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Container connectivity

Learn how containers on the same network communicate.

Connecting containers

A running container can be connected to other networks later or removed from a network; a container can be on several networks at once. This is used, for example, to temporarily put a debugging tool into the back-end network:

docker network connect arka api
docker network disconnect arka api
docker inspect -f '{{range $ad, $_ := .NetworkSettings.Networks}}{{$ad}} {{end}}' api
# on arka

Alias

--network-alias gives the container an extra DNS name inside the network. Several containers can carry the same alias; the DNS query returns all their IPs (simple load distribution):

docker run -d --network uygulama-agi --network-alias arama elasticsearch:8.15.0
docker run -d --network uygulama-agi --network-alias arama elasticsearch:8.15.0
docker run --rm --network uygulama-agi alpine nslookup arama   # two addresses

Practice Join and leave

Write three commands: connect the running api container to the arka network; run ping -c 1 db inside api; then disconnect api from the arka network.

Starter code

terminal
# buraya yaz

Sample solution

terminal
docker network connect arka api
docker exec api ping -c 1 db
docker network disconnect arka api

Expected output

PING db (172.20.0.2): 56 data bytes
64 bytes from 172.20.0.2: seq=0 ttl=64 time=0.098 ms

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 If a container is attached to two networks, which containers can it reach?

  • Only the host
  • Containers on both networks Correct answer
  • Only those on the first network
  • None

Explanation: It gets a separate interface and IP for each network.

2 Connect a running container to a network.

Question 
docker network ____ arka api
Answer 
docker network connect arka api

Explanation: connect is written in network container order.

3 Match.

Term Meaning
network connect Join a network
network disconnect Leave a network
--network-alias Extra DNS name
nslookup DNS query

All lessons in Beginner

1 What is Docker?

  1. What is Docker?
  2. Containers and virtual machines
  3. Images and containers
  4. Docker architecture
  5. Use cases
  6. Summary - What is Docker

2 Installation and first commands

  1. Installing Docker
  2. docker version
  3. docker info
  4. The hello-world container
  5. Docker Desktop
  6. Summary - Installation

3 First container

  1. docker run basics
  2. Running in the background
  3. docker ps
  4. docker stop and rm
  5. docker logs
  6. Summary - First container

4 Dockerfile basics

  1. What is a Dockerfile?
  2. The FROM instruction
  3. The RUN instruction
  4. COPY and ADD
  5. CMD and ENTRYPOINT
  6. Summary - Dockerfile

5 Build and images

  1. docker build
  2. Tagging images
  3. Build context
  4. Layers and cache
  5. .dockerignore
  6. Summary - Build

6 docker run options

  1. Interactive mode
  2. Naming containers
  3. Restart policy
  4. Working directory
  5. Switching users
  6. Summary - docker run

7 Image management

  1. docker images
  2. docker rmi
  3. docker pull
  4. docker push
  5. docker tag
  6. Summary - Image management

8 Port mapping

  1. The port concept
  2. The -p flag
  3. Multiple ports
  4. Random ports
  5. IP binding
  6. Summary - Port mapping

9 Volumes and data

  1. Data persistence
  2. Named volumes
  3. The -v flag
  4. Bind mounts
  5. Volume management
  6. Summary - Volumes

10 Environment variables

  1. The -e flag
  2. The ENV instruction
  3. The ARG instruction
  4. env-file
  5. The WORKDIR instruction
  6. Summary - Environment

11 Project: docker-compose

  1. Compose design
  2. Service definition
  3. Database service
  4. Volume and network
  5. Putting it together - compose

12 Docker Hub and registry

  1. What is Docker Hub?
  2. docker login
  3. Searching images
  4. Official images
  5. Private registry
  6. Summary - Registry

13 Network basics

  1. Network drivers
  2. docker network ls
  3. Bridge network
  4. Container connectivity
  5. Publishing ports
  6. Summary - Network

14 Beginner final

  1. Challenge - Dockerfile
  2. Challenge - Build and run
  3. Challenge - Volume
  4. Challenge - Compose
  5. Challenge - Beginner project