Zipkin works well locally but not in docker container. All the microservices are registered well in the Eureka and they can communicate well. But the only problem is Zipkin. I am getting the following error:

org.springframework.web.client.ResourceAccessException: I/O error on POST request for "http://localhost:9411/api/v2/spans": Connect to http://localhost:9411 [localhost/127.0.0.1] failed: Connection refused

my docker-compose.yaml is as follows:

version: '3.8'

services:
  currency-exchange:
    image: samankt/springboot-udemy-currency-exchange:0.0.1-SNAPSHOT
    mem_limit: 512m
    ports:
      - '8000:8000'
    networks:
      - saman-network  
    environment:
      EUREKA.CLIENT.SERVICE-URL.DEFAULTZONE: http://naming-server:8761/eureka
      EUREKA.INSTANCE.PREFERIPADDRESS: true  
      SPRING.ZIPKIN.BASE-URL: http://zipkin-server:9411/
      RABBIT_URI: amqp://guest:guest@rabbitmq:5672
      SPRING_RABBITMQ_HOST: rabbitmq
      SPRING_ZIPKIN_SENDER_TYPE: rabbit
    depends_on:
      - naming-server
      - rabbitmq
      
  api-gateway:
    image: samankt/springboot-udemy-currency-api-gateway:0.0.1-snapshot
    mem_limit: 512m
    ports:
      - '8765:8765'
    networks:
      - saman-network  
    environment:
      EUREKA.CLIENT.SERVICE-URL.DEFAULTZONE: http://naming-server:8761/eureka
      EUREKA.INSTANCE.PREFERIPADDRESS: true  
      SPRING.ZIPKIN.BASE-URL: http://zipkin-server:9411/
      RABBIT_URI: amqp://guest:guest@rabbitmq:5672
      SPRING_RABBITMQ_HOST: rabbitmq
      SPRING_ZIPKIN_SENDER_TYPE: rabbit 
    depends_on:
      - naming-server
      - rabbitmq
      
  currency-converter:
    image: samankt/currency-conversion:0.0.1-SNAPSHOT
    mem_limit: 700m
    ports:
      - '8100:8100'
    networks:
      - saman-network  
    environment:
      EUREKA.CLIENT.SERVICE-URL.DEFAULTZONE: http://naming-server:8761/eureka
      EUREKA.INSTANCE.PREFERIPADDRESS: true  
      SPRING.ZIPKIN.BASE-URL: http://zipkin-server:9411/
      SPRING.ZIPKIN.DISCOVERYCLIENTENABLED: true 
      RABBIT_URI: amqp://guest:guest@rabbitmq:5672
      SPRING_RABBITMQ_HOST: rabbitmq
      SPRING_ZIPKIN_SENDER_TYPE: rabbit     
    depends_on:
      - naming-server
      - rabbitmq
      
  naming-server:
    image: samankt/naming-server:0.0.1-SNAPSHOT
    mem_limit: 512m
    ports:
      - '8761:8761'
    environment:
      SPRING.ZIPKIN.BASE-URL: http://zipkin-server:9411/    
    networks:
      - saman-network
      
  zipkin-server:
    image: openzipkin/zipkin:latest
    mem_limit: 400m
    ports:
      - '9411:9411'
    networks:
      - saman-network   
    environment:
      RABBIT_URI: amqp://guest:guest@rabbitmq:5672
    depends_on:
      - rabbitmq
    restart: always
      
      
  rabbitmq:
    image: rabbitmq:3.8.12-management
    ports:
      - '5672:5672'
      - '15672:15672'
    networks:
      - saman-network
    
networks: 
  saman-network:

SOLUTION 1

The error is telling you that the connection on http://localhost:9411/api/v2/spans is refused. When you run your application inside a docker container you need to send the request not with localhost, but through the docker network (in your case saman-network). To access the docker container which runs zipkin you need to send the request to http://zipkin-server:9411/api/v2/spans.

From Spring Boot 3 Sleuth is deprecated and you will need to use micrometer. To use tracing with zipkin you can add the following 4 dependencies to you pom.xml file

<dependency>
     <groupId>io.micrometer</groupId>
     <artifactId>micrometer-tracing-bridge-brave</artifactId>
</dependency>
<dependency>
    <groupId>io.micrometer</groupId>
    <artifactId>micrometer-observation-test</artifactId>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>io.zipkin.reporter2</groupId>
    <artifactId>zipkin-reporter-brave</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

Finally to change the Zipkin endpoint to send the tracing information you need to add the following to your application.yaml file.

management:
  tracing:
    sampling:
      probability: 1.0 # only for testing purpose, switch back to 0.1 for production code
  zipkin:
    tracing:
      endpoint: http://localhost:9411/api/v2/spans

It is good practice to use Spring profiles, therefore you copy your application.yaml file and name it application-docker.yaml. In this new file you can modify the Zipkin endpoint with the zipkin container name zipkin-server.

management:
  tracing:
    sampling:
      probability: 1.0 # only for testing purpose, switch back to 0.1 for production code
  zipkin:
    tracing:
      endpoint: http://zipkin-server:9411/api/v2/spans

You can activate the Spring profiles passing SPRING_PROFILES_ACTIVATE=docker in the environment.

SOLUTION 2

For you this solution is faster to implement but I didn't test it. You pass the Zipkin url as an environment value SPRING.ZIPKIN.BASE-URL inside the docker-compose.yml. This property was used with the Sleuth package. With Spring Boot 3 and micrometer try to pass instead: MANAGEMENT.ZIPKIN.TRACING.ENDPOINT: http://zipkin-server:9411/api/v2/spans