今天，我在访问 ubuntu.com 域上的任何网站时遇到了问题。
Hmm. We’re having trouble finding that site.
We can’t connect to the server at ubuntu.com.
另外，我无法下载更新。
Err:1 http://security.ubuntu.com/ubuntu hirsute-security InRelease
Temporary failure resolving 'security.ubuntu.com'
Err:2 http://nl.archive.ubuntu.com/ubuntu hirsute InRelease
Temporary failure resolving 'nl.archive.ubuntu.com'
Err:3 http://nl.archive.ubuntu.com/ubuntu hirsute-updates InRelease
Temporary failure resolving 'nl.archive.ubuntu.com'
Err:4 http://nl.archive.ubuntu.com/ubuntu hirsute-backports InRelease
Temporary failure resolving 'nl.archive.ubuntu.com'
status.canonical.com也无法加载（以及 canonical.com 域上的其他网站）。
ubuntu.com 的 DNS 解析：
;; ANSWER SECTION:
ubuntu.com. 18 IN A 91.189.88.181
ubuntu.com. 18 IN A 91.189.88.180
但这些地址无法 ping 通。
更新
重启后，我可以ubuntu.com在 Firefox 中打开并运行apt update。但是，pingubuntu.com仍然不起作用。现在看来ubuntu.com可以通过 IPv6 访问，但不能通过 IPv4 访问。
$ dig a ubuntu.com
<...>
;; ANSWER SECTION:
ubuntu.com. 51 IN A 91.189.88.181
ubuntu.com. 51 IN A 91.189.88.180
<...>
$ ping 91.189.88.181
PING 91.189.88.181 (91.189.88.181) 56(84) bytes of data.
^C
--- 91.189.88.181 ping statistics ---
5 packets transmitted, 0 received, 100% packet loss, time 4076ms
$ tracepath 91.189.88.181
1?: [LOCALHOST] pmtu 1500
1: _gateway 4.869ms
1: _gateway 3.077ms
2: no reply
3: gv-rc0011-cr101-et119-201.core.as33915.net 20.414ms
4: asd-rc0001-cr101-be156-10.core.as9143.net 15.348ms
5: nl-srk03a-ri1-ae50-0.core.as9143.net 14.500ms
6: 213.46.182.142 15.869ms asymm 7
7: ae-2-3210.edge3.London1.Level3.net 22.430ms asymm 9
8: no reply
9: no reply
10: no reply
11: no reply
12: no reply
13: no reply
14: no reply
但是，如果我使用 IPv6：
$ dig aaaa ubuntu.com
;; ANSWER SECTION:
ubuntu.com. 44 IN AAAA 2001:67c:1360:8001::2c
ubuntu.com. 44 IN AAAA 2001:67c:1360:8001::2b
$ ping 2001:67c:1360:8001::2c
PING 2001:67c:1360:8001::2c(2001:67c:1360:8001::2c) 56 data bytes
64 bytes from 2001:67c:1360:8001::2c: icmp_seq=1 ttl=54 time=21.9 ms
64 bytes from 2001:67c:1360:8001::2c: icmp_seq=2 ttl=54 time=23.7 ms
64 bytes from 2001:67c:1360:8001::2c: icmp_seq=3 ttl=54 time=24.3 ms
64 bytes from 2001:67c:1360:8001::2c: icmp_seq=4 ttl=54 time=23.3 ms
^C
--- 2001:67c:1360:8001::2c ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3006ms
rtt min/avg/max/mdev = 21.873/23.277/24.251/0.876 ms