解决Some index files failed to download.They have been ignored, or old ones used instead报错

使用sudo apt-get update后报错：

Err:1 http://ppa.launchpad.net/wireshark-dev/stable/ubuntu bionic InRelease
  Temporary failure resolving 'ppa.launchpad.net'
Err:2 http://security.ubuntu.com/ubuntu bionic-security InRelease
  Temporary failure resolving 'security.ubuntu.com'
Err:3 http://archive.ubuntu.com/ubuntu bionic InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Err:4 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Err:5 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
  Temporary failure resolving 'archive.ubuntu.com'
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/wireshark-dev/stable/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.

使用ping www.baidu.com测试一下网络，如果出现：ping: www.baidu.com: Temporary failure in name resolution就是网络问题了，

解决方法

首先

sudo vim /etc/systemd/resolved.conf

修改DNS如下：

[Resolve]
DNS=8.8.8.8
#FallbackDNS=
#Domains=
#LLMNR=no
#MulticastDNS=no
#DNSSEC=no
#Cache=yes
#DNSStubListener=yes

修改下如下配置文件

vim /etc/resolv.conf 

添加如下内容：

nameserver 8.8.8.8

然后重启：

reboot

重启完成后更新一下源就行：

sudo apt-get update