出现如下错误:
Err:1 http://wiki.t-firefly.com/firefly-rk3399-repo bionic InRelease
Could not resolve 'wiki.t-firefly.com'
Err:2 https://download.docker.com/linux/ubuntu bionic InRelease
Could not resolve 'download.docker.com'
Err:3 http://mirrors.aliyun.com/ubuntu-ports bionic InRelease
Could not resolve 'mirrors.aliyun.com'
Err:4 http://mirrors.aliyun.com/ubuntu-ports bionic-updates InRelease
Could not resolve 'mirrors.aliyun.com'
Err:5 http://mirrors.aliyun.com/ubuntu-ports bionic-backports InRelease
Could not resolve 'mirrors.aliyun.com'
Err:6 http://mirrors.aliyun.com/ubuntu-ports bionic-security InRelease
Could not resolve 'mirrors.aliyun.com'
Reading package lists... Done
W: Failed to fetch http://mirrors.aliyun.com/ubuntu-ports/dists/bionic/InRelease Could not resolve 'mirrors.aliyun.com'
W: Failed to fetch http://mirrors.aliyun.com/ubuntu-ports/dists/bionic-updates/InRelease Could not resolve 'mirrors.aliyun.com'
W: Failed to fetch http://mirrors.aliyun.com/ubuntu-ports/dists/bionic-backports/InRelease Could not resolve 'mirrors.aliyun.com'
W: Failed to fetch http://mirrors.aliyun.com/ubuntu-ports/dists/bionic-security/InRelease Could not resolve 'mirrors.aliyun.com'
W: Failed to fetch https://download.docker.com/linux/ubuntu/dists/bionic/InRelease Could not resolve 'download.docker.com'
W: Failed to fetch http://wiki.t-firefly.com/firefly-rk3399-repo/dists/bionic/InRelease Could not resolve 'wiki.t-firefly.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.
解决方法:设置妥当DNS服务器即可，如电信网络的设置如下所示
(参考:https://blog.csdn.net/npy_lp/article/details/42080553
https://blog.csdn.net/weixin_30449239/article/details/98918706)
$ sudo vi /etc/resolv.conf
修改为114.114.114.114
添加:
cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 8.8.8.8 nameserver 8.8.4.4
重启网络：
sudo /etc/init.d/networking restart
再执行命令:sudo apt-get update
执行完成,然后:
sudo apt-get upgrade