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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.

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

in my case (macOS), updating jdk to 8 and reinstall jenkins solved the problem.

The issue is that for some reason Jenkins doesn't have access to the certificates chain of the Jenkins updates service.
On Ubuntu 18.04 I had this issue and the next links and instructions helped me to start Jenkins properly without modifying the default url or using a proxy of some sort.

Installing a new JDK 8(u101+) with JAVA_HOME should do the trick like at:

• https://www.digitalocean.com/community/tutorials/how-to-install-java-with-apt-on-ubuntu-18-04

is making sure that Lets Encrypt certificates are installed and available on the OS/SYSTEM.
To test it you can use the instructions at:

• Does Java support Let's Encrypt certificates?

And to install manually if required the Let's encrypt root chain using the next script:

• https://gist.github.com/Firefishy/109b0f1a90156f6c933a50fe40aa777e

On MacOS: 10.11.6

Step-1: Go to /Users//.jenkins

Step-2: Open the file: hudson.model.UpdateCenter.xml

Step-3: CHange the value of "default" from "https://xxx" to "http://xxx"

I make no claims about this, especially Java related (I personally cannot stand the language), but on my Ubuntu 18.04 L/T, I needed to stand-up a standalone Jenkins instance; So I installed both the Java on which Jenkins appears to be dependent and Jenkins itself

sudo apt install openjdk-8-jdk jenkins

Then I updated the Jenkins defaults

sudo gvim /etc/default/jenkins

... inserting the following at the top of the file - only because editing /etc/init.d/jenkins appeared to serve no purpose as it had no effect whatsoever ...

JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64/jre/bin/

PATH=$JAVA_HOME:$PATH

Finally, save & quit ... then attempt Jenkins restart

sudo service jenkins restart

HTH - it certainly worked for me :-)

There is something wrong with the internet connection of update center.I have met the same situation with Jenkins installed on macOS.

You can try to modify the hudson.model.UpdateCenter.xml file.

Jenkins installed with .pkg package on macOS,the file path is:
/Users/Shared/Jenkins/Home/hudson.model.UpdateCenter.xml

Jenkins installed by Homebrew on macOS,the file path is:
/Users/{username}/.jenkins/hudson.model.UpdateCenter.xml

Open the file:

<?xml version='1.1' encoding='UTF-8'?>

<sites>

  <site>

    <id>default</id>

    <url>https://updates.jenkins.io/update-center.json</url>

  </site>

</sites>

You can try to change https://updates.jenkins.io/update-center.json to:

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

or

// If you are in China or near China area,https and http are both OK.

https://mirrors.tuna.tsinghua.edu.cn/jenkins/updates/current/update-center.json

And then you can restart Jenkins by:

//loading the URL to restart Jenkins.

http://localhost:8080/restart

or

// Use Homebrew command to restart Jenkins if you installed Jenkins LTS version by Homebrew.  

brew services restart jenkins-lts

For Windows OS,
Follow below steps

1. Go to C:Users*your user name*.jenkins

2. Open the file: hudson.model.UpdateCenter.xml

3. Update the value of "url" from "https://xxx" to "http://xxx"

Enter this before running Jenkins.

# iptalbes -F

It may be because of the firewall.

jenkins_enable="YES"

jenkins_home="/usr/local/jenkins"

jenkins_user="jenkins"

jenkins_args="--webroot=${jenkins_home}/war --httpListenAddress=*.*.*.*  --httpPort=8180"



#jenkins_java_opts="-Djavax.net.ssl.trustStore=/usr/local/jenkins/cacerts -Djava.net.preferIPv6Addresses=true"



#jenkins_java_opts="-Djavax.net.ssl.trustStore=/usr/local/jenkins/cacerts -Djava.net.preferIPv4Addresses=true"