Can ping but not wget between Docker containers

Question

I am using docker-compose to set up a NextJS-app that fetches data from the Wordpress REST-API running in separate containers.

Problem is, I get ECONNREFUSED when I try to fetch or WGET the wordpress-container http://wordpress:8000 from the NextJS-container. I can ping wordpress:8000 without any problems.

If I use Postman or try to fetch the REST-API from another host (i.e. not the machine running docker-compose) using the public ip, it works perfectly.

I'm suspecting some docker configuration issue, but I'm quite lost as the pinging works but not the wget.

Anyone with an idea on what the culprit could be?

My docker-compose.yml:

version: '2'
  services:
  db:
   image: mysql:5.7
   volumes:
     - db_data:/var/lib/mysql
   restart: always
   environment:
     MYSQL_ROOT_PASSWORD: ***REMOVED***
     MYSQL_DATABASE: ***REMOVED***
     MYSQL_USER: ***REMOVED***
     MYSQL_PASSWORD: ***REMOVED***
   networks:
     - back
   wordpress:
     depends_on:
     - db
     image: wordpress:latest
   volumes:
     - ./wp-content:/var/www/html/wp-content 
   ports:
     - "8000:80"
   restart: always
   environment:
     WORDPRESS_DB_HOST: db:3306
     WORDPRESS_DB_USER: root
     WORDPRESS_DB_PASSWORD: ***REMOVED****
   networks:
     - back
   phpmyadmin:
   depends_on:
     - db
   image: phpmyadmin/phpmyadmin
   restart: always
   ports: 
     - 8080:80
   environment:
     PMA_HOST: db
     MYSQL_ROOT_PASSWORD: ***REMOVED***
   networks:
     - back
   next-app:
     depends_on:
       - wordpress
   build:
     context: ./next-app
     dockerfile: Dockerfile
   volumes:
     - './next-app:/usr/src/app'
     - '/usr/src/app/node_modules'
   ports:
     - '80:3000'
   networks:
     - back
 express-server:
   build:
   context: ./express-server
   dockerfile: Dockerfile
   ports:
     - '3001:3001'
networks:
   back:
   driver: bridge
volumes:
   db_data:

Answer 1

From inside the NextJS container, you need to use port 80. Port 8000 is at your's (host) machine.

That means you need to use http://wordpress:80 from inside the docker containers.

---

Ports section in you docker-compose file

   ports:
     - "8000:80"

mean: "Map my local (host machine) port 8000 to container's ports 80", but inside the docker network, it's still port 80

You can ping because ping doesn't use ports. Ports that we are talking about are TCP/UDP ports, see Wikipedia | Port. Ping uses ICMP (Internet Control Message Protocol), not TCP or UDP; ICMP doesn't use ports at all. See Wikipeadia | Ping