root@Debian:~# apt update
Err:1 http://mirrors.ustc.edu.cn/debian stretch InRelease
  Temporary failure resolving 'mirrors.ustc.edu.cn'
Reading package lists... Done
Building dependency tree... Done
All packages are up to date.
W: Failed to fetch http://mirrors.ustc.edu.cn/debian/dists/stretch/InRelease  Temporary failure resolving 'mirrors.ustc.edu.cn'
W: Some index files failed to download. They have been ignored, or old ones used instead.
root@Debian:~# ping mirrors.ustc.edu.cn
PING mirrors.USTC.edu.cn (218.104.71.170) 56(84) bytes of data.
64 bytes from 170.71.104.218.adsl-pool.ah.cnuninet.net (218.104.71.170): icmp_seq=1 ttl=54 time=35.7 ms
64 bytes from 170.71.104.218.adsl-pool.ah.cnuninet.net (218.104.71.170): icmp_seq=2 ttl=54 time=33.9 ms
^C
--- mirrors.USTC.edu.cn ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 33.938/34.830/35.723/0.911 ms