【環境】
Azure/Ubuntu 18.04.1 LTS(Bionic Beaver)

【症状】
上記環境で、「apt-get update」が失敗します。

以下のような感じ
>sudo apt-get update
--------------------------------結果---------------------------------------------------------------------------------------------------------
Err:1 http://archive.ubuntu.com/ubuntu bionic InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Err:2 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Err:3 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Err:4 http://security.ubuntu.com/ubuntu bionic-security InRelease
  Temporary failure resolving 'security.ubuntu.com'
Err:5 http://ppa.launchpad.net/ethereum/ethereum/ubuntu bionic InRelease
  Temporary failure resolving 'ppa.launchpad.net'
Reading package lists... Done
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-backports/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/bionic-security/InRelease  Temporary failure resolving 'security.ubuntu.com'
W: Failed to fetch http://ppa.launchpad.net/ethereum/ethereum/ubuntu/dists/bionic/InRelease  Temporary failure resolving 'ppa.launchpad.net'
W: Some index files failed to download. They have been ignored, or old ones used instead.
------------------------------------------------------------------------------------------------------------------------------------------------

【やったこと】
①resolved.conf の修正
　『/etc/systemd/resolved.conf』をviで開き
　＃DNS=
　　　↓
　DNS=8.8.8.8
に編集し保存
　＞sudo systemctl restart systemd-resolved
特に変わり無。

-------------
どの様な事を確認し、設定すればよいかアドバイスよろしくお願いします。