Thank you for you reply!
I check my firewall status and it is off
sudo ufw status
Status: inactive
I can open github on my browser,and I try to ping github ,it works:
ping githu.com
PING github.com (140.82.112.3) 56(84) bytes of data.
64 bytes from github.com (140.82.112.3): icmp_seq=1 ttl=43 time=263 ms
64 bytes from github.com (140.82.112.3): icmp_seq=2 ttl=43 time=263 ms
64 bytes from github.com (140.82.112.3): icmp_seq=3 ttl=43 time=264 ms
I really don't know what to do,can anyone help me ?