apt-get update错误之：Could not handshake:The TLS connection was non-properly terminated.[IP:xxxx]

今天在把电脑从Ubuntu16.04利用do-release-upgrade升级到了Ubuntu18.04，惯例update一下，结果就开始报错。具体信息如下：

hurricanezj@xx-Alienware-17-R4:~$ sudo apt-get update
Ign:1 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic InRelease
Ign:2 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates InRelease
Ign:3 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports InRelease
Ign:4 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security InRelease
Err:5 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic Release
Could not handshake: The TLS connection was non-properly terminated. [IP: 127.0.0.1 12333]
Err:6 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates Release
Could not handshake: The TLS connection was non-properly terminated. [IP: 127.0.0.1 12333]
Err:7 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports Release
Could not handshake: The TLS connection was non-properly terminated. [IP: 127.0.0.1 12333]
Err:8 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security Release
Could not handshake: The TLS connection was non-properly terminated. [IP: 127.0.0.1 12333]

显然，主要的报错就是TLS连接中断了，一开始报IP以为是自己代理导致的问题。
后来发现，其实是清华源崩了。更换成阿里云就没事了。

Ubuntu18.04更换国内源

解决之前，还尝试过修改DNS，修改设置里的代理模式，但均不奏效。