Why does Jenkins say "This Jenkins instance appears to be offline"

Question

I have a fresh install of Jenkins 2.32.2 on Ubuntu. Opened a browser on the instance and get to a page titled Offline, with the message "This Jenkins instance appears to be offline" and offers options to "Configure Proxy" or "Skip Plugin Installation"

The machine clearly isn't offline as I just used the internet connection to do the installation. I also had a previous installation, done exactly the same way, that was working. I removed the previous installation as Pipelines wouldn't work and Google told me it was because of JDK 9. So I removed all the JDK/JRE installations as well as jenkins, fresh installed JDK 8 only, installed Jenkins, and got to here.

Jenkins has worked on this box in the past so what makes it think it is offline?

Update: Found a log file /var/lib/jenkins/logs/tasks/Download metadata.log and it starts with a message FATAL: Connection refused (Connection refused) I would think that indicates that the internet connection is OK, but something else is stopping it getting data?

Update: Based on other research I checked the Update Site in the Plugin Manager. This is listed as http://updates.jenkins-ci.org/update-center.json and, from the command line a wget of this file succeeds. So most definitely not a connection issue. Worked with both http and https, jenkins works with neither.

Answer 1

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.

Answer 2

For macOS users:

Step -1: Edit /Users/Shared/Jenkins/Home/hudson.model.UpdateCenter.xml and change url to use http instead of https.

Step -2: Restart jenkins (for instance by going to http:localhost:8080) and reload the website

Step -3(Optional): If it asks for user and password and if you don't remember creating one; default user is admin and copy the password from

sudo cat /Users/Shared/Jenkins/Home/secrets/initialAdminPassword 

credits - @Alex in this post and @Tuan Pham here and @Joe Walsh for the comment on macOS directory path