I am runningJenkins Master & K8s-Master on same server. Jenkins running through tomcat Apache(not on K8s cluster). I have another server for K8s-Worker-Node, On both the server CentOS-8 OS installed. I have configured Jenkins Kubernetes Plugin version - 1.26.4 But while running pipeline job i always getting an error, Below is K8s cluster Jenkins agent pod log.

[root@K8s-Master /]# kubectl logs -f pipeline-test-33-sj6tl-r0clh-g559d -c jnlp

Aug 08, 2020 8:37:21 AM hudson.remoting.jnlp.Main createEngine
INFO: Setting up agent: pipeline-test-33-sj6tl-r0clh-g559d
Aug 08, 2020 8:37:21 AM hudson.remoting.jnlp.Main$CuiListener <init>
INFO: Jenkins agent is running in headless mode.
Aug 08, 2020 8:37:21 AM hudson.remoting.Engine startEngine
INFO: Using Remoting version: 4.3
Aug 08, 2020 8:37:21 AM org.jenkinsci.remoting.engine.WorkDirManager initializeWorkDir
INFO: Using /home/jenkins/agent/remoting as a remoting work directory
Aug 08, 2020 8:37:21 AM org.jenkinsci.remoting.engine.WorkDirManager setupLogging
INFO: Both error and output logs will be printed to /home/jenkins/agent/remoting
Aug 08, 2020 8:37:21 AM hudson.remoting.jnlp.Main$CuiListener status
INFO: Locating server among [http://jenkins-serverjenkins/]
Aug 08, 2020 8:37:41 AM hudson.remoting.jnlp.Main$CuiListener error
SEVERE: Failed to connect to http://jenkins-server/jenkins/tcpSlaveAgentListener/: jenkins-server
java.io.IOException: Failed to connect to http://jenkins-serverjenkins/tcpSlaveAgentListener/: jenkins-server
        at org.jenkinsci.remoting.engine.JnlpAgentEndpointResolver.resolve(JnlpAgentEndpointResolver.java:217)
        at hudson.remoting.Engine.innerRun(Engine.java:693)
        at hudson.remoting.Engine.run(Engine.java:518)
Caused by: java.net.UnknownHostException: jenkins-server
        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 org.jenkinsci.remoting.engine.JnlpAgentEndpointResolver.resolve(JnlpAgentEndpointResolver.java:214)
        ... 2 more

Below settings configuration already enabled.

Manage Jenkins --> Configure Global Security --> Agents Random [Enabled]
 

I am successfully able to communicate from my Jenkins to the K8s master cluster(Verified in Jenkins Cloud section).

Even in K8s master all the namespace pods are running. weave-net CNI installed, Don't know what is causing problem while agent provisioning through Jenkins.

My Jenkins/K8s master & K8s-Worker-Node /etc/hosts as follows.

# cat /etc/hosts
127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6

75.76.77.5  jenkins-server jenkins-server.company.domain.com
75.76.77.6  k8s-node-1 k8s-node-1.company.domain.com

Below output getting in K8s-Worker node. It looks there is no problem in connecting jenkins-master from K8s-worker node.

#  curl -I http://jenkins-server/jenkins/tcpSlaveAgentListener/
HTTP/1.1 200
Server: nginx/1.14.1
Date: Fri, 28 Aug 2020 06:13:34 GMT
Content-Type: text/plain;charset=UTF-8
Connection: keep-alive
Cache-Control: private
Expires: Thu, 01 Jan 1970 00:00:00 GMT
X-Content-Type-Options: nosniff
X-Hudson-JNLP-Port: 40021
X-Jenkins-JNLP-Port: 40021
X-Instance-Identity: MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAnkgz8Av2x8R9R2KZDzWm1K11O01r7VDikW48rCNQlgw/pUeNSPJu9pv7kH884tOE65GkMepNdtJcOFQFtY1qZ0sr5y4GF5TOc7+U/TqfwULt60r7OQlKcrsQx/jJkF0xLjR+xaJ64WKnbsl0AiZhd8/ynk02UxFXKcgwkEP2PGpGyQ1ps5t/yj6ueFiPAHX2ssK8aI7ynVbf3YyVrtFOlqhnTy11mJFoLAZnpjYRCJsrX5z/xciVq5c2XmEikLzMpjFl0YBAsDo7JL4eBUwiBr64HPcSKrsBBB9oPE4oI6GkYUCAni8uOLfzoNr9B1eImaETYSdVPdSKW/ez/OeHjQIDAQAB
X-Jenkins-Agent-Protocols: JNLP4-connect, Ping
X-Remoting-Minimum-Version: 3.14

# curl http://jenkins-server:40021/
Jenkins-Agent-Protocols: JNLP4-connect, Ping
Jenkins-Version: 2.235.3
Jenkins-Session: 4455fd45
Client: 75.76.77.6
Server: 75.76.77.5
Remoting-Minimum-Version: 3.14

It looks Kubernetes DNS not resolving the name. So any pointers to resolve this problem will help. Thanks.