Before upgrading swarm-plugin to 3.24, we are able to bring up swarm-nodes by specifying the jenkins master docker's service name in -master. An example docker-compose template is follows and it works for 3.23 and earlier versions.

 
services:
    jenkins_master:
        image: jenkins_image
        ...
    ...
  
    swarm_node:
        image: image that contains swarm-client.jar

        command: java -jar swarm-client.jar -master http://jenkins_master:8080 -executors 1
 
When starting up the docker-compose.
 
3.23 successfully connected to the master jenkins:
INFO: Client invoked with: -executors 1 -master http://jenkins_master:8080 -password ***** -username *****
Apr 21, 2021 5:10:50 PM hudson.plugins.swarm.Client run
INFO: Connecting to Jenkins master
Apr 21, 2021 5:10:50 PM hudson.plugins.swarm.Client run
INFO: Attempting to connect to http://jenkins_master:8080/
Apr 21, 2021 5:10:50 PM hudson.remoting.jnlp.Main createEngine
INFO: Setting up agent: ***
Apr 21, 2021 5:10:50 PM hudson.remoting.jnlp.Main$CuiListener <init>
INFO: Jenkins agent is running in headless mode.
Apr 21, 2021 5:10:50 PM hudson.remoting.Engine startEngine
INFO: Using Remoting version: 4.5
 
But 3.24 complained target host is not specified:
Client invoked with: -executors 1 -master http://jenkins_master:8080

hudson.plugins.swarm.Client runINFO: Connecting to Jenkins masterApr 21, 2021 5:18:25 PM hudson.plugins.swarm.Client runINFO: Attempting to connect to http://jenkins_master:8080/Apr 21, 2021 5:18:25 PM hudson.plugins.swarm.Client run
SEVERE: An error occurredshaded.org.apache.hc.client5.http.ClientProtocolException: Target host is not specified at shaded.org.apache.hc.client5.http.impl.classic.InternalHttpClient.doExecute(InternalHttpClient.java:181) at shaded.org.apache.hc.client5.http.impl.classic.CloseableHttpClient.execute(CloseableHttpClient.java:75) at hudson.plugins.swarm.SwarmClient.getJenkinsVersion(SwarmClient.java:590) at hudson.plugins.swarm.SwarmClient.createSwarmAgent(SwarmClient.java:379) at hudson.plugins.swarm.Client.run(Client.java:170) at hudson.plugins.swarm.Client.main(Client.java:53)Caused by: shaded.org.apache.hc.core5.http.ProtocolException: Target host is not specified at shaded.org.apache.hc.client5.http.impl.routing.DefaultRoutePlanner.determineRoute(DefaultRoutePlanner.java:65) at shaded.org.apache.hc.client5.http.impl.classic.InternalHttpClient.determineRoute(InternalHttpClient.java:125) at shaded.org.apache.hc.client5.http.impl.classic.InternalHttpClient.doExecute(InternalHttpClient.java:169) ... 5 more
Apr 21, 2021 5:18:25 PM hudson.plugins.swarm.Client run