一，确认是否能ping通

$ ping github.com
Pinging github.com [20.205.243.166] with 32 bytes of data:
Request timed out.

如果不能ping通,先处理ping问题，详见如下链接
Github无法ping通的解决策略

二，clone项目(第4步才成功)

1，git clone https://…
$ git clone https://github.com/tom-wong168/knowledge-system.git
Cloning into 'knowledge-system'...
fatal: unable to connect to github.com:
github.com[0: 140.82.113.3]: errno=Unknown error

2，改https为git，git clone git://…

Cloning into 'knowledge-system'...
fatal: unable to access 'https://github.com/tom-wong168/knowledge-system.git/': Failed to connect to github.com port 443 after 21069 ms: Couldn't connect to server

3，重置 http.proxy

$ git config --global http.proxy http://127.0.0.1:1080
$ git config --global https.proxy https://127.0.0.1:1080
$ git config --global --unset http.proxy
$ git config --global --unset https.proxy
$ git clone git://github.com/tom-wong168/knowledge-system.git
Cloning into 'knowledge-system'...
fatal: unable to access 'https://github.com/tom-wong168/knowledge-system.git/': Failed to connect to github.com port 443 after 21071 ms: Couldn't connect to server

4，设置 http.sslVerify 为 false

$ git config --global http.sslVerify "false"
$ git config --global https.sslVerify "false"
$ git clone git://github.com/tom-wong168/knowledge-system.git
Cloning into 'knowledge-system'...
remote: Enumerating objects: 69, done.
remote: Counting objects: 100% (69/69), done.
remote: Compressing objects: 100% (50/50), done.
remote: Total 69 (delta 14), reused 57 (delta 8), pack-reused 0
Receiving objects: 100% (69/69), 4.52 MiB | 42.00 KiB/s, done.
Resolving deltas: 100% (14/14), done.