tomcat 方式启动jenkins时，报：This Jenkins instance appears to be offline and offers options to "Configure Proxy" or "Skip Plugin Installation"
网上有个帖子：
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服务器 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]
确实是因为https的问题，把它修改成http，重启jenkins就OK了