apt更新失败并显示以下错误消息：
$ apt update
Err:1 http://archive.ubuntu.com/ubuntu bionic InRelease
Connection failed [IP: 91.189.88.152 80]
Err:2 http://archive.ubuntu.com/ubuntu bionic-security InRelease
Connection failed [IP: 91.189.88.161 80]
Err:3 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Connection failed [IP: 91.189.88.152 80]
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic/InRelease Connection failed [IP: 91.189.88.152 80]
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-security/InRelease Connection failed [IP: 91.189.88.161 80]
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease Connection failed [IP: 91.189.88.152 80]
W: Some index files failed to download. They have been ignored, or old ones used instead.
但wget对于相同的 url 有效：
wget http://archive.ubuntu.com/ubuntu/dists/bionic/InRelease
--2018-09-26 12:12:17-- http://archive.ubuntu.com/ubuntu/dists/bionic/InRelease
Resolving archive.ubuntu.com (archive.ubuntu.com)... 91.189.88.152, 91.189.88.149, 91.189.88.161, ...
Connecting to archive.ubuntu.com (archive.ubuntu.com)|91.189.88.152|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 242048 (236K)
Saving to: ‘InRelease’
InRelease 100%[==========================================================================================>] 236.38K 1.42MB/s in 0.2s
2018-09-26 12:12:17 (1.42 MB/s) - ‘InRelease’ saved [242048/242048]