Could not resolve host:

报错如下：
root@ubuntu:~# apt-get update
Err:1 /ubuntu xenial InRelease
  Could not resolve host:
Err:2 /ubuntu xenial-updates InRelease
  Could not resolve host:
Err:3 /ubuntu xenial-backports InRelease
  Could not resolve host:
Err:4 /ubuntu xenial-security InRelease
  Could not resolve host:
Reading package lists... Done
W: Failed to fetch /ubuntu/dists/xenial/InRelease  Could not resolve host:
W: Failed to fetch /ubuntu/dists/xenial-updates/InRelease  Could not resolve host:
W: Failed to fetch /ubuntu/dists/xenial-backports/InRelease  Could not resolve host:
W: Failed to fetch /ubuntu/dists/xenial-security/InRelease  Could not resolve host:
W: Some index files failed to download. They have been ignored, or old ones used instead.