ubuntu20.04换清华源后不能更新、更新失败、Certificate verification failed

• • • 原因与官方解决方案：
    • 快速解决问题
    • 附录：问题复现：

务必注意时效性！
日期：2021.12.26

原因与官方解决方案：

https://github.com/tuna/issues/issues/1342

快速解决问题

1. 换回默认的官方源
2. 更新/安装 ca-certificates

ml@ml-virtual-machine:/etc/apt$ sudo apt install ca-certificates
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be upgraded:
  ca-certificates
...
done.

3. 然后换上清华源，不出意外的话就能用了

ml@ml-virtual-machine:/etc/apt$ sudo cp sources.list.th.bak sources.list
ml@ml-virtual-machine:/etc/apt$ sudo apt update
Get:1 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal InRelease [265 kB]
Get:2 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-updates InRelease [114 kB]
...
149 packages can be upgraded. Run 'apt list --upgradable' to see them.

附录：问题复现：

1. 换清华源
2. apt update
3. BOOM！

ml@ml-virtual-machine:/etc/apt$ sudo apt update
Ign:1 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal InRelease
Ign:2 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-updates InRelease
Ign:3 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-backports InRelease
Ign:4 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-security InRelease
Err:5 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal Release
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Err:6 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-updates Release
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Err:7 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-backports Release
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Err:8 https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-security Release
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Reading package lists... Done
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-backports Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-security Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.