Jenkins instance appears offline on Centos - jenkins

Question

I Installed Docker, Docker Compose and then Jenkins on CentOS 8. Seems Jenkins is installed correctly. However I could see the message Jenkins appears to be offline and get an exception as mentioned below. I changed the URL https://updates.jenkins.io/update-center.json in hudson.model.UpdateCenter.xml from https to http. Still the exception reappears. Plugins are not getting upgraded.
Any help on this is appreciated. Thanks.
Exception:
$: docker logs -f jenkins
Running from: /usr/share/jenkins/jenkins.war
webroot: EnvVars.masterEnvVars.get("JENKINS_HOME")
2020-05-16 07:08:57.939+0000 [id=1] INFO org.eclipse.jetty.util.log.Log#initialized: Logging initialized #4453ms to org.eclipse.jetty.util.log.JavaUtilLog
2020-05-16 07:09:02.052+0000 [id=1] INFO winstone.Logger#logInternal: Beginning extraction from war file
2020-05-16 07:09:04.197+0000 [id=1] WARNING o.e.j.s.handler.ContextHandler#setContextPath: Empty contextPath
2020-05-16 07:09:04.990+0000 [id=1] INFO org.eclipse.jetty.server.Server#doStart: jetty-9.4.27.v20200227; built: 2020-02-27T18:37:21.340Z; git: a304fd9f351f337e7c0e2a7c28878dd536149c6c; jvm 1.8.0_242-b08
2020-05-16 07:09:12.128+0000 [id=1] INFO o.e.j.w.StandardDescriptorProcessor#visitServlet: NO JSP Support for /, did not find org.eclipse.jetty.jsp.JettyJspServlet
2020-05-16 07:09:12.439+0000 [id=1] INFO o.e.j.s.s.DefaultSessionIdManager#doStart: DefaultSessionIdManager workerName=node0
2020-05-16 07:09:12.439+0000 [id=1] INFO o.e.j.s.s.DefaultSessionIdManager#doStart: No SessionScavenger set, using defaults
2020-05-16 07:09:12.476+0000 [id=1] INFO o.e.j.server.session.HouseKeeper#startScavenging: node0 Scavenging every 600000ms
2020-05-16 07:09:14.143+0000 [id=1] INFO hudson.WebAppMain#contextInitialized: Jenkins home directory: /var/jenkins_home found at: EnvVars.masterEnvVars.get("JENKINS_HOME")
2020-05-16 07:09:14.794+0000 [id=1] INFO o.e.j.s.handler.ContextHandler#doStart: Started w.#2235eaab{Jenkins v2.237,/,file:///var/jenkins_home/war/,AVAILABLE}{/var/jenkins_home/war}
2020-05-16 07:09:14.871+0000 [id=1] INFO o.e.j.server.AbstractConnector#doStart: Started ServerConnector#5315b42e{HTTP/1.1, (http/1.1)}{0.0.0.0:8080}
2020-05-16 07:09:14.872+0000 [id=1] INFO org.eclipse.jetty.server.Server#doStart: Started #21390ms
2020-05-16 07:09:14.881+0000 [id=20] INFO winstone.Logger#logInternal: Winstone Servlet Engine running: controlPort=disabled
2020-05-16 07:09:18.067+0000 [id=26] INFO jenkins.InitReactorRunner$1#onAttained: Started initialization
2020-05-16 07:09:18.316+0000 [id=25] INFO jenkins.InitReactorRunner$1#onAttained: Listed all plugins
2020-05-16 07:09:22.668+0000 [id=26] INFO jenkins.InitReactorRunner$1#onAttained: Prepared all plugins
2020-05-16 07:09:22.694+0000 [id=26] INFO jenkins.InitReactorRunner$1#onAttained: Started all plugins
2020-05-16 07:09:22.881+0000 [id=25] INFO jenkins.InitReactorRunner$1#onAttained: Augmented all extensions
2020-05-16 07:09:24.037+0000 [id=25] INFO jenkins.InitReactorRunner$1#onAttained: System config loaded
2020-05-16 07:09:24.037+0000 [id=25] INFO jenkins.InitReactorRunner$1#onAttained: System config adapted
2020-05-16 07:09:24.037+0000 [id=25] INFO jenkins.InitReactorRunner$1#onAttained: Loaded all jobs
2020-05-16 07:09:24.038+0000 [id=26] INFO jenkins.InitReactorRunner$1#onAttained: Configuration for all jobs updated
2020-05-16 07:09:24.194+0000 [id=39] INFO hudson.model.AsyncPeriodicWork#lambda$doRun$0: Started Download metadata
2020-05-16 07:09:24.255+0000 [id=39] INFO hudson.util.Retrier#start: Attempt #1 to do the action check updates server
2020-05-16 07:09:27.052+0000 [id=25] INFO o.s.c.s.AbstractApplicationContext#prepareRefresh: Refreshing org.springframework.web.context.support.StaticWebApplicationContext#6069d6b7: display name [Root WebApplicationContext]; startup date [Sat May 16 07:09:27 UTC 2020]; root of context hierarchy
2020-05-16 07:09:27.053+0000 [id=25] INFO o.s.c.s.AbstractApplicationContext#obtainFreshBeanFactory: Bean factory for application context [org.springframework.web.context.support.StaticWebApplicationContext#6069d6b7]: org.springframework.beans.factory.support.DefaultListableBeanFactory#6481ce76
2020-05-16 07:09:27.084+0000 [id=25] INFO o.s.b.f.s.DefaultListableBeanFactory#preInstantiateSingletons: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory#6481ce76: defining beans [authenticationManager]; root of factory hierarchy
2020-05-16 07:09:27.739+0000 [id=25] INFO o.s.c.s.AbstractApplicationContext#prepareRefresh: Refreshing org.springframework.web.context.support.StaticWebApplicationContext#3b4860bb: display name [Root WebApplicationContext]; startup date [Sat May 16 07:09:27 UTC 2020]; root of context hierarchy
2020-05-16 07:09:27.739+0000 [id=25] INFO o.s.c.s.AbstractApplicationContext#obtainFreshBeanFactory: Bean factory for application context [org.springframework.web.context.support.StaticWebApplicationContext#3b4860bb]: org.springframework.beans.factory.support.DefaultListableBeanFactory#5c405df3
2020-05-16 07:09:27.747+0000 [id=25] INFO o.s.b.f.s.DefaultListableBeanFactory#preInstantiateSingletons: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory#5c405df3: defining beans [filter,legacy]; root of factory hierarchy
2020-05-16 07:09:27.955+0000 [id=25] INFO jenkins.InitReactorRunner$1#onAttained: Completed initialization
2020-05-16 07:09:28.492+0000 [id=19] INFO hudson.WebAppMain$3#run: Jenkins is fully up and running
***2020-05-16 07:09:44.531+0000 [id=39] INFO hudson.util.Retrier#start: The attempt #1 to do the action check updates server failed with an allowed exception:
java.net.UnknownHostException: updates.jenkins.io***
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:184)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:607)
at sun.net.NetworkClient.doConnect(NetworkClient.java:175)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:463)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:558)
at sun.net.www.http.HttpClient.<init>(HttpClient.java:242)
at sun.net.www.http.HttpClient.New(HttpClient.java:339)
at sun.net.www.http.HttpClient.New(HttpClient.java:357)
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1226)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1162)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1056)
at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:990)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1570)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1498)
at hudson.model.DownloadService.loadJSON(DownloadService.java:114)
at hudson.model.UpdateSite.updateDirectlyNow(UpdateSite.java:212)
at hudson.model.UpdateSite.updateDirectlyNow(UpdateSite.java:207)
at hudson.PluginManager.checkUpdatesServer(PluginManager.java:1767)
at hudson.util.Retrier.start(Retrier.java:63)
at hudson.PluginManager.doCheckUpdatesServer(PluginManager.java:1738)
at jenkins.DailyCheck.execute(DailyCheck.java:93)
at hudson.model.AsyncPeriodicWork.lambda$doRun$0(AsyncPeriodicWork.java:100)
at java.lang.Thread.run(Thread.java:748)
2020-05-16 07:09:44.536+0000 [id=39] INFO hudson.util.Retrier#start: Calling the listener of the allowed exception 'updates.jenkins.io' at the attempt #1 to do the action check updates server
2020-05-16 07:09:44.544+0000 [id=39] INFO hudson.util.Retrier#start: Attempted the action check updates server for 1 time(s) with no success
2020-05-16 07:09:44.547+0000 [id=39] SEVERE hudson.PluginManager#doCheckUpdatesServer: Error checking update sites for 1 attempt(s). Last exception was: UnknownHostException: updates.jenkins.io
2020-05-16 07:09:44.566+0000 [id=39] INFO hudson.model.AsyncPeriodicWork#lambda$doRun$0: Finished Download metadata. 20,358 ms
2020-05-16 07:10:31.488+0000 [id=56] INFO hudson.model.AsyncPeriodicWork#lambda$doRun$0: Started Periodic background build discarder
2020-05-16 07:10:31.492+0000 [id=56] INFO hudson.model.AsyncPeriodicWork#lambda$doRun$0: Finished Periodic background build discarder. 2 ms

Answer 1

I got the same exception with Kubernetes environment.
My Setup:
-Docker CE
-Kubernetes
Deployment of Jenkins:
deployment.apps/jenkins created
persistentvolume/jenkins created
persistentvolumeclaim/jenkins-claim created
serviceaccount/jenkins created
role.rbac.authorization.k8s.io/jenkins created
rolebinding.rbac.authorization.k8s.io/jenkins created
service/jenkins created
In the pod's log, I could saw this
2020-06-19 05:04:12.590+0000 [id=39] INFO hudson.util.Retrier#start: The attempt #1 to do the action check updates server failed with an allowed exception:
java.net.UnknownHostException: updates.jenkins.io
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:184)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:607)
at sun.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:666)
at sun.net.NetworkClient.doConnect(NetworkClient.java:175)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:463)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:558)
at sun.net.www.protocol.https.HttpsClient.<init>(HttpsClient.java:264)
at sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:367)
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:191)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1162)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1056)
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:177)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1570)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1498)
at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:268)
at hudson.model.DownloadService.loadJSON(DownloadService.java:114)
at hudson.model.UpdateSite.updateDirectlyNow(UpdateSite.java:212)
at hudson.model.UpdateSite.updateDirectlyNow(UpdateSite.java:207)
at hudson.PluginManager.checkUpdatesServer(PluginManager.java:1767)
at hudson.util.Retrier.start(Retrier.java:63)
at hudson.PluginManager.doCheckUpdatesServer(PluginManager.java:1738)
at jenkins.DailyCheck.execute(DailyCheck.java:93)
at hudson.model.AsyncPeriodicWork.lambda$doRun$0(AsyncPeriodicWork.java:100)
at java.lang.Thread.run(Thread.java:748)
<b>2020-06-19 05:04:12.591+0000 [id=39] INFO hudson.util.Retrier#start: Calling the listener of the allowed exception 'updates.jenkins.io' at the attempt #1 to do the action check updates server</b>
2020-06-19 05:04:12.593+0000 [id=39] INFO hudson.util.Retrier#start: Attempted the action check updates server for 1 time(s) with no success
2020-06-19 05:04:12.594+0000 [id=39] SEVERE hudson.PluginManager#doCheckUpdatesServer: Error checking update sites for 1 attempt(s). Last exception was: UnknownHostException: updates.jenkins.io
2020-06-19 05:04:12.597+0000 [id=39] INFO hudson.model.AsyncPeriodicWork#lambda$doRun$0: Finished Download metadata. 20,180 ms
After some time if logs are fetched, you'd see the token for first time Jenkins setup.
get the token and in my case, I had to do a port-forward to 8080.
Once you access Jenkins on the browser, fill in the token and see if Jenkins is offline.
if it is, goto manage Jenkins then advanced tab, scroll down and locate the update.jenkins.io URL at last.
Here you just need to click on submit without tempering anything and apply.
Now check for the updates in the plugins
If this does not work then you can choose HTTP over HTTPS for updates.jenkins.io URL in the same advanced section and submit, apply the changes. Again check for updates in plugins.
If above 2 don't work then it is very much possible you'd have to configure the proxy settings of Jenkins so that Jenkins instance can reach to internet.
The exception is simply saying that Jenkins instance could not fetch the available plugins/updates from updates.jenkins.io site since it can not resolve the URL.
The main idea here is to resolve the URL.
There are a couple of issues related to this URL exception on git issue trackers as well.
Guess you've just hit the common issue.
In my case I had to consider https://kubernetes.io/docs/tasks/debug-application-cluster/dns-debugging-resolution/ for proper debugging of dns.
I guess you can check if you have the network interface with nameserver 8.8.8.8 or the /etc/resolve.conf has the same.

Answer 2

After trying different things, these commands worked for me:
Disable and stop the firewall on my Centos Host
sudo systemctl disable firewalld
sudo systemctl stop firewalld
Then, restart docker
sudo service docker restart