backports/InRelease Temporary failure resolving 'us.archive.ubuntu.com' W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/cosmic-security/InRelease Temporary failure resolving 'security.ubuntu.com' W: Some index files failed to download. They have been ignored, or old ones used instead. root@a:~# sudo apt-get purge docker OR sudo apt-get remove --purge docker sudo: unable to resolve host a.com: Resource temporarily unavailable Reading package lists... Done Building dependency tree
Reading state information... Done

No apt package "docker", but there is a snap with that name. Try "snap install docker"

E: Unable to locate package docker E: Unable to locate package OR E: Unable to locate package apt-get E: Unable to locate package remove E: Unable to locate package docker root@a:~# snap install docke

error: cannot install "docke": Post https://api.snapcraft.io/v2/snaps/refresh: dial tcp: lookup api.snapcraft.io on [::1]:53: read udp [::1]:52028->[::1]:53: read: connection refused root@a:~# root@a:~# sudo apt-mark hold docker sudo: unable to resolve host a.com: Resource temporarily unavailable E: Unable to locate package docker E: No packages found root@a:~# sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup sudo: unable to resolve host a.com: Resource temporarily unavailable root@a:~# editor /etc/apt/sources.list root@a:~# sudo apt-get update sudo: unable to resolve host a.com: Resource temporarily unavailable Err:1 http://security.ubuntu.com/ubuntu cosmic-security InRelease Temporary failure resolving 'security.ubuntu.com' Err:2 http://us.archive.ubuntu.com/ubuntu cosmic InRelease Temporary failure resolving 'us.archive.ubuntu.com' Err:3 http://us.archive.ubuntu.com/ubuntu cosmic-updates InRelease Temporary failure resolving 'us.archive.ubuntu.com' Err:4 http://us.archive.ubuntu.com/ubuntu cosmic-backports InRelease Temporary failure resolving 'us.archive.ubuntu.com' Reading package lists... Done W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/cosmic/InRelease Temporary failure resolving 'us.archive.ubuntu.com' W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/cosmic-updates/InRelease Temporary failure resolving 'us.archive.ubuntu.com' W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/cosmic-backports/InRelease Temporary failure resolving 'us.archive.ubuntu.com' W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/cosmic-security/InRelease Temporary failure resolving 'security.ubuntu.com' W: Some index files failed to download. They have been ignored, or old ones used instead.