参考：

1. Git 报错：Failed to connect to github.com port 443: Timed out 解决办法
2. https://blog.csdn.net/twi_twi/article/details/124692509

git 克隆 yolov5 时出现下面错误：

$ git clone https://github.com/ultralytics/yolov5
Cloning into 'yolov5'...
fatal: unable to access 'https://github.com/ultralytics/yolov5/': Failed to connect to github.com port 443: Connection timed out

同时github.com 也打不开
参考 Git 报错：Failed to connect to github.com port 443: Timed out 解决办法 的解决步骤

sudo gedit /etc/hosts

在 hosts 的后面添加：

# GitHub Start
140.82.112.4 github.com
199.232.69.194 github.global.ssl.fastly.net
185.199.108.153 assets-cdn.github.com
185.199.109.153 assets-cdn.github.com
185.199.110.153 assets-cdn.github.com
185.199.111.153 assets-cdn.github.com
# GitHub End

保存后退出：

$ ping github.com
PING github.com (140.82.112.4) 56(84) bytes of data.
64 bytes from github.com (140.82.112.4): icmp_seq=1 ttl=46 time=258 ms
64 bytes from github.com (140.82.112.4): icmp_seq=2 ttl=46 time=243 ms
64 bytes from github.com (140.82.112.4): icmp_seq=3 ttl=46 time=267 ms
64 bytes from github.com (140.82.112.4): icmp_seq=4 ttl=46 time=250 ms
^C
--- github.com ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3002ms
rtt min/avg/max/mdev = 243.457/254.622/267.248/8.937 ms

成功！！！