看起来这台主机尤其遇到了数据包丢失：
$ ping security.ubuntu.com
PING security.ubuntu.com (91.189.88.162) 56(84) bytes of data.
64 bytes from yukinko.canonical.com (91.189.88.162): icmp_seq=3 ttl=52 time=38.7 ms
64 bytes from yukinko.canonical.com (91.189.88.162): icmp_seq=5 ttl=52 time=40.2 ms
64 bytes from yukinko.canonical.com (91.189.88.162): icmp_seq=8 ttl=52 time=40.0 ms
64 bytes from yukinko.canonical.com (91.189.88.162): icmp_seq=10 ttl=52 time=41.2 ms
64 bytes from yukinko.canonical.com (91.189.88.162): icmp_seq=17 ttl=52 time=39.6 ms
64 bytes from yukinko.canonical.com (91.189.88.162): icmp_seq=19 ttl=52 time=40.3 ms
^C
--- security.ubuntu.com ping statistics ---
19 packets transmitted, 6 received, 68,4211% packet loss, time 18260ms
rtt min/avg/max/mdev = 38.682/39.970/41.187/0.752 ms