Status returned by url [] is not valid: [503] -> [Help 1]

Hi All,

env :sonarqube 7.9.2 community edition

Need your help on troubleshoot below error .
Doing source code analysis through Jenkins pipeline on Jenkins slave container. To download the maven dependencies we have configured proxy on settings.xml, Due to the proxy on slave container the sonarqube url is not reachable .

Jenkins pipeline snippet

  stage('Analysis') {
        steps {
            script {
				def mvnHome = tool 'maven'
				sh 'mvn clean package sonar:sonar'

Downloaded from central: (625 kB at 311 kB/s)
[INFO] User cache: /root/.sonar/cache
[ERROR] SonarQube server [] can not be reached
[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 01:57 min
[INFO] Finished at: 2020-03-24T16:03:55Z
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.sonarsource.scanner.maven:sonar-maven-plugin: (default-cli) on project demo_web_project: Unable to execute SonarQube: Fail to get bootstrap index from server: Status returned by url [] is not valid: [503] -> [Help 1]


A 503 indicates a server error. What do your server logs say?