Redes do Docker

In this lesson you connect containers to each other. A network lets containers find one another by name, and you prove it by running a command in one container that reaches a database in another. You also see why a container on the default network cannot do the same thing.

Docker’s own guide to networking is at Docker networking. Two ways of reaching a container are easy to mix up:

• A published port connects your computer to a container. Your browser uses a published port such as 8080 to reach Nginx.
• A network path connects one container to another. A container reaches a second container by the second container’s name and its own port, such as db:5432. No port of your computer is involved.

Step 1: Create a network

Type this command and press Enter:

docker network create course-net

Docker prints a long identifier. The network is called course-net, and every container you attach to it can find the others by name.

Step 2: Start a database on the network

Type this command and press Enter:

docker run -d --name db --network course-net -e POSTGRES_PASSWORD=learner-only-change-me postgres:17

The new part is --network course-net, which attaches the container to your network. The container is named db. This lesson uses the tag 17 for PostgreSQL, as in the Run PostgreSQL lesson. Wait about 10 seconds for the database to start before the next step.

Step 3: Reach the database by name from a second container

Type this command and press Enter. It starts a second, short-lived container on the same network. That container runs the psql command line and asks the database db for one line of text:

docker run --rm --network course-net -e PGPASSWORD=learner-only-change-me postgres:17 psql -h db -U postgres -c "SELECT 'reached db by name' AS result;"

The parts of the command are:

• --rm removes the container as soon as its command finishes.
• -e PGPASSWORD=... gives psql the password, so it does not stop to ask for one.
• psql -h db means “connect to the host named db”. The name is resolved to the database container’s address because both containers are on course-net.

The result is:

        result
----------------------
 reached db by name
(1 row)

The second container reached the first one by its name, with no published port in between.

Step 4: Try the same thing on the default network

Docker also has a default network that every container joins when you do not choose one. Run the same kind of command without --network course-net:

docker run --rm -e PGPASSWORD=learner-only-change-me postgres:17 psql -h db -U postgres -c "SELECT 1;"

This time the command fails. psql reports that it cannot translate the host name db:

psql: error: could not translate host name "db" to address: Name or service not known

The database is still running, but this container is on the default network, and the default network does not let containers find one another by name. Only a network you create yourself, like course-net, provides that. The wording after the host name can differ a little between systems, but the message always names db.

Step 5: Know which path is which

Your browser reaches a container through a published port. The database in this lesson publishes no port, so nothing on your computer can connect to it, and only containers on course-net can reach it. Publishing a database port, such as -p 5432:5432, would let programs on your computer connect as well. Do that only when you need it, and set a strong password when you do.

Containers on the same network use the name and the port inside the container. Inside course-net, the name db and the port 5432 are the address of the database.

Step 6: Clean up

Stop and remove the database, then remove the network. A network can only be removed when no containers are attached to it:

docker stop db
docker rm db
docker network rm course-net

What you learned

• A user-defined network, such as course-net, lets containers find each other by name.
• The default network does not provide name lookups between containers.
• A published port connects your computer to a container. A network connects one container to another.

What comes next

Go on to Docker Compose, which creates a network for you and starts several services together.