Container to container communication in Docker - Commands & Configuration

Introduction

Sometimes, you run multiple containers that need to talk to each other. Container to container communication lets these containers share data or services easily on the same machine or network.

When you want a web app container to get data from a database container on the same host.

When you run a backend service in one container and a frontend in another, and they need to exchange information.

When you want to split your app into smaller parts that work together but run separately.

When you need to connect a logging container to other containers to collect their logs.

When you want to test how two containers interact before deploying them together.

Config File - docker-compose.yml

docker-compose.yml

version: '3.8'
services:
  webapp:
    image: nginx:1.23
    ports:
      - "8080:80"
    networks:
      - app-network
  database:
    image: mysql:8.0
    environment:
      MYSQL_ROOT_PASSWORD: examplepass
    networks:
      - app-network
networks:
  app-network:
    driver: bridge

This file defines two containers: webapp running Nginx and database running MySQL.

Both containers are connected to the same custom network called app-network. This network allows them to communicate by container name.

The ports section exposes the webapp on the host machine's port 8080.

Commands

This command creates a custom network named app-network so containers can communicate on it.

Terminal

docker network create app-network

Expected Output

app-network

Starts a MySQL database container named database connected to app-network with a root password set.

Terminal

docker run -d --name database --network app-network -e MYSQL_ROOT_PASSWORD=examplepass mysql:8.0

Expected Output

a1b2c3d4e5f6g7h8i9j0k1l2m3n4o5p6q7r8s9t0u1v2w3x4y5z6a7b8c9d0e1f2

→

-d - Run container in background (detached mode)

→

--name - Assign a name to the container

→

--network - Connect container to a specific network

Starts an Nginx web server container named webapp connected to app-network and exposes port 8080 on the host.

Terminal

docker run -d --name webapp --network app-network -p 8080:80 nginx:1.23

Expected Output

f1e2d3c4b5a697887766554433221100ffeeddccbbaa99887766554433221100

→

-d - Run container in background (detached mode)

→

--name - Assign a name to the container

→

--network - Connect container to a specific network

→

-p - Map host port to container port

Runs a ping command inside the webapp container to check connectivity to the database container by its name.

Terminal

docker exec webapp ping -c 3 database

Expected Output

PING database (172.18.0.2): 56 data bytes

64 bytes from 172.18.0.2: icmp_seq=0 ttl=64 time=0.123 ms

64 bytes from 172.18.0.2: icmp_seq=1 ttl=64 time=0.110 ms

64 bytes from 172.18.0.2: icmp_seq=2 ttl=64 time=0.105 ms



--- database ping statistics ---

3 packets transmitted, 3 packets received, 0% packet loss

round-trip min/avg/max = 0.105/0.112/0.123 ms

→

-c 3 - Send 3 ping packets then stop

Key Concept

Containers on the same user-defined network can communicate using their container names as hostnames.

Common Mistakes

Not connecting containers to the same custom network

Containers on different networks or the default bridge network cannot resolve each other's names, so they cannot communicate by name.

Create a user-defined network and connect all related containers to it.

Using container IP addresses directly for communication

Container IPs can change when containers restart, causing communication failures.

Use container names as hostnames within the same network for stable communication.

Not exposing or mapping ports when external access is needed

Without port mapping, services inside containers are not reachable from the host or outside.

Use the -p flag to map container ports to host ports when needed.

Summary

Create a user-defined Docker network to allow containers to communicate by name.

Run containers attached to this network so they can find each other easily.

Use container names as hostnames inside the network for stable communication.

Verify connectivity by running commands like ping inside one container targeting another.