This Jenkins instance appears to be offline

tomcat 方式啓動jenkins時，報：This Jenkins instance appears to be offline and offers options to "Configure Proxy" or "Skip Plugin Installation"

網上有個帖子：

In my case, it has something to do with SSL. I manage to fix it by editing `/var/lib/jenkins/hudson.model.UpdateCenter.xml` and change url to use `http` instead of `https`.

Restart jenkins and reload the website, it no longer shows offline.

[root@server-test jenkins]# find / -name "hudson.model.UpdateCenter.xml"
/work/data/jenkins/hudson.model.UpdateCenter.xml

　

<?xml version='1.0' encoding='UTF-8'?>
<sites>
  <site>
    <id>default</id>
    <url>https://updates.jenkins.io/update-center.json</url>
  </site>
</sites>

在jenkins服務器 直接 wget https://updates.jenkins.io/update-center.json

[root@server-test jenkins]# wget https://updates.jenkins.io/update-center.json
--2017-09-19 12:35:45--  https://updates.jenkins.io/update-center.json
Resolving updates.jenkins.io... 52.202.51.185
Connecting to updates.jenkins.io|52.202.51.185|:443... connected.
ERROR: certificate common name 「pkg.jenkins.io」 doesn’t match requested host name 「updates.jenkins.io」.
To connect to updates.jenkins.io insecurely, use ‘--no-check-certificate’.

wget http://updates.jenkins.io/update-center.json

[root@server-test jenkins]# wget http://updates.jenkins.io/update-center.json
--2017-09-19 12:36:30--  http://updates.jenkins.io/update-center.json
Resolving updates.jenkins.io... 52.202.51.185
Connecting to updates.jenkins.io|52.202.51.185|:80... connected.
HTTP request sent, awaiting response... 301 Moved Permanently
Location: http://updates.jenkins.io/current/update-center.json [following]
--2017-09-19 12:36:30--  http://updates.jenkins.io/current/update-center.json
Reusing existing connection to updates.jenkins.io:80.
HTTP request sent, awaiting response... 302 Found
Location: http://mirrors.jenkins-ci.org/updates/current/update-center.json [following]
--2017-09-19 12:36:31--  http://mirrors.jenkins-ci.org/updates/current/update-center.json
Resolving mirrors.jenkins-ci.org... 52.202.51.185
Reusing existing connection to updates.jenkins.io:80.
HTTP request sent, awaiting response... 302 Found
Location: http://mirrors.tuna.tsinghua.edu.cn/jenkins/updates/current/update-center.json [following]
--2017-09-19 12:36:31--  http://mirrors.tuna.tsinghua.edu.cn/jenkins/updates/current/update-center.json
Resolving mirrors.tuna.tsinghua.edu.cn... 101.6.6.178, 2402:f000:1:416:101:6:6:177
Connecting to mirrors.tuna.tsinghua.edu.cn|101.6.6.178|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1229441 (1.2M) [application/json]
Saving to: 「update-center.json」

100%[=====================================================================================================================================================>] 1,229,441   2.43M/s   in 0.5s    

2017-09-19 12:36:32 (2.43 MB/s) - 「update-center.json」 saved [1229441/1229441]

　　確實是由於https的問題，把它修改爲http，重啓jenkins就OK了